Glossary

Key terms

Every term is defined in the lesson that introduces it. The glossary is generated from the lessons, so it grows as they do.

21-centimetre line

A faint radio signal from neutral hydrogen. Stretched by expansion to wavelengths of metres, it is the only way to observe the Dark Ages directly, and nobody has yet done so convincingly. From The Dark Ages and the first light

4.2 ka event

An abrupt weakening of monsoon rainfall around 4,200 years ago, recorded in several proxies, coinciding with the end of the urban phase. From The Indus Valley civilisation

Abiogenesis

The origin of life from non-living chemistry. The word names the problem; it does not name a solution. From How chemistry became biology

Absolute magnitude

How bright a star would look from a standard distance of ten parsecs. The astronomer's scale for luminosity, running backwards: smaller numbers are brighter. From One chart for every star

Absorption line

A dark gap in a spectrum where atoms of one element in the star's atmosphere have swallowed light of one exact colour. Each element leaves its own pattern, a barcode. From Reading starlight

Abundance

How much of an element the universe contains. It spans twelve orders of magnitude, and its pattern is a direct record of nuclear physics. From The periodic table as a history book

Accretion disc

The flat, spinning disc of gas and dust that forms around a protostar because the infalling cloud was rotating. Planets form in it. From Gravity versus pressure

Acheulean

The teardrop-shaped handaxe tradition, from about 1.76 million years ago, requiring a mental template and persisting with little change for over a million and a half years. From Fire, tools and cooking

Acoustic peaks

The characteristic sizes of the ripples, set by sound waves that ran through the early plasma. Their pattern encodes the shape and contents of the universe. From The afterglow

Adaptation

A trait shaped by selection because it improved reproduction. Not every trait is one; some are by-products, some are historical accidents. From Evolution by natural selection

Aerobic respiration

Using oxygen to extract energy from food. It yields roughly an order of magnitude more energy per molecule than the anaerobic alternatives, which is why it eventually made large active organisms possible. From The Great Oxidation Event

Agrarian state

A polity whose revenue comes overwhelmingly from taxing farmers. Almost every state between the first cities and industrialisation was one, which constrains what any of them could do. From Empires and exchange

Airglow

Faint light emitted by molecules high in Earth's atmosphere. At the darkest sites on Earth, airglow and sunlight scattered by solar-system dust outshine all the stars and galaxies combined. From Why is the night sky dark?

Alkaline hydrothermal vent

A seafloor vent producing warm alkaline fluid through porous rock. It supplies natural proton gradients across thin mineral walls, which resembles how all life makes energy. From How chemistry became biology

Allotrope

A different structural form of the same element. Carbon alone gives diamond, graphite, graphene, nanotubes and buckyballs, all pure carbon with wildly different properties. From Why carbon?

Alpha process

Adding helium nuclei one at a time to build oxygen, neon, magnesium, silicon and so on. It explains why elements with even atomic numbers are more common than their odd neighbours. From Where the elements come from

Ancestral North Indian and Ancestral South Indian

Two statistically inferred ancestral components identified in 2009, present in almost all Indian groups in varying proportions. They are analytical constructs, not ancient peoples with names. From Out of Africa, into India

Ancient DNA

Genetic material recovered from ancient remains. It preserves poorly in warm climates, which is why South Asia has far fewer ancient genomes than Europe or Siberia. From Out of Africa, into India

Angular momentum

The rotational momentum of a system. The Earth-Moon system has an unusually large amount for its mass, which any explanation of the Moon must account for. From Earth's violent birth

Anisotropy

A difference from place to place. The afterglow's temperature varies across the sky by about one part in a hundred thousand, and those variations carry the story of the early universe. From The afterglow

Anthropocentric framing

The habit of stating cosmic facts in terms of human bodies. Useful for teaching, provided the claim is checked rather than simply repeated. From Are you really made of stardust?

Antimatter

Matter's mirror image: particles with the same mass but opposite charge. When a particle meets its antiparticle, both vanish into light. The early universe held almost equal amounts of each. From The first three minutes

Antimatter

For every particle a mirror partner with opposite charge. Matter and antimatter annihilate on contact, which is why the imbalance in the early universe matters so much. From From quarks to the Higgs

Archaea

The third domain of life, superficially like bacteria but more closely related to us. The host cell in the merger was an archaeon. From The complex cell

Ardipithecus

A hominin from about 4.4 million years ago, described in 2009 from a partial skeleton. It had a grasping big toe and walked upright, which complicated the idea of a chimpanzee-like ancestor. From Primates and hominins

Armstrong limit

The altitude, around 19 kilometres, where pressure falls low enough that body fluids boil at body temperature. Mars's surface pressure is far below it. From Leaving Earth

Asgard archaea

A group of archaea, discovered in deep-sea sediments from 2015 onwards, carrying genes previously thought to be exclusively eukaryotic. The closest known relatives of the host lineage. From The complex cell

Asthenosphere

The weak, partly mobile upper mantle layer on which the rigid plates slide. Still solid: less than a few percent of it is molten anywhere. From Core, mantle, crust

Atmospheric escape

The loss of gas to space. Light gases escape most easily, which is why Earth has no primordial hydrogen atmosphere and why Mars lost most of its air. From Oceans and air

Atomic number

The number of protons in a nucleus, which defines the element. Henry Moseley showed in 1913 that this, not atomic weight, is what orders the table. From The periodic table as a history book

Attirampakkam

A site in Tamil Nadu with Acheulean tools from about 1.5 million years ago and a long later sequence. Evidence that hominins were in South Asia very early. From Out of Africa, into India

Attribution

Determining how much of an observed change, or the probability of a specific event, is due to human influence. It is now a routine, quantitative branch of the science. From The next century: climate

Australopithecus

The genus including Lucy, spanning roughly 4 to 2 million years ago. Fully bipedal, brains close to chimpanzee size, and almost certainly ancestral to our own genus. From Primates and hominins

Autocatalytic set

A group of molecules that collectively catalyse each other's production. It sustains itself and grows, and most people would not call it alive. From What is life?

Axion

A very light hypothetical particle, proposed for other reasons, that would also make a good dark matter candidate. Searches are under way. From Open questions

B²FH

The 1957 paper by Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle that set out how stars build the elements. Still the framework for the whole field. From Where the elements come from

Background extinction

The ordinary rate at which species disappear. Most species that have ever lived went extinct without a catastrophe. From Mass extinctions

Background limit

The average distance a line of sight must travel before it ends on a star, of the order of a hundred thousand billion billion light-years given how sparse stars are. Light has travelled only 13.8 billion. From Why is the night sky dark?

Band

The typical residential group, roughly 25 to 50 people, with flexible membership. People move between bands routinely, which is part of how conflict is managed. From Foraging lifeways

Banded iron formation

Layered iron-rich sedimentary rock that records the chemistry of an ocean without free oxygen. Much of the world's iron ore, including in Odisha and Jharkhand, is of this kind. From Oceans and air

Banded iron formation

Layered iron-rich sediment deposited when dissolved iron met oxygen and rusted out of seawater. Most of the world's iron ore, including India's, is this rock. From The Great Oxidation Event

Baryon acoustic oscillations

The imprint of the sound waves of the early universe on where galaxies sit today: a slight preference for pairs of galaxies about 150 megaparsecs apart, predicted before it was found. From How we know

Baryon-to-photon ratio

The number of matter particles for every photon of light, about six for every ten billion. It is the one number that sets the whole primordial recipe. From The first three minutes

Behavioural modernity

The package of traits taken to indicate a fully modern mind. The concept is increasingly criticised, because the traits appear at different times in different places rather than as a package. From Language and symbolic thought

Benchmark leakage

When test material has appeared in training data, so a reported score measures memorisation rather than capability. It is one of several reasons published performance numbers routinely overstate real performance. From Machines that learn

Bhimbetka

Rock shelters in Madhya Pradesh with painted panels spanning many thousands of years, inscribed by UNESCO in 2003. The Indian record of image-making at scale. From Language and symbolic thought

Big Bang nucleosynthesis

The nuclear fusion of the first minutes, which turned about a quarter of ordinary matter by mass into helium and left the rest as hydrogen. From Was the Big Bang an explosion?

Big Bang nucleosynthesis

The few minutes of nuclear fusion, starting about three minutes after the Big Bang, that turned a quarter of ordinary matter by mass into helium and left the rest as hydrogen. From The first three minutes

Big Bang theory

The model in which the universe has expanded and cooled for about 13.8 billion years from an extremely hot, dense early state. It describes the expansion, not the first instant. From Was the Big Bang an explosion?

Binding energy per nucleon

How tightly the particles in a nucleus are held. It rises from hydrogen to a peak near iron and nickel, then falls. Fusion releases energy below the peak, fission above it. From Where the elements come from

Biogeochemical cycle

The circulation of an element through living and non-living reservoirs. Carbon, nitrogen, phosphorus, sulphur and oxygen each have one, and life runs most of them. From Earth as a system

Biomass

The total mass of living things. Humans and our livestock now account for the overwhelming majority of mammal biomass on Earth. From The Great Acceleration

Biosignature

A pattern that life makes and non-life does not. Almost every candidate has a non-living explanation too, which is the central difficulty of the whole field. From What is life?

Bipedalism

Habitual walking on two legs. It appears in the hominin line millions of years before brain expansion, which reverses the order most people assume. From Primates and hominins

Black hole

The remnant of the heaviest stars, where gravity has overwhelmed every known form of pressure. Pairs of them have been heard merging. From The lives of stars, sorted by mass

Blackbody spectrum

The exact mix of colours emitted by anything in perfect thermal balance. The afterglow's spectrum is the most perfect blackbody ever measured, which is how we know it is thermal. From The afterglow

Blombos Cave

A South African site with engraved ochre from about 77,000 to 100,000 years ago and a pigment-processing workshop at 100,000 years. Among the earliest widely accepted symbolic material. From Language and symbolic thought

Body composition by atom count

A different answer entirely: hydrogen about 62 percent of atoms, oxygen 24 percent, carbon 12 percent. Hydrogen is light, so it dominates the count and not the weight. From Are you really made of stardust?

Body composition by mass

Oxygen about 65 percent, carbon 18 percent, hydrogen 10 percent, nitrogen 3 percent, calcium 1.4 percent, phosphorus 1.1 percent, and the rest under one percent. From Are you really made of stardust?

Bomb spike

The sharp global rise in radioactive fallout, especially plutonium-239, from atmospheric thermonuclear testing between 1952 and 1963. The most abrupt and synchronous global marker in recent sediments. From The Anthropocene

Bond energy

The energy needed to break a bond. Carbon-carbon at about 350 kilojoules per mole sits in the useful middle: stable at everyday temperatures, breakable by enzymes. From Why carbon?

Boring Billion

The period from roughly 1.8 to 0.8 billion years ago when oxygen, climate and evolution all appear unusually static. The name is affectionate and increasingly disputed. From The Great Oxidation Event

Broad-spectrum revolution

A shift before farming towards a wider range of smaller, harder-to-process foods, including grasses and shellfish. It is the behavioural groundwork for cultivation. From Why farm?

Brown dwarf

An object below about eight percent of the Sun's mass, too light to sustain hydrogen fusion. It glows faintly from its own contraction and then cools for ever. From Gravity versus pressure

Bullae and tokens

Clay counters sealed inside clay envelopes to record a transaction. Marking the outside to show the contents is plausibly how writing began. From Villages to cities

Burgess Shale

A Canadian deposit, about 508 million years old, preserving soft tissue in extraordinary detail. It is why we know these animals had guts, eyes and legs rather than just shells. From The Cambrian explosion

Calcium-aluminium inclusion

The oldest solids in the solar system, white specks in primitive meteorites that condensed directly from the cooling nebula. Their age, 4.567 billion years, defines time zero. From From nebula to planets

Cambrian explosion

The appearance in the fossil record, over roughly 20 million years from 538.8 million years ago, of almost every major animal body plan alive today. From The Cambrian explosion

Carbon budget

The total remaining carbon dioxide that can be emitted for a given chance of staying below a temperature level. It is a total, not a rate, which is why delay is expensive. From The next century: climate

Carbon dioxide versus silicon dioxide

Carbon dioxide is a gas that dissolves in water and circulates. Silicon dioxide is sand, a solid that does not. This single difference rules out a silicon-based biosphere on a planet like ours. From Why carbon?

Carbon-14

A radioactive isotope of carbon made continuously in the atmosphere by cosmic rays. Some of the carbon in you is only a few thousand years old, and some was made this century. From Are you really made of stardust?

Carbonaceous chondrite

A primitive meteorite that has never been strongly heated, preserving the organic chemistry of the early solar system. The Murchison meteorite is the most studied. From Molecules in space

Carbonaceous chondrite

A primitive meteorite type containing water-bearing minerals and organic compounds. Its hydrogen isotope ratio is the closest match to Earth's oceans. From Oceans and air

Carbonate-silicate cycle

The slow loop in which carbon dioxide dissolves in rain, weathers silicate rock, is buried as carbonate on the seafloor, and is returned to the air by volcanoes. From Earth as a system

Carboniferous

A geological period roughly 359 to 299 million years ago, when vast swamp forests laid down much of the world's coal. The name means coal-bearing. From Fossil Fuels and Industry

Carrying capacity

The population a given way of life can support in a given place. Farming raises it enormously per unit of land, which is why the transition is irreversible in practice. From Why farm?

Cash crop

A crop grown for sale rather than subsistence. Sugar, tobacco, cotton and indigo drove the plantation system and the demand for coerced labour. From The Columbian Exchange

CASP

Critical Assessment of Structure Prediction, a biennial blind competition in protein structure prediction running since 1994. It is why the AlphaFold result is a measurement rather than a press release. From Machines that learn

Çatalhöyük

A large Anatolian settlement of about 9,000 years ago, with thousands of residents, no streets and no evidence of a ruling class. A town that is not a city. From Villages to cities

Catenation

The ability of an element to bond to itself in chains and rings. Carbon does this better than any other element, which is why molecules of thousands of carbon atoms exist. From Why carbon?

Cepheid variable

A pulsating star whose rhythm reveals its true brightness. Henrietta Leavitt found the rule in 1912; Hubble used it to measure the distance to Andromeda. From Reading starlight

Chandrasekhar limit

About 1.4 solar masses, the most a white dwarf can weigh before its own gravity overwhelms it. Worked out by Subrahmanyan Chandrasekhar in 1930, on the voyage from India to Cambridge. From The lives of stars, sorted by mass

Chattel slavery

Owning a person as property. It requires an economy where a captive can be made to produce more than they consume, which foraging generally does not permit. From The price of farming

Chicxulub

A crater about 180 kilometres across buried beneath the Yucatán Peninsula, produced by an impact 66 million years ago. From Mass extinctions

Chilli

A South American domesticate that reached India through Portuguese trade in the sixteenth century. Almost every dish now thought of as traditionally Indian postdates its arrival. From The Columbian Exchange

Chirality

The handedness of a molecule. Life on Earth uses left-handed amino acids almost exclusively; meteorite amino acids are mostly balanced, with a slight excess in some samples. From Molecules in space

Chirality

Handedness. Life uses left-handed amino acids and right-handed sugars almost exclusively, while ordinary chemistry makes both. Why remains unexplained. From How chemistry became biology

Chloroplast

The compartment that does photosynthesis in plants and algae. A second, later capture, this time of a cyanobacterium. From The complex cell

Chondrule

A millimetre-sized bead of once-molten rock making up much of a primitive meteorite. Something flash-heated dust to melting point and let it cool within hours, and nobody is sure what. From From nebula to planets

Closed ecological system

A habitat that recycles air, water and food indefinitely without resupply. No one has yet built one that works for people at any scale for long. From Leaving Earth

Closure temperature

The temperature below which a mineral stops exchanging parent or daughter atoms with its surroundings. It is the moment a rock's clock starts, and different minerals start at different times. From Deep time

CNO cycle

A faster route to the same result, using carbon, nitrogen and oxygen nuclei as catalysts. It dominates in stars hotter than the Sun and was detected in the Sun itself in 2020. From What a star is

Collapse

The rapid loss of an established level of political complexity. It usually means administration failing rather than population disappearing, which is why the word misleads. From Empires and exchange

Collective learning

The accumulation of knowledge across generations, so that each one starts from where the last finished rather than from the beginning. It is the threshold this chapter is named for. From Collective learning

Collective learning

From lesson 6.5: information accumulating outside individual brains, transmitted with enough fidelity that each generation starts where the last stopped. The question of this lesson is whether machine learning extends it or changes it. From Machines that learn

Colour–magnitude diagram

The practical version of the HR diagram, with colour standing in for temperature and magnitude for brightness, which is what telescopes measure directly. From One chart for every star

Columbian Exchange

Alfred Crosby's term for the transfer of organisms, diseases and people between the Americas and the rest of the world after 1492. The biology mattered more than the politics. From The Columbian Exchange

Commensal pathway

Domestication beginning with an animal attracted to human refuse rather than captured. It is the leading account of dogs, and probably of cats and some birds. From Domestication

Committed emissions

The carbon dioxide that existing infrastructure will emit over its normal operating life if it runs as designed. It is already spent unless something is retired early. From Energy after fossil fuels

Common descent

The claim that all living things share ancestors. It is a separate claim from natural selection, and the evidence for it is separate too. From Evolution by natural selection

Concordance model

Also called Lambda-CDM: the six-parameter description of the universe, with a cosmological constant and cold dark matter, that fits the afterglow, galaxy surveys, supernovae and lensing at once. From How we know

Confinement

Why free quarks are never seen: pulling two apart costs so much energy that new quarks are created from it instead, giving two new particles rather than one loose quark. From From quarks to the Higgs

Continental collision

What happens when two continents meet: neither can subduct, because continental crust is too buoyant, so the crust thickens and mountains rise. From Plate tectonics and the Himalaya

Continental drift

Wegener's 1912 proposal that the continents have moved. Correct in its conclusion, rejected for fifty years because he could suggest no mechanism. From Plate tectonics and the Himalaya

Controlled experiment

Deliberately varying one thing while holding others fixed, to isolate a cause. It is the single most powerful item in the method and it is not obvious. From The Scientific Revolution

Convective zone

The outer layer of a Sun-like star where energy is carried by rising hot gas and sinking cool gas, like water boiling in a pot. From What a star is

Cooking hypothesis

Richard Wrangham's argument that control of fire was the change that made humans, by raising the energy available from food. From Fire, tools and cooking

Cool early Earth

The proposal, based on zircon chemistry, that Earth's surface had liquid water and moderate temperatures within about 150 million years of formation. From Earth's violent birth

Core accretion

The standard explanation for giant planets: a rocky core of about ten Earth masses forms first, then captures gas from the disc, provided it does so before the disc disperses. From From nebula to planets

Core-collapse supernova

The explosion of a star of more than about eight solar masses when its iron core can no longer support itself, briefly outshining its whole galaxy. From The lives of stars, sorted by mass

Core-collapse supernova

The death of a massive star. It scatters the oxygen, magnesium and silicon made during the star's life, and makes iron in the explosion itself. From Making gold

Corona

The Sun's outer atmosphere, visible during total eclipses, at one to three million kelvin: hundreds of times hotter than the surface beneath it, for reasons still argued over. From The Sun, a typical star

Coronal mass ejection

A billion-tonne cloud of magnetised gas hurled from the corona into space. When one hits Earth it lights the aurora and can knock out power grids and satellites. From The Sun, a typical star

Corvée

Unpaid labour owed to a ruler or state, typically for building and irrigation. It is the commonest form of extraction in early agrarian states, and it precedes money. From The price of farming

Cosmic Dawn

The period when the first stars and galaxies formed and their light began to change the universe around them, roughly 100 to 500 million years after the Big Bang. From The Dark Ages and the first light

Cosmic microwave background

The afterglow of the early universe, released about 380,000 years after the Big Bang and now cooled to 2.7 kelvin. It arrives from every direction. From Was the Big Bang an explosion?

Cosmic microwave background

The oldest light in the universe, released about 380,000 years after the Big Bang when the universe first became transparent, now stretched to microwaves at 2.7 kelvin and arriving from every direction. From The afterglow

Cosmic optical background

The visible-light part of the extragalactic background, measured directly from beyond Pluto by the New Horizons spacecraft at about 11 nanowatts per square metre per steradian. From Why is the night sky dark?

Cosmic web

The largest structure there is: galaxies strung along filaments and sheets around vast empty voids, grown by gravity from the ripples in the afterglow. From Galaxies

Cosmic-ray spallation

The chipping of heavy nuclei by high-energy particles, which made most of the lithium, beryllium and boron in your body. From Are you really made of stardust?

Cosmological constant

Einstein's symbol, Lambda, for an energy density of empty space that does not change with time. The simplest form dark energy could take, and so far the one that fits. From Open questions

Covalent bond

A bond in which two atoms share a pair of electrons. Carbon forms four of them, and they are strong enough to be stable and weak enough to be broken by life's chemistry. From Why carbon?

CRISPR-Cas9

A bacterial defence system, repurposed as a programmable tool. A guide RNA specifies a DNA sequence and the Cas9 protein cuts there. The programmability is the invention; the system is billions of years old. From Editing life

Crowd disease

An infection needing a large, dense host population to survive, because it kills or immunises its hosts quickly. Measles, smallpox and mumps are examples, and none could persist among foragers. From The price of farming

Cumulative culture

The technical term for the same thing: culture that ratchets upward rather than resetting. It requires high-fidelity transmission, because anything copied imperfectly decays. From Collective learning

Cumulative technology

Tools that improve across generations because each learner starts from what the last one achieved. Its near-absence for two million years is the puzzle of this lesson. From Fire, tools and cooking

Cuneiform

The wedge-shaped script of Mesopotamia. The earliest tablets are overwhelmingly accounts: quantities of grain, animals and labour, with names attached. From Villages to cities

Dark Ages

The stretch after the afterglow, before the first stars, when the universe was transparent but nothing in it shone. From The afterglow

Dark Ages

The stretch after the afterglow faded and before the first stars formed, when the universe was transparent but nothing in it shone. Roughly the first hundred million years. From The Dark Ages and the first light

Dark energy

Whatever is making the expansion of the universe speed up. It behaves like an energy belonging to empty space itself, and it makes up about two thirds of the universe's contents. From Open questions

Dark matter

Matter that neither emits nor absorbs light and reveals itself only by gravity. About five times more of it than ordinary matter. Its existence is established; its identity is not. From Open questions

Dark matter halo

The invisible envelope in which every galaxy sits, extending far beyond its stars and holding most of its mass. Galaxies form inside halos, not the other way round. From Galaxies

Darwinian evolution

Replication with variation, where the variation affects how much replication happens. It is the only item on the standard list of life's properties that non-living chemistry has never been shown to do on its own. From What is life?

De-urbanisation

The dispersal of urban populations into smaller settlements. The preferred term for what happened after 1900 BCE, because the population did not vanish. From The Indus Valley civilisation

Deccan Traps

One of the largest volcanic outpourings in Earth's history, erupted across peninsular India about 66 million years ago as the plate passed over a hotspot. From Plate tectonics and the Himalaya

Deccan Traps

The vast flood basalts of peninsular India, erupted across the Cretaceous-Palaeogene boundary. The main rival and possible partner to the impact. From Mass extinctions

Deep time

The vast timescale of geology, first argued for in the eighteenth century, in which ordinary slow processes have had enough time to build and destroy mountains repeatedly. From Deep time

Degenerate era

The long period after star formation ends, when the universe consists mainly of stellar remnants: white dwarfs, neutron stars and black holes. From The deep future

Deindustrialisation

The collapse of an existing manufacturing sector. India's textile industry, the largest in the world in 1750, is the standard case. From Fossil Fuels and Industry

Delayed-return economy

Investing in stored food, fish traps or other assets that pay off later. It permits accumulation, and with accumulation comes inequality. From Foraging lifeways

Delivery

The idea that comets and asteroids brought water and organic molecules to the young Earth after its surface had cooled. From Molecules in space

Delta-v

The change in velocity a manoeuvre requires. It is the currency of spaceflight: every destination has a price in delta-v, and the price does not negotiate. From Leaving Earth

Demand sharing

A norm in which asking for something obliges giving. It circulates food and dampens accumulation, and it can be experienced as an obligation rather than a generosity. From Foraging lifeways

Demographic hypothesis

The proposal that the apparent explosion of complexity was driven by population size and connection rather than by a change in the brain. From Collective learning

Denisovan

A population identified in 2010 from DNA in a finger bone rather than from anatomy. A human group discovered by genome sequencing before anyone knew what it looked like. From From habilis to sapiens

Density wave

A spiral arm is not a fixed set of stars but a pattern, a traffic jam moving through the disc, in which gas piles up and new stars are born. From Galaxies

Deuterium

Heavy hydrogen: a nucleus of one proton and one neutron. It is the first step of every fusion chain and the first thing the early universe had to build. From The first three minutes

Deuterium bottleneck

The delay before fusion could start. Until about three minutes, the light was intense enough to break every deuterium nucleus apart the moment it formed. From The first three minutes

Deuterium-to-hydrogen ratio

The proportion of heavy hydrogen in water. It differs between asteroids, comets and Earth's oceans, and is the main tool for tracing where the water came from. From Oceans and air

Dholavira

A city in the Rann of Kutch, Gujarat, inscribed by UNESCO in 2021. Built on an island with no perennial river, it survived on an extraordinary system of reservoirs cut into rock. From The Indus Valley civilisation

Differentiated teeth

Incisors, canines, premolars and molars doing different jobs in one mouth. Reptile teeth are mostly interchangeable pegs. Chewing is a mammalian invention and it is why fossil teeth identify species. From Mammals in the shadows

Differentiation

The separation of a planet into layers by density, with metal sinking and rock floating. It happens once, while the planet is hot enough to be partly molten. From Core, mantle, crust

Dipole

The afterglow looks slightly warmer in one direction and cooler in the opposite one because the solar system is moving through it at about 370 kilometres per second. From The afterglow

Displacement

The ability to refer to what is not present: last year, the next valley, a person who died. Almost no animal communication does this, and all human language does. From Language and symbolic thought

Distance ladder

The chain of methods, each calibrated by the one below, that carries measured distances from the nearest stars to the edge of the observable universe. From Reading starlight

Dmanisi

A site in Georgia with five skulls around 1.8 million years old, whose variation within one population is as large as the variation used to separate several named species. From From habilis to sapiens

Domestication

A multi-generational relationship in which a population becomes genetically adapted to living with humans. It is a process rather than an event, and both parties change. From Domestication

Domestication syndrome

The recurring set of changes seen across domesticated species: reduced size in some, smaller brains, retained juvenile features, altered coat or seed coat, and reduced fear responses. From Domestication

Doppler shift

Light from something moving away is stretched to the red; from something approaching, squeezed to the blue. The shift of a star's spectral lines gives its speed along the line of sight. From Reading starlight

Dunkelflaute

A German term for a still, dark spell lasting days over a large region. It is the specific problem that makes the last few per cent of a wind-and-solar grid hard. From Energy after fossil fuels

Earth observation

Measuring the planet from orbit. It is where most of the practical value of spaceflight actually is, and it is why the groundwater result in lesson 8.4 was possible at all. From Leaving Earth

Earth system

The planet treated as one set of interacting parts: rock, water, air, ice and life, exchanging matter and energy. Studying any one alone gives wrong answers. From Earth as a system

Ecological release

Rapid expansion into niches vacated by others. It is the mechanism behind the mammal radiation, and it required no change in mammals themselves. From Mammals in the shadows

Ecumene

The connected zone within which goods, diseases, technologies and ideas circulate. By 1000 CE most of Afro-Eurasia was one; the Americas were another. From Empires and exchange

Ediacaran biota

Soft-bodied organisms from 575 to 539 million years ago, mostly frond-like and immobile. Some may be animals, some may be nothing alive today, and most vanish at the boundary. From The Cambrian explosion

Effective population size

The number of people actually exchanging ideas, which depends on connection as well as headcount. It determines whether an innovation is retained or lost. From Collective learning

Electromagnetism

The force between charged particles, carried by photons. It builds every atom, every molecule and every chemical bond in this chapter. From From quarks to the Higgs

Empire

A state ruling over other peoples who retain some distinct identity. The word implies unequal incorporation, which is what distinguishes it from a large kingdom. From Empires and exchange

Empiricism

Grounding claims in observation rather than in authority or pure reasoning. The idea is ancient; making it the rule rather than one option among several is what changed. From The Scientific Revolution

Enamel hypoplasia

Horizontal defects in tooth enamel recording an interruption to growth in childhood. Because enamel does not remodel, it preserves a permanent record of early stress. From The price of farming

Encephalisation

Brain size relative to body size. It rises through the genus Homo, and it is a poor guide to capability on its own, since Neanderthals exceeded us. From From habilis to sapiens

Endogamy

Marriage within a group. Genetic evidence indicates it became widespread in India within the last two thousand years, which is much more recent than the population mixing that preceded it. From Out of Africa, into India

Endogenous retrovirus

A viral sequence inserted into a genome and inherited since. Humans and chimpanzees share thousands at identical positions, which is very hard to explain except by shared ancestry. From Evolution by natural selection

Endosymbiosis

One cell living inside another and the arrangement becoming permanent. Lynn Margulis argued in 1967 that this is where mitochondria came from, and was widely dismissed. From The complex cell

Endothermy

Generating body heat internally. It is expensive, requiring roughly ten times the food of a comparable reptile, and it buys activity at night and in the cold. From Mammals in the shadows

Energy per gene

The argument that internalising power generation gave eukaryotes orders of magnitude more energy available per gene, which is what large genomes require. From The complex cell

Energy poverty

Consumption too low to support health, education and productive work. Roughly a third of humanity is in this position, and the transition has to raise their energy use, not lower it. From Energy after fossil fuels

Energy return on investment

How much usable energy you get back for the energy you spend getting it. Early coal and oil returned enormously more than they cost, which is what made everything else possible. From Fossil Fuels and Industry

Energy versus power

Power is a rate, in watts; energy is an amount, in watt-hours. Batteries are cheap for power and expensive for energy, which is why they cover hours well and seasons badly. From Energy after fossil fuels

Epidemiological transition

A shift in the dominant causes of death. The first one, with farming, brought crowd infections that require dense populations to persist. From The price of farming

Epoch

A formal division of the geological timescale, below a period and above an age. The Holocene is an epoch; so is the Pleistocene. From The Anthropocene

Equation of state

The relation between a component's pressure and its energy density, written as w. For dark energy, whether w equals exactly minus one, and whether it changes with time, determines how the universe ends. From The deep future

Equilibrium climate sensitivity

How much the planet warms, at equilibrium, for a doubling of atmospheric carbon dioxide. It is the single most important number in climate science, and its uncertainty range was stuck for forty years before recently narrowing. From The next century: climate

Ethnographic analogy

Using living foragers to interpret the past. Necessary, and dangerous, because living foragers occupy marginal land and have long histories of contact. From Foraging lifeways

Eukaryote

A cell with a nucleus and internal compartments. Every animal, plant, fungus and alga is made of them, and they all descend from one merger. From The complex cell

Event versus epoch

The alternative that prevailed. An event is a described happening with a duration; an epoch is a formal chronostratigraphic unit with a defined base. Geology uses both. From The Anthropocene

Expensive tissue hypothesis

The proposal that brains could enlarge only because guts shrank, since both are metabolically costly and the total energy budget did not change. From Fire, tools and cooking

Externalised digestion

Doing outside the body what a gut would otherwise do inside it. Cutting, pounding and cooking all pre-process food, which is why human guts are small for a primate. From Fire, tools and cooking

Externalised memory

Storing knowledge outside any individual: in tools, in traditions, in writing, in machines. It is why a society can know more than any of its members. From Collective learning

Extraction

Moving the agricultural surplus from those who produce it to those who rule. Every agrarian state is a machine for doing this, and the differences between them are largely differences in method. From Empires and exchange

Extragalactic background light

The combined glow of every star and galaxy outside our own, summed over the whole sky: about 50 nanowatts per square metre per steradian in the optical and infrared together. From Why is the night sky dark?

Faint young Sun problem

The Sun was about 30 percent dimmer when Earth formed, which should have left the planet frozen, yet there is evidence for liquid water. Resolving it requires a much stronger greenhouse effect. From Oceans and air

Falsifiable

Capable of being shown wrong by a measurement. A claim that no observation could ever contradict is not scientific, however appealing. From How we know

Fertile Crescent

The arc from the Levant through southeastern Anatolia to the Zagros foothills, where wheat, barley, lentils, sheep and goats were domesticated. The earliest and best-documented centre, not the only one. From Why farm?

Fetal haemoglobin

The oxygen-carrying protein made before birth, normally switched off afterwards. Switching it back on is the mechanism behind the first approved CRISPR therapy — a repair that does not touch the broken gene at all. From Editing life

Figurative art

Images of recognisable things. The oldest securely dated examples are in Indonesia, not Europe, which corrects a long-standing bias in the field. From Language and symbolic thought

Firm low-carbon resource

A generator that can supply power on demand for long periods without emitting: nuclear fission, geothermal, hydro with storage, or combustion with carbon capture. Whether they are needed is one of the field's live arguments. From Energy after fossil fuels

Fission-fusion

Groups splitting and recombining seasonally with resources. It means the social unit is not fixed, and it makes census figures for foraging societies awkward. From Foraging lifeways

Fitness

Expected number of surviving offspring in a particular environment. Not strength, speed or complexity. A parasite that loses its gut and its eyes can be fitter than its free-living ancestor. From Evolution by natural selection

Forager

Someone who lives by gathering, hunting and fishing rather than by farming or herding. Preferred to hunter-gatherer because gathering usually supplies more of the food than hunting. From Foraging lifeways

Foramen magnum

The hole where the spinal cord enters the skull. In a bipedal animal it sits underneath, balancing the head on the spine; in a quadruped it sits at the back. From Primates and hominins

Founder crops

The eight species domesticated together in the Fertile Crescent: emmer and einkorn wheat, barley, lentil, pea, chickpea, bitter vetch and flax. From Domestication

Founder effect

The loss of genetic diversity when a small group leaves a large population. It is why the whole world outside Africa is genetically a subset of Africa. From Out of Africa, into India

FOXP2

A gene whose disruption causes severe speech and language impairment. It was once called the language gene, which was always an overstatement and is now understood as one component among many. From Language and symbolic thought

Free-fall time

How long a cloud would take to collapse if nothing resisted gravity: a few hundred thousand years for a dense core. Real clouds take longer, because something always resists. From Gravity versus pressure

Functional group

A small cluster of atoms, such as hydroxyl or amino, attached to a carbon skeleton, which gives the molecule a particular behaviour. Life builds by combining skeletons with groups. From Why carbon?

Gaia hypothesis

Lovelock and Margulis's proposal that life actively regulates the planet's surface conditions. Influential, partly right, and disputed in its stronger forms. From Earth as a system

Galactic cosmic rays

High-energy charged particles from outside the Solar System. They are hard to shield against because stopping them produces showers of secondary particles, and light shielding can make the dose worse. From Leaving Earth

Galaxy

A gravitationally bound system of stars, gas, dust and dark matter, from a few million to several trillion stars. The Milky Way is one of hundreds of billions. From Galaxies

Galaxy cluster

A gravitationally bound swarm of hundreds to thousands of galaxies in hot gas and dark matter. The nearest large one is in Virgo, about fifty million light-years away. From Galaxies

Gelatinisation

What heat does to starch, opening its structure so digestive enzymes can reach it. It is the chemistry behind why cooked food yields more calories. From Fire, tools and cooking

Gene drive

A genetic element engineered to be inherited by nearly all offspring rather than half, so it spreads through a wild population. It is the first technology explicitly designed to alter a species in the wild, and it cannot be recalled. From Editing life

Genetic drift

Change in gene frequencies by chance alone. It dominates in small populations and is why not everything that spreads is an improvement. From Evolution by natural selection

Geodynamo

The process by which convection in the liquid outer core, organised by Earth's rotation, sustains the magnetic field. First simulated successfully in 1995. From Core, mantle, crust

Germline edit

A change to eggs, sperm or early embryos. It is inherited by every subsequent generation, and the people affected cannot consent. From Editing life

Ghaggar-Hakra

A seasonal river system in northwest India and Pakistan along which a large number of Harappan sites lie. Its ancient flow regime is central to arguments about the civilisation's rise and decline. From The Indus Valley civilisation

Ghost acres

Kenneth Pomeranz's term for land elsewhere that supplies what your own land cannot. Coal was ghost acres of a different kind: it substituted for forest Britain did not have. From Fossil Fuels and Industry

Giant branch

The region above and to the right of the main sequence occupied by stars that have swollen after exhausting their core hydrogen: cool but bright because they are enormous. From One chart for every star

Giant impact hypothesis

The explanation for the Moon: a Mars-sized body struck the young Earth, and the Moon formed from the debris thrown into orbit. From Earth's violent birth

Gini coefficient

A measure of inequality from 0 to 1. Archaeologists have applied it to house-size distributions to compare inequality between ancient settlements. From Villages to cities

Gondwana

The southern supercontinent that included India, Africa, South America, Antarctica and Australia. India broke away from it about 130 million years ago. From Plate tectonics and the Himalaya

Grain surface chemistry

Reactions between atoms stuck to the surface of a cold dust grain. The grain acts as a meeting place and a heat sink, and makes reactions possible that could never happen in the gas. From Molecules in space

Grand Tack

A model in which Jupiter migrated inward to about the present orbit of Mars, then turned back outward when caught by Saturn, explaining why Mars is so small. From From nebula to planets

Gravity

By far the weakest force, and the only one not described by the Standard Model. It is negligible between particles and dominant between planets, because it never cancels out. From From quarks to the Higgs

Great Oxidation Event

The rise of free oxygen in the atmosphere around 2.4 billion years ago, from negligible levels to perhaps a few percent of today's. From The Great Oxidation Event

Green Revolution

The spread from the 1960s of high-yielding cereal varieties with fertiliser, irrigation and pesticides. In India it turned a food-importing country into a food-exporting one within two decades. From The Great Acceleration

Greenhouse effect

The warming produced when gases absorb outgoing infrared radiation. Without it Earth's average surface temperature would be about 33 degrees colder. From Oceans and air

Groundwater depletion

Extraction of water from aquifers faster than they recharge. Measurable from orbit by the tiny gravity changes the missing mass causes. From The Great Acceleration

Group and period

A column and a row. Elements in a group have the same outer-electron arrangement and so behave alike; a period is one filling of an electron shell. From The periodic table as a history book

GSSP

Global Boundary Stratotype Section and Point, informally the golden spike. A single physical location whose layers define the start of a time unit for the whole world. From The Anthropocene

Gunn–Peterson trough

A stretch of a distant quasar's spectrum blacked out by neutral hydrogen. Its appearance in quasars beyond redshift 6 marks the end of reionisation. From The Dark Ages and the first light

GW170817

The neutron-star merger detected on 17 August 2017 in gravitational waves and then in light by seventy observatories: the first event seen in both. From Making gold

Haber-Bosch process

Industrial synthesis of ammonia from atmospheric nitrogen and hydrogen. It broke the nitrogen limit on agriculture and is why the population curve looks the way it does. From The Great Acceleration

Habitability

A planet's capacity to sustain liquid water and life. On Earth it has depended on tectonics, a magnetic field, a large moon and a working thermostat. From Earth as a system

Hadean

Earth's first geological eon, from 4.57 to 4.0 billion years ago, named for the Greek underworld. Almost no rock survives from it. From Earth's violent birth

Half-life

The time for half the atoms of a radioactive isotope to decay. It is a property of the nucleus alone, which is what makes it a reliable clock. From Deep time

Harappan

The standard name for the civilisation, after Harappa, the first site excavated. Also called the Indus or Indus-Sarasvati civilisation, and the naming itself is contested. From The Indus Valley civilisation

Hard-to-abate

Uses where electrification is difficult: aviation, long-distance shipping, steelmaking, cement, and high-temperature industrial heat. Together they are roughly a quarter of emissions. From Energy after fossil fuels

Hawking radiation

Thermal radiation predicted to be emitted by black holes, causing them to evaporate over immense timescales. Never directly observed, and the theoretical basis is solid enough to build on and not settled. From The deep future

Helioseismology

Reading the Sun's interior from the sound waves that make its surface ring, the way seismologists read the Earth from earthquakes. From The Sun, a typical star

Hertzsprung–Russell diagram

A plot of stars by luminosity against temperature, found independently by Ejnar Hertzsprung and Henry Norris Russell around 1911 to 1914. Stars do not scatter randomly on it; they fall into bands that tell their story. From One chart for every star

Hierarchical formation

Galaxies grew from the bottom up, small clumps merging into larger ones over billions of years. The Milky Way's own history of mergers is written in the motions of its stars. From Galaxies

Higgs field

A field filling all of space that gives mass to the particles moving through it. Proposed in 1964, confirmed in 2012 by finding its associated particle, the Higgs boson. From From quarks to the Higgs

High-fidelity transmission

Copying accurately enough that complexity is not lost each generation. Teaching, language and imitation of process rather than outcome are what supply it. From Collective learning

High-wage economy

Robert Allen's argument that British wages were high and energy cheap, so machines that replaced labour paid off in Britain and not elsewhere. From Fossil Fuels and Industry

Holocene

The current geological epoch, beginning about 11,700 years ago. It is unusually stable in climate compared with the preceding hundred thousand years, and that stability is the precondition for farming. From Why farm?

Homeostasis

Holding internal conditions steady while the outside world changes. It costs energy continuously, which is why life is a process rather than a substance. From What is life?

Hominin

The group containing humans and all our extinct relatives since the split from chimpanzees. Chimpanzees are not hominins and are not our ancestors; they are our cousins. From Primates and hominins

Homo erectus

The first hominin to leave Africa, appearing around 2 million years ago and surviving until perhaps 110,000 years ago. The longest-lived human species by a wide margin. From From habilis to sapiens

Homo floresiensis

A small-bodied human from Flores in Indonesia, about a metre tall with a brain around 400 cubic centimetres, surviving until roughly 50,000 years ago. From From habilis to sapiens

Homo habilis

Named in 1964 for fossils at Olduvai Gorge associated with stone tools. The name means handy man, and whether it belongs in our genus at all is still argued. From From habilis to sapiens

Homology

The same structure inherited from a common ancestor and put to different uses, such as the identical bone arrangement in a human arm, a bat wing and a whale flipper. From Evolution by natural selection

Horizon problem

Opposite sides of the sky show the afterglow at the same temperature to one part in a hundred thousand, though in a plain Big Bang they could never have exchanged heat. Inflation was invented largely to explain this. From Open questions

Horizontal gene transfer

Passing genes sideways between organisms rather than to offspring. It is routine among prokaryotes and is why the earliest branches of the tree of life are tangled. From The complex cell

Hoyle state

An excited state of the carbon-12 nucleus at 7.65 MeV, predicted by Fred Hoyle in 1953 because carbon exists, and found in the laboratory soon after. Without it, the triple-alpha process would be far too slow. From Where the elements come from

Hubble sequence

Hubble's 1926 classification of galaxies into ellipticals, spirals and irregulars, still used, though it is not the evolutionary sequence he once suspected. From Galaxies

Hubble tension

The expansion rate measured from the afterglow disagrees with the rate measured directly in nearby galaxies, 67 against 73 kilometres per second per megaparsec, by more than the errors allow. From Open questions

Hubble–Lemaître law

Recession speed is proportional to distance: v = H₀d. Every observer in the universe sees the same rule. From Was the Big Bang an explosion?

Huronian glaciation

A series of severe global glaciations between about 2.4 and 2.2 billion years ago, overlapping the oxidation event and plausibly caused by it. From The Great Oxidation Event

Hydrogen hypothesis

The proposal that the original partnership was metabolic: the host consumed hydrogen that the bacterium produced, and only later did the arrangement become about oxygen. From The complex cell

Hydrostatic equilibrium

The balance, at every depth inside a star, between the weight of the layers above and the pressure of the hot gas below. It holds for the whole of a star's stable life. From What a star is

Hyoid bone

A small bone anchoring the tongue muscles. Neanderthal hyoids are essentially modern, which shows the vocal apparatus was not the limiting factor. From Language and symbolic thought

Ibn al-Haytham

An eleventh-century scholar in Cairo whose Book of Optics tested theories of vision by experiment and argued that claims must be checked against observation. From The Scientific Revolution

Ice-albedo feedback

A positive feedback: ice reflects sunlight, so a colder planet grows more ice and gets colder still. It is what makes a snowball possible. From Earth as a system

Immediate-return economy

Consuming what is obtained within days, with little storage. It is associated with strong sharing norms and with egalitarian social arrangements. From Foraging lifeways

Independent evidence

Measurements that rely on different physics and different instruments. The Big Bang rests on several lines that could each have failed separately. From How we know

Independent origin

Farming beginning in a region without being learned from elsewhere. At least eleven are recognised, which turns the question from why once to why then. From Why farm?

Indus script

Around 400 to 700 distinct signs, appearing in inscriptions averaging about five signs long. Undeciphered, and whether it encodes a language at all is disputed. From The Indus Valley civilisation

Inflation

A hypothesised burst of extremely rapid expansion in the first fraction of a second. Well supported by observations, but not yet confirmed. From Was the Big Bang an explosion?

Inflation

A hypothesised burst of enormous expansion in the first fraction of a second. It explains why the universe is so uniform and so flat, and predicts the ripple pattern we see. Its decisive signature has not been found. From Open questions

Initial mass function

The recipe of stellar masses a cloud produces. For every star like the Sun it makes several smaller ones and, rarely, one much larger. From Gravity versus pressure

Inner core

A solid ball of iron alloy about 1,220 kilometres in radius, solid despite being hotter than the outer core, because the pressure there is immense. From Core, mantle, crust

Innovation rate

How often something new is invented. It rises with the number of connected minds, because innovation is largely recombination of what already exists. From Collective learning

Instrument

A device that extends the senses. The telescope, microscope, barometer, thermometer and air pump all arrive within a century, and each opened a domain that argument alone could not reach. From The Scientific Revolution

Interstellar dust

Solid grains, mostly silicates and carbon, about a ten-thousandth of a millimetre across. They make up about one percent of the interstellar medium by mass and block starlight. From Molecules in space

Interstellar ice

Layers of water, carbon monoxide, methanol, ammonia and carbon dioxide frozen onto dust grains in cold clouds. Most interstellar water exists as ice on grains, not as gas. From Molecules in space

Interstellar medium

The gas and dust between the stars: mostly hydrogen and helium, on average about one atom per cubic centimetre, but gathered into clouds a million times denser. From Gravity versus pressure

Introgression

Genes entering one population from another through interbreeding. Most living people outside Africa carry a small percentage of Neanderthal DNA. From From habilis to sapiens

Inverse-square law

A star twice as far away appears a quarter as bright. In the paradox, this is exactly cancelled by the four times as many stars in a shell twice as far away. From Why is the night sky dark?

Iridium anomaly

A worldwide spike of a metal rare in Earth's crust and common in asteroids, found in a thin clay layer at the Cretaceous-Palaeogene boundary. The discovery that started the impact hypothesis. From Mass extinctions

Iron peak

The end of the line for fusion. Iron-56 and nickel-62 sit at the top of the binding-energy curve, so fusing them costs energy instead of releasing it. From Where the elements come from

Island of stability

A predicted region of superheavy elements with longer half-lives, near proton number 114 to 126. Whether it exists as predicted is unresolved. From The periodic table as a history book

Isochron

A plotting method that gives an age without assuming how much daughter isotope was present at the start, by using several samples with different parent concentrations. From Deep time

Isomer

One of several molecules with the same atoms arranged differently. The number of possible isomers grows explosively with the number of carbon atoms, which is where chemical variety comes from. From Why carbon?

Isotope

A version of an element with a different number of neutrons. Isotope ratios are what let scientists trace where a particular batch of atoms came from. From Are you really made of stardust?

Isotopic twin

The finding that Earth and Moon rocks have nearly identical oxygen and titanium isotope ratios, unlike any meteorite group, which constrains the impact models severely. From Earth's violent birth

IUGS

The International Union of Geological Sciences, the body whose ratification makes a change to the geological timescale official. From The Anthropocene

Jeans mass

The smallest mass of gas that can collapse under its own gravity against its own pressure. It rises with temperature and falls with density, which is why stars form in cold, dense places. From Gravity versus pressure

Jebel Irhoud

A Moroccan site whose fossils, redated in 2017 to about 300,000 years, pushed the origin of our species back by roughly 100,000 years and moved it out of East Africa alone. From From habilis to sapiens

Jwalapuram

A site in Andhra Pradesh with stone tools in layers directly below and above the Toba ash, showing continuity of occupation across the eruption. From Out of Africa, into India

Kelvin–Helmholtz contraction

Shrinking under gravity and radiating the released energy as light. It powers a protostar for millions of years before fusion starts. From Gravity versus pressure

Kerala school

A lineage of mathematicians in south India from the fourteenth century who developed infinite series for trigonometric functions roughly two centuries before comparable European results. From The Scientific Revolution

Kilonova

The glow, a thousand times brighter than a nova, from a neutron-star merger, powered by the radioactive decay of the freshly made heavy elements. From Making gold

Knapping

Striking stone to remove flakes in a controlled way. It requires understanding fracture mechanics through the hands, and it is harder to learn than it looks. From Fire, tools and cooking

Konservat-Lagerstätte

A deposit with exceptional preservation of soft tissue. They are rare and patchy, which is why the fossil record's apparent sudden starts often reflect preservation rather than events. From The Cambrian explosion

Lactase persistence

Continuing to digest milk sugar into adulthood. It evolved independently several times in herding populations, and it is one of the clearest cases of a culture reshaping human genetics. From Domestication

Laetoli footprints

Trails preserved in volcanic ash in Tanzania, about 3.6 million years old, made by hominins walking upright. Direct evidence of gait rather than inference from bones. From Primates and hominins

Lagrange point L1

A spot 1.5 million kilometres sunward of Earth where the pulls of Sun and Earth balance, so a spacecraft can hover with an unbroken view of the Sun. Aditya-L1 lives there. From The Sun, a typical star

Lambda-CDM

The standard model of cosmology: a cosmological constant plus cold dark matter, described by six numbers. Precise, successful, and mostly made of things we have not identified. From Open questions

Large igneous province

An enormous volcanic outpouring, hundreds of thousands to millions of cubic kilometres of lava over a geologically short time. Four of the five big extinctions coincide with one. From Mass extinctions

Last universal common ancestor

The population from which every living thing descends. Not the first life, but the last one whose descendants are all still here. From What is life?

Last universal common ancestor

The population from which all life descends. It was already sophisticated, which means it is far downstream of the origin and cannot tell us directly how the origin worked. From How chemistry became biology

Late veneer

The material added to Earth after the core had formed, which explains why the mantle contains more gold and platinum than core formation should have left. From Earth's violent birth

Learning rate

The percentage cost reduction per doubling of cumulative production. Solar photovoltaics have sustained one of the highest sustained learning rates ever measured for an energy technology. From Energy after fossil fuels

Lepton

A fundamental particle that does not feel the strong force. The electron is one; so are the muon, the tau and the three neutrinos. From From quarks to the Higgs

Light-year and parsec

A light-year is the distance light travels in a year, about 9.5 trillion kilometres. A parsec is 3.26 light-years, the distance at which a star's parallax is one second of arc. From Reading starlight

Line of sight

A straight line from your eye out into space. The paradox asks what each one eventually hits. From Why is the night sky dark?

Local Group

The small cluster of galaxies the Milky Way belongs to: Andromeda, Triangulum and dozens of dwarfs, spread over about ten million light-years. From Galaxies

Lomagundi excursion

A large, sustained carbon-isotope anomaly shortly after the oxidation event, indicating a burst of organic carbon burial and possibly a transient oxygen overshoot. From The Great Oxidation Event

Lomekwian

Stone tools from Lomekwi in Kenya dated to 3.3 million years ago, older than our genus. Whoever made them, it was not a member of Homo. From Fire, tools and cooking

Long-term evolution experiment

A study running since 1988 in which populations of E. coli have passed more than 80,000 generations under identical conditions, with frozen samples allowing the past to be replayed. From Evolution by natural selection

Lookback time

How long ago the light you see set out. Looking far away is looking into the past, and there is a limit to how far back there is anything to see. From Why is the night sky dark?

Luminosity

A star's true brightness: the total energy it radiates each second. Not to be confused with how bright it looks, which also depends on distance. From One chart for every star

Magma ocean

A globally molten outer Earth, hundreds of kilometres deep, created by the heat of accretion and the giant impact. It froze within a few million years. From Earth's violent birth

Magnetic reversal

A flip of the magnetic field's polarity, recorded in rocks. It has happened hundreds of times, irregularly, and the record is what proved seafloor spreading. From Core, mantle, crust

Magnetic stripes

Bands of alternately magnetised ocean floor, symmetrical about a ridge, recording reversals of Earth's magnetic field as the floor formed. The decisive evidence, published in 1963. From Plate tectonics and the Himalaya

Main Himalayan Thrust

The great fault surface on which India slides beneath the Himalaya. It ruptures in large earthquakes, including the 2015 Gorkha earthquake in Nepal. From Plate tectonics and the Himalaya

Main sequence

The long, stable phase in which a star fuses hydrogen in its core. Ninety percent of a star's life, and the phase the Sun is in now. From The lives of stars, sorted by mass

Main sequence

The diagonal band from hot, bright stars to cool, faint ones where stars spend their hydrogen-fusing lives. It is really a sequence of mass, and nine in ten stars are on it. From One chart for every star

Main sequence brightening

The steady increase in a star's luminosity as it fuses hydrogen. The Sun is about thirty per cent brighter than when Earth formed and continues to brighten. From The deep future

Main-sequence turn-off

In a star cluster, the point where the main sequence ends because heavier stars have already left it. Its position gives the cluster's age. From One chart for every star

Maize

A Mesoamerican domesticate that became a staple across Africa, southern Europe and Asia, and is now one of the three crops supplying most human calories. From The Columbian Exchange

Malthusian trap

The pattern in which productivity gains are absorbed by population growth, leaving living standards unchanged. It describes most of the agrarian era reasonably well. From The price of farming

Mammaliaform

The broader group containing mammals and their closest extinct relatives. Useful because the boundary of what counts as a mammal is a matter of definition rather than a fact about the animals. From Mammals in the shadows

Mantle

The 2,900-kilometre layer of silicate rock between crust and core. It is solid, but flows over millions of years, which is what drives plate tectonics. From Core, mantle, crust

Marginal returns

The declining benefit from each additional unit of investment. Joseph Tainter's account of collapse is built on complexity eventually costing more than it returns. From Empires and exchange

Mass extinction

A short interval in which extinction rates rise far above background across many unrelated groups worldwide. Five are conventionally recognised in the last 540 million years. From Mass extinctions

Mass-independent fractionation of sulphur

An isotopic signature produced only when ultraviolet light reaches sulphur gases in an atmosphere without an ozone layer. Its disappearance is the sharpest single marker of the oxidation event. From The Great Oxidation Event

Mass–luminosity relation

More massive main-sequence stars are far brighter: roughly, luminosity rises as mass to the power three and a half. This is why heavy stars burn out fast. From What a star is

Mathematisation

Expressing regularities as equations that predict quantities. It is what let Newton's laws be tested against measurements rather than merely believed. From The Scientific Revolution

Mature Harappan

The urban phase, about 2600 to 1900 BCE, defined by standardised bricks, weights, seals and city planning across an enormous area. From The Indus Valley civilisation

Metabolism

The chemistry a living thing runs to stay out of equilibrium with its surroundings. Stop it and the thing dies, even though every molecule is still present. From What is life?

Metabolism-first

The family of hypotheses in which self-sustaining chemical cycles came before any genetic molecule, and heredity was added later. From How chemistry became biology

Metallicity

An astronomer's word for the fraction of anything heavier than helium. The first stars had a metallicity of zero. From The Dark Ages and the first light

Metallicity

The fraction of a star made of anything heavier than helium. It rises through cosmic time as generations of stars enrich the gas, so it is a rough clock. From Where the elements come from

Methane greenhouse

The warming that kept the early Earth from freezing under a fainter Sun. Oxygen destroys methane, which is the proposed link between oxidation and glaciation. From The Great Oxidation Event

Metric expansion

The growth of space itself, which carries galaxies apart. Galaxies do not move through space away from a centre; the distances between them increase. From Was the Big Bang an explosion?

Middle ear bones

Three tiny bones that transmit sound. Two of them were jaw bones in mammal ancestors, and the transition is documented in fossils and repeated in mammal embryos. From Mammals in the shadows

Migration

The movement of a planet's orbit as it exchanges angular momentum with the disc or with other planets. Jupiter and Saturn almost certainly did not form where they now sit. From From nebula to planets

Milky Way

Our galaxy: a barred spiral with a disc about a hundred thousand light-years across, a few hundred billion stars, a central black hole of four million solar masses, and a dark halo holding most of its mass. From Galaxies

Minihalo

A clump of dark matter about a million times the mass of the Sun, the smallest structure in which gas could gather and cool enough to make the first stars. From The Dark Ages and the first light

Mismatch

The idea that some modern health problems arise because our physiology suits conditions that no longer hold. It has real support in some cases and is frequently overstated. From Foraging lifeways

Mitochondrion

The compartment that burns food with oxygen. It has its own DNA, its own ribosomes, a double membrane, and divides on its own schedule, because it used to be a free-living bacterium. From The complex cell

Model collapse

Progressive degradation when a model is trained on data generated by earlier models, with rare cases disappearing first. It is direct evidence about whether the loop can close without new human input. From Machines that learn

Modern synthesis

The mid-twentieth-century unification of Darwin's selection with Mendelian genetics, which supplied the mechanism of heredity Darwin lacked. From Evolution by natural selection

Modified gravity

The alternative to dark matter: perhaps gravity itself changes at large scales. It reproduces the rotation of galaxies but fails for clusters and for the afterglow. From Open questions

Mohenjo-daro

The largest excavated city, in Sindh, Pakistan. Its name means mound of the dead, given in the twentieth century; nobody knows what its inhabitants called it. From The Indus Valley civilisation

Mohorovičić discontinuity

The boundary between crust and mantle, found in 1909 from the way earthquake waves speed up. Usually called the Moho. From Core, mantle, crust

Molecular clock

Dating divergences by counting genetic differences and calibrating against known fossils. It consistently places the origin of animal groups earlier than their first fossils. From The Cambrian explosion

Molecular clock

Dating lineage splits from genetic differences. For mammals it tends to place major divergences before the extinction, while fossils place the radiation after it. From Mammals in the shadows

Molecular clock

Estimating divergence dates from genetic differences. It put the human-chimpanzee split far more recently than palaeontologists had assumed, and it was right. From Primates and hominins

Molecular cloud

A cold, dense cloud in which hydrogen atoms have paired into molecules. Temperatures of ten to twenty kelvin, and the only places where stars form. From Gravity versus pressure

Molecular cloud

A cold, dense region of the interstellar medium where hydrogen exists as molecules and dust shields the interior from ultraviolet starlight. The site of both star formation and chemistry. From Molecules in space

Molecular hydrogen cooling

The only way the first gas clouds could shed heat, since there was no carbon, oxygen or dust. It is inefficient, which kept the clouds warm and made the first stars huge. From The Dark Ages and the first light

Monsoon trade

Seasonal Indian Ocean sailing using reversing winds. It made regular long-distance sea trade possible a millennium before the Atlantic was crossed. From Empires and exchange

Monumental architecture

Building far beyond practical need. It demonstrates the ability to command labour, which is a claim about power rather than about engineering. From Villages to cities

Mosaic evolution

Different features evolving at different rates rather than as a package. It is why early hominins can have modern feet and ancient skulls at the same time. From Primates and hominins

Mosaicism

When an edit reaches some cells of an embryo and not others, producing an organism that is a patchwork. It is one of the specific technical reasons germline editing is not ready. From Editing life

Multi-messenger astronomy

Observing one event through more than one channel: light, gravitational waves, neutrinos, cosmic rays. GW170817 was its founding moment. From Making gold

Multituberculate

An extinct order of small mammals, superficially rodent-like, that lasted over 100 million years and diversified before the dinosaur extinction rather than after it. From Mammals in the shadows

Mutation

A change in a genetic sequence. Mutations are random with respect to what the organism needs, which is the only part of evolution that is random. From Evolution by natural selection

NASA working definition

A self-sustaining chemical system capable of Darwinian evolution. Deliberately called working rather than final, because it is a tool for searching, not a settled answer. From What is life?

Natural selection

Differential survival and reproduction of heritable variants. Three conditions are enough: variation, heredity, and variation that affects reproductive success. From Evolution by natural selection

Neanderthal

A human species across Europe and western Asia until about 40,000 years ago, with brains slightly larger than ours, controlled fire, made tools and buried some of their dead. From From habilis to sapiens

Negative feedback

A response that opposes the change that caused it, producing stability. Positive feedback amplifies instead, producing runaway change. From Earth as a system

Neolithic

The period defined by farming, pottery and settled life. The word means new stone age, which names the least important thing about it. From Why farm?

Neutrino

A particle with almost no mass that barely interacts with anything. Neutrinos stopped interacting one second after the Big Bang and have been streaming through the universe ever since. From The first three minutes

Neutrino

A nearly massless particle produced by fusion. Neutrinos leave the Sun's core within seconds; about sixty-five billion pass through every square centimetre of you each second. From What a star is

Neutron star

The collapsed core left by most supernovae: more than the Sun's mass in a sphere about twenty-four kilometres across, spinning, and often visible as a pulsar. From The lives of stars, sorted by mass

Neutron star merger

Two collapsed stellar cores spiralling together and colliding. Confirmed in 2017 as a site of the r-process, and now the best-established one. From Making gold

Neutron-to-proton ratio

How many neutrons there were for every proton. It froze at about one to six around one second and fell to one to seven by the time fusion began, and it sets how much helium the universe made. From The first three minutes

Nice model

A model in which the giant planets shifted orbits later, scattering small bodies throughout the solar system. Named after the French city where it was developed. From From nebula to planets

Nickel-56

The radioactive isotope made in bulk by a supernova. Its decay to cobalt and then iron powers the explosion's visible light for months, and it is where most of the universe's iron comes from. From Making gold

Noble gas

An element with a full outer electron shell, chemically almost inert. Helium is the second most abundant element in the universe and one of the rarest gases on Earth, because it escapes into space. From The periodic table as a history book

Nocturnal bottleneck

The hypothesis that a long period of night-time living shaped mammalian senses, which is why most mammals have poor colour vision and excellent hearing and smell. From Mammals in the shadows

Nuclear fusion

Joining light nuclei into heavier ones. Four hydrogen nuclei fused into one helium nucleus lose 0.7 percent of their mass, which becomes energy. This, not fire, is what powers a star. From What a star is

Nucleon

A proton or a neutron, the two particles that make up atomic nuclei. From The first three minutes

Nucleosynthesis

The making of atomic nuclei. Big Bang nucleosynthesis made hydrogen, helium and a trace of lithium; stellar nucleosynthesis made almost everything else. From Where the elements come from

Observable universe

The region from which light has had time to reach us. Its radius is about 46 billion light-years, larger than 13.8 billion because space expanded while the light travelled. From Was the Big Bang an explosion?

Ocean acidification

The drop in seawater pH when carbon dioxide dissolves. It dissolves carbonate shells, and it is a recurring mechanism across several extinctions and today. From Mass extinctions

Ochre

Iron-rich pigment. It has practical uses, including as an adhesive and a sunscreen, which is exactly why its presence alone is not proof of symbolism. From Language and symbolic thought

Olbers' paradox

The puzzle that in an infinite, unchanging universe full of stars, every line of sight would end on a star and the whole sky would be as bright as the Sun's surface. Named after Heinrich Olbers, who discussed it in 1823, though Kepler saw it two centuries earlier. From Why is the night sky dark?

Oldowan

The earliest widespread stone toolkit, from about 2.6 million years ago: cores struck to produce sharp flakes. Simple, effective, and almost unchanged for a million years. From Fire, tools and cooking

Orbis spike

A proposed marker for the start of the Anthropocene at 1610, based on a dip in atmospheric carbon dioxide recorded in ice cores, attributed to forest regrowth after the American population collapse. From The Columbian Exchange

Orbis spike

A dip in atmospheric carbon dioxide recorded in Antarctic ice around 1610, proposed as an Anthropocene start date and linked to reforestation after the American depopulation. From The Anthropocene

Organic chemistry

The chemistry of carbon compounds. The name is a leftover from the belief that such compounds could be made only by living things, disproved in 1828. From Why carbon?

Origin key

A colouring of the periodic table by where each element was made: Big Bang, cosmic rays, dying low-mass stars, exploding massive stars, exploding white dwarfs, merging neutron stars, or human laboratories. From The periodic table as a history book

Outer core

A layer of liquid iron and nickel about 2,260 kilometres thick. Its motion generates Earth's magnetic field. From Core, mantle, crust

Outgassing

The release of volatiles from a planet's interior through volcanism. It supplied much of Earth's early atmosphere, and continues today. From Oceans and air

Overimitation

Children copying even the unnecessary steps of a demonstrated action. Chimpanzees skip the useless steps and do better on that task; children copy everything and thereby acquire things they could not have worked out. From Collective learning

Oxygen oasis

A local patch of oxygenated water produced by cyanobacteria long before the atmosphere as a whole changed. Evidence suggests these existed for hundreds of millions of years first. From The Great Oxidation Event

Oxygenic photosynthesis

Splitting water with sunlight to get electrons, releasing oxygen as waste. It was invented once, by cyanobacteria, and every plant does it with borrowed machinery. From The Great Oxidation Event

Palaeodemography

Estimating past mortality from skeletal age distributions. It is difficult, biased by preservation and by how ages are estimated, and it underlies most claims about life expectancy. From Foraging lifeways

Pan-African origin

The view that Homo sapiens emerged from interconnected populations across Africa rather than from a single region, which the fossil and genetic evidence increasingly supports. From From habilis to sapiens

Parallax

The tiny shift in a nearby star's apparent position as the Earth moves around the Sun. The only direct way to measure a star's distance, and the bottom rung of the distance ladder. From Reading starlight

Parent and daughter

The decaying isotope and the product it becomes. Measuring the ratio of the two gives the time since the clock started. From Deep time

Patriarchy

Systematic male authority over women in property, marriage and public life. It is not universal in foraging societies and becomes widespread with intensive agriculture. From The price of farming

Peer review

Evaluation by other specialists before publication. It is much more recent than most people assume, becoming standard only in the twentieth century. From The Scientific Revolution

Per capita versus cumulative emissions

Two ways of counting responsibility for atmospheric carbon. They give sharply different answers, and which one is used is a political choice, not a scientific one. From The Great Acceleration

Periodic table

The arrangement of elements by atomic number, in which chemically similar elements fall into columns. Published in a usable form by Dmitri Mendeleev in 1869. From The periodic table as a history book

Photosphere

The Sun's visible surface: not a solid but the layer, a few hundred kilometres thick, where the gas becomes transparent and light escapes, at about 5,800 kelvin. From The Sun, a typical star

Photosynthesis

The process that captured the sunlight now stored in coal, oil and gas. Every fossil fuel is buried ancient sunlight, which is not a metaphor. From Fossil Fuels and Industry

Phylum

A major category of body plan: arthropods, molluscs, chordates and so on. Nearly all of them first appear in this interval, and no new ones have appeared since. From The Cambrian explosion

Placental

The group containing most living mammals, which nourish young through a placenta. Their relationships and timing of diversification are actively disputed. From Mammals in the shadows

Planetary boundaries

A proposed framework identifying processes with thresholds beyond which the Earth system may shift state. Useful as a framing; the specific numbers are contested. From The Great Acceleration

Planetary nebula

The glowing shell of gas a dying Sun-like star sheds in its last hundred thousand years. Nothing to do with planets; the name is a historical accident. From The lives of stars, sorted by mass

Planetesimal

A body a kilometre or more across, large enough for its own gravity to matter. Asteroids and comets are survivors of this stage. From From nebula to planets

Plate tectonics

The theory that Earth's rigid outer shell is broken into plates that move over the weaker mantle beneath, making earthquakes, volcanoes, ocean basins and mountains. From Plate tectonics and the Himalaya

Plough hypothesis

The proposal that societies historically using the plough developed stronger norms of male dominance, because ploughing rewards upper-body strength and removes women from field work. From The price of farming

Polarisation

A preferred orientation of the light's vibration, imprinted on the afterglow as it scattered for the last time. The E-mode pattern has been measured; the B-mode pattern that inflation would leave has not. From The afterglow

Polycyclic aromatic hydrocarbon

A flat sheet of fused carbon rings, like a fragment of graphene with hydrogen around the edge. These are thought to hold a large share of all the carbon in the galaxy. From Molecules in space

Polygenic trait

A characteristic influenced by thousands of genetic variants each of tiny effect, usually interacting with environment. Height and cognitive test scores are polygenic, which is precisely why they are not editable in the way films imply. From Editing life

Population III stars

The first generation of stars, made only of hydrogen and helium. Predicted to be far more massive, hotter and shorter-lived than stars today. None has been confirmed by observation yet. From The Dark Ages and the first light

Potato

An Andean domesticate that yields far more calories per hectare than European grains and grows in poor soil. Its adoption is associated with substantial population growth in Europe. From The Columbian Exchange

Prebiotic chemistry

Chemical reactions that could have happened on the early Earth before life, producing molecules life uses. The field is well developed; the step from molecules to life is not. From How chemistry became biology

Prebiotic molecule

A compound that is a building block or precursor of biological chemistry, such as an amino acid or a sugar. Several have been found in space and in meteorites. From Molecules in space

Predation

One animal eating another. Its first clear evidence, in the form of bored shells and gut contents, coincides with the radiation, and it may be part of the cause. From The Cambrian explosion

Prediction

A statement about a measurement not yet made. The strongest evidence for a theory is a prediction made before the data existed and confirmed afterwards. From How we know

Presolar grain

A speck of dust that condensed around a dying star before the Sun existed, survived the making of the solar system, and can be picked out of a meteorite and analysed. From Where the elements come from

Prime editing

A method that writes a specified sequence without cutting both DNA strands, reducing some of the risks of the original approach. From Editing life

Primordial abundances

The proportions of hydrogen, helium, deuterium and lithium the universe had before any star existed. Measured in gas that stars have never polluted. From The first three minutes

Primordial hydrogen

Hydrogen made in the first twenty minutes of the universe. It has never been made in quantity since, so essentially all of yours is 13.8 billion years old. From Are you really made of stardust?

Pristine myth

The mistaken belief that the pre-Columbian Americas were a lightly inhabited wilderness. The landscape Europeans described was substantially a managed one whose managers had recently died. From The Columbian Exchange

Prokaryote

A cell without a nucleus: bacteria and archaea. Small, simple in structure, and metabolically far more versatile than we are. From The complex cell

Proper motion

A star or galaxy's apparent motion across the sky, as distinct from its motion toward or away from us. Measuring it for Andromeda is what turned its future into a calculation. From The deep future

Protocell

A simple membrane-bounded compartment that can grow and divide. Fatty acid vesicles do this spontaneously, which is why compartments are considered the easy part. From How chemistry became biology

Proton decay

A predicted but never observed process in which protons eventually disintegrate. Whether it happens determines whether ordinary matter has a finite lifetime, and experiments have only set lower limits. From The deep future

Proton–proton chain

The sequence of reactions by which Sun-like stars fuse hydrogen into helium. It starts with the slowest step in the universe: two protons fusing, which takes a given proton about a billion years to manage. From What a star is

Protoplanetary disc

The flattened, rotating disc of gas and dust around a young star, left over from the collapsing cloud. Planets form in it, and it lasts only a few million years. From From nebula to planets

Protostar

A collapsing core that has become opaque and hot enough to glow but not yet hot enough to fuse hydrogen. It shines by shrinking. From Gravity versus pressure

Publication

Putting a claim and its evidence where strangers can attack it. The Philosophical Transactions began in 1665 and is the oldest continuously published scientific journal. From The Scientific Revolution

Quantum tunnelling

The ability of particles to pass through a barrier they lack the energy to climb. Without it, the Sun's core would be far too cool for fusion, and no star would shine. From What a star is

Quark

One of the fundamental particles that make up protons and neutrons, three at a time. Quarks moved freely only in the first ten microseconds; they have been locked inside protons and neutrons ever since. From The first three minutes

Quark

A fundamental particle that feels the strong force. Six kinds exist; only the lightest two, up and down, make up ordinary matter, three at a time inside protons and neutrons. From From quarks to the Higgs

r-process

Rapid neutron capture. A seed nucleus absorbs neutrons faster than it can decay, racing far from stability before decaying back. It makes gold, platinum, uranium and about half of everything heavier than iron. From Making gold

Rachis

The stem holding the seed head together. A tough rachis keeps seeds attached until harvest, which is what a farmer wants and what a wild plant must never do. From Domestication

Radiative zone

The region outside the core where energy travels outward as light, scattered so often that a photon takes on the order of a hundred thousand years to escape. From What a star is

Radio astronomy of molecules

Molecules rotate at frequencies set by their structure, emitting radio and millimetre waves. Each has a unique set of lines, so a radio telescope can identify a molecule by its fingerprint. From Molecules in space

Radioactive dating of the elements

Uranium and thorium made by the r-process decay at known rates, so their ratio in old stars gives a rough age for when they were made. From Making gold

Radioactive decay

The spontaneous transformation of an unstable nucleus into another, at a rate that is fixed for each isotope and unaffected by heat, pressure or chemistry. From Deep time

Radiocarbon dating

Dating of once-living material from carbon-14, which is made in the atmosphere and decays with a half-life of 5,730 years. It reaches back about 50,000 years. From Deep time

Radiogenic

Produced by radioactive decay. Some of the argon in the air you breathe was made by potassium decaying in rock, long after the solar system formed. From Are you really made of stardust?

Rakhigarhi individual

The one Indus Valley individual from whom a genome has been recovered, published in 2019. It carried no steppe ancestry and no Iranian-farmer-related ancestry of the Anatolian type. From Out of Africa, into India

Rare earth element

The lanthanides, made mostly by neutron capture. Not especially rare in the crust, but hard to separate, and essential to modern electronics and magnets. From The periodic table as a history book

Ratchet

A change that is easy to make and hard to reverse. Farming has this shape: each step is locally sensible and the accumulated result cannot be undone. From Why farm?

RCP8.5

A very high emissions scenario, widely and wrongly labelled business as usual in media coverage and in some papers. Current policy trajectories are well below it. From The next century: climate

Recombination

The moment, at about 3,000 kelvin, when electrons bound to nuclei to make neutral atoms and the universe became transparent to light. From Was the Big Bang an explosion?

Recombination

The period, ending about 380,000 years after the Big Bang, when electrons bound to nuclei to make the first neutral atoms. The name is a historical accident: the electrons had never been combined before. From The afterglow

Recovery interval

The time before diversity returns to pre-extinction levels. After the end-Permian it was on the order of five to ten million years, far longer than the killing itself. From Mass extinctions

Red dwarf

A star below about half the Sun's mass. Faint, cool, and so frugal with its fuel that it can live for trillions of years. No red dwarf has ever died. From The lives of stars, sorted by mass

Red giant

A Sun-like star late in life, swollen to a hundred times its former size after its core hydrogen runs out and fusion moves to a shell around the core. From The lives of stars, sorted by mass

Red giant branch

The stage after core hydrogen is exhausted, when the star swells enormously. For the Sun this arrives in roughly seven and a half billion years. From The deep future

Redshift

The stretching of light to longer, redder wavelengths. Cosmological redshift is caused by space expanding while the light travels. From Was the Big Bang an explosion?

Redshift as a clock

Astronomers label early epochs by redshift, z. Redshift 1,090 is the afterglow; 20 to 30 is the first stars; 14 is the earliest confirmed galaxy; 6 is the end of reionisation. From The Dark Ages and the first light

Reionisation

The period when ultraviolet light from the first stars and galaxies stripped the electrons back off the hydrogen between them, ending about a billion years after the Big Bang. From The Dark Ages and the first light

Replication

Repeating someone else's result independently. It converts a personal claim into a public one, and it is what makes science collective rather than individual. From The Scientific Revolution

Replication-first

The family of hypotheses in which a self-copying molecule came first, and metabolism was built around it. From How chemistry became biology

Representation learning

Discovering useful features from data rather than being given them by a programmer. It is the core idea of deep learning, and it is what distinguishes these systems from earlier hand-built ones. From Machines that learn

Residence time

How long a substance stays in a reservoir. Carbon in the atmosphere turns over in years; in rock, in hundreds of millions. From Earth as a system

Resource class

A ranking of how much digital text and annotation exists for a language. Most of the world's languages, including most Indian ones, sit near the bottom, which determines who these systems work for. From Machines that learn

Reverse dominance

Christopher Boehm's term for a group actively suppressing anyone who tries to dominate it, through ridicule, disobedience, desertion or violence. Egalitarianism as maintenance rather than as absence. From Foraging lifeways

Ribozyme

An RNA molecule that acts as a catalyst. Their discovery in the early 1980s made the RNA world plausible, because it showed one molecule could do both jobs. From How chemistry became biology

RNA world

The hypothesis that RNA came before both DNA and proteins, because RNA can both carry information and catalyse reactions. The strongest single idea in the field, and incomplete. From How chemistry became biology

Rocket equation

The relation showing that required propellant grows exponentially with delta-v. It is why rockets are almost entirely fuel, and why nothing in this lesson gets easier with better engineering alone. From Leaving Earth

Runaway accretion

The stage where the largest body in a region grows fastest, because its gravity sweeps up an ever-widening area, so it quickly outpaces its neighbours. From From nebula to planets

s-process

Slow neutron capture, in which a nucleus absorbs a neutron every few thousand years and has time to decay in between. It builds about half the elements heavier than iron, in the outer layers of dying giant stars. From Where the elements come from

Saha equation

Meghnad Saha's 1920 formula for how many atoms in a hot gas are ionised at a given temperature and pressure. It turned spectral lines from a catalogue into a thermometer. From Reading starlight

Sahelanthropus

A fossil skull from Chad, dated to around 7 million years ago, close to the estimated split with chimpanzees. Whether it was bipedal is argued. From Primates and hominins

Salinisation

Accumulation of salt in irrigated soil as water evaporates. It is the classic self-inflicted wound of early irrigation agriculture and it is largely irreversible on human timescales. From The price of farming

Savannah hypothesis

The old explanation that hominins stood up when forests thinned into grassland. Fossils from wooded settings have weakened it considerably. From Primates and hominins

Scenario

A description of a possible emissions future, used as an input to a model. A scenario is an assumption, not a prediction, and confusing the two causes most public misunderstanding of climate projections. From The next century: climate

Scientific theory

Not a guess but the opposite: a framework that has made predictions which could have failed, and survived them. Gravity, evolution and the Big Bang are theories in this sense. From How we know

Seafloor spreading

The creation of new ocean floor at mid-ocean ridges, which then moves outward. The mechanism Wegener lacked. From Plate tectonics and the Himalaya

Second atmosphere

The carbon dioxide and nitrogen atmosphere that followed the loss of the original hydrogen and helium and preceded the rise of oxygen. From Oceans and air

Secondary products revolution

The shift to using animals for milk, wool, traction and transport rather than only meat. It roughly doubles what a herd is worth and enables the plough. From Empires and exchange

Sedentism

Living in one place year-round. In several regions it comes before farming rather than after, which reverses the intuitive order. From Why farm?

Seed nucleus

The starting point for neutron capture, usually an iron-peak nucleus. The r-process piles neutrons onto seeds in about a second. From Making gold

Seismic wave

A vibration travelling through the Earth from an earthquake. P-waves are compressions and pass through liquid; S-waves are shear waves and do not. That difference revealed the liquid core. From Core, mantle, crust

Selective sweep

The rapid spread of a favoured variant through a population, leaving a detectable signature in the genome. It is how ancient selection can be read from modern DNA. From Domestication

Self-play

Training by playing against copies of yourself, generating your own data. It works where a perfect simulator and a clean measure of success exist, which is rare outside games. From Machines that learn

Shadow biosphere

Hypothetical life on Earth with a different biochemistry, which our tests would not detect because every test assumes our own chemistry. From What is life?

Shattering

The ripe seed head of a wild cereal breaking apart to scatter seed. Non-shattering is the single most important change in cereal domestication, and it makes the plant dependent on humans. From Domestication

Shell beads

Perforated shells worn on the body, found across Africa and the Levant from at least 100,000 years ago. Ornament implies an audience, and an audience implies shared meaning. From Language and symbolic thought

Siberian Traps

The largest known continental volcanic event, erupted across Siberia at the end of the Permian. The leading cause of the worst extinction on record. From Mass extinctions

Siderophile

Iron-loving. Elements such as gold, platinum and nickel that dissolve preferentially in molten iron and were carried into the core. From Core, mantle, crust

Sievert

The unit of radiation dose weighted for biological effect. Context: a chest CT is a few millisieverts; the cruise to Mars and back is measured in hundreds. From Leaving Earth

Silicon

Carbon's neighbour below in group 14, with the same four bonds. It is far more abundant in Earth's crust, and its chemistry is far less versatile. From Why carbon?

Silk Roads

The network of overland and maritime routes linking East Asia, South Asia, the Middle East and Europe. Better understood as many short relays than as one long road. From Empires and exchange

Singularity

The point at which the equations of general relativity predict infinite density and stop working. It marks the limit of the theory, not a known physical event. From Was the Big Bang an explosion?

Sixth extinction

The proposal that current human-driven losses constitute a sixth mass extinction. The rate is alarming; whether the magnitude has yet reached the threshold is contested. From Mass extinctions

Small shelly fauna

Millimetre-scale mineralised fragments that appear just before the main radiation. The first widespread evidence that organisms had started building hard parts. From The Cambrian explosion

Snow line

The distance from a young star beyond which water can freeze, around three times the Earth-Sun distance in our system. Inside it, planets are rock and metal; outside, they can gather ice and gas. From From nebula to planets

Snow line

The distance from the Sun beyond which water ice could condense. Earth formed inside it, which is why the origin of its water is a genuine question. From Oceans and air

Snowball Earth

Episodes, most clearly around 700 million years ago, when ice may have reached the tropics. Evidence that the thermostat can be overwhelmed. From Earth as a system

Solar constant

The power the Sun delivers to each square metre facing it at Earth's distance: 1,361 watts. It varies by only a tenth of a percent over the solar cycle. From The Sun, a typical star

Solar cycle

The roughly eleven-year rise and fall in sunspots, flares and eruptions, found by Heinrich Schwabe in 1843 after seventeen years of daily drawings. Driven by the Sun's magnetic dynamo. From The Sun, a typical star

Solar wind

The stream of charged particles that blows outward from the corona at hundreds of kilometres per second, filling the solar system. Predicted by Eugene Parker in 1958. From The Sun, a typical star

Somatic edit

A change to cells in one person's body. It affects that person and is not inherited. From Editing life

Sound horizon

How far a sound wave could travel in the plasma before the afterglow was released: about 145 megaparsecs today. It is the ruler that sets the size of the largest ripples. From The afterglow

Southern route

The proposed coastal dispersal from the Horn of Africa along Arabia and South Asia to Southeast Asia and Australia. It fits the Australian dates better than any inland route. From Out of Africa, into India

Space weather

The changing conditions in near-Earth space driven by the Sun's activity. Forecasting it is the practical reason nations send spacecraft to watch the Sun. From The Sun, a typical star

Specialisation

People doing one job rather than everything. It requires that somebody else grows their food, which requires a system for moving food to them. From Villages to cities

Spectral class

The letters O, B, A, F, G, K, M, from hottest to coolest, assigned from the pattern of lines in a star's spectrum. The sequence was built at Harvard, largely by Annie Jump Cannon, who classified a quarter of a million stars. From One chart for every star

Spectrum

Light spread out by wavelength, from red to violet and beyond. A star's spectrum carries its temperature, composition, motion, rotation and magnetic field. From Reading starlight

Standard candle

An object of known brightness, such as a type Ia supernova, whose apparent brightness gives its distance. From How we know

Standard candle

Any object of known true brightness. Compare that with how bright it appears and you have its distance. Cepheids and type Ia supernovae are the two most important. From Reading starlight

Standard Model

The theory of the particles and three of the four forces, assembled between the 1960s and 1970s. It has passed every laboratory test for fifty years, and is known to be incomplete. From From quarks to the Higgs

Standardised weights

Cubes of chert in a consistent binary and decimal series, found across the whole area with remarkable precision. Evidence of shared administration or shared trade convention, or both. From The Indus Valley civilisation

Stardust

In its literal sense, matter made inside stars. Applied to a human body it is accurate for most of the mass but not for most of the atoms. From Are you really made of stardust?

Steady-state theory

The Big Bang's serious rival from 1948 to the mid-1960s: an expanding universe with no beginning, kept at constant density by the continuous creation of new matter. From How we know

Steam engine

A machine converting heat into motion. Newcomen's engine of 1712 was used to pump water out of coal mines: coal, from the beginning, powered its own extraction. From Fossil Fuels and Industry

Stefan–Boltzmann law

A star's luminosity equals its surface area times the fourth power of its temperature. On the diagram, this means position gives size: two stars of equal temperature differ in brightness only by radius. From One chart for every star

Steppe ancestry

A genetic component originating in Central Asian pastoralist populations, which spread into Europe and South Asia in the Bronze Age. Its arrival in India is dated to roughly the second millennium BCE. From Out of Africa, into India

Steppe frontier

The boundary between farming states and mobile pastoralists. It is the most persistent military and cultural interface in Eurasian history. From Empires and exchange

Storage

Keeping food for later. It is the pivot of the whole chapter, because a stored surplus can be counted, owned, taxed and stolen. From Why farm?

Stratification

Durable, inherited differences in wealth and status. Burials are the usual evidence, because grave goods differ long before written records exist. From Villages to cities

Stratigraphy

The study of rock layers and their order. It gives relative ages: which is older. Radiometric methods supply the absolute numbers. From Deep time

Stromatolite

A layered rock built by mats of microbes trapping sediment. Some of the oldest claimed evidence for life is of this kind, and it is contested because chemistry can make similar layers. From What is life?

Strong force

The force that binds quarks into protons and neutrons and holds nuclei together. It grows stronger with distance, which is why a quark can never be pulled out on its own. From From quarks to the Higgs

Subduction

The sinking of dense oceanic plate back into the mantle at a trench. It is what pulls plates along, and it is why no ocean floor is older than about 200 million years. From Plate tectonics and the Himalaya

Substrate revolution

The change from seafloors sealed by microbial mats to seafloors churned by burrowing animals. It transformed marine chemistry and destroyed the Ediacaran way of life. From The Cambrian explosion

Sunspot

A dark patch on the photosphere where a strong magnetic field chokes off the flow of heat. Cooler than its surroundings, and still brighter than an arc lamp. From The Sun, a typical star

Supermassive black hole

A black hole of millions to billions of solar masses at the centre of nearly every large galaxy. The Milky Way's, Sagittarius A*, was photographed in 2022. From Galaxies

Surface brightness

How bright a surface looks per unit area on the sky. Distance does not change it: the Sun's disc would look just as bright per patch from twice as far, only smaller. That is why a sky covered in stellar surfaces would be as bright as the Sun. From Why is the night sky dark?

Surface of last scattering

The shell around us from which the afterglow's photons last bounced off an electron before travelling freely. It is not a physical surface; it is the distance at which we see the universe as it was 380,000 years in. From The afterglow

Surface-brightness dimming

In an expanding universe a source at redshift z looks dimmer by a factor of (1+z) to the fourth power. It makes the night sky about 40 percent darker than it would otherwise be, but it is not the main reason the sky is dark. From Why is the night sky dark?

Symbol

Something that stands for something else by agreement rather than resemblance. The agreement is the hard part, because it must be shared. From Language and symbolic thought

Synapsid

The lineage that includes mammals and their ancestors, distinguished by a single opening behind the eye socket. Synapsids dominated land before the dinosaurs and were nearly wiped out at the end-Permian. From Mammals in the shadows

Technetium

Element 43, with no stable isotope. Its presence in the atmospheres of some giant stars proves that nucleosynthesis is happening in them now. From The periodic table as a history book

Technofossil

A manufactured object durable enough to persist in the sediment record. Concrete, aluminium, plastic and glass all qualify. From The Anthropocene

Tell

A mound formed by generations of mudbrick buildings collapsing and being rebuilt on the same spot. Many Near Eastern cities sit on tens of metres of their own rubble. From Villages to cities

Terraforming

Deliberately altering a planet to make it habitable. For Mars the limiting factor is not engineering ambition but inventory: the required gas is not there. From Leaving Earth

Tethys

The ocean that lay between India and Asia and was consumed as India moved north. Marine fossils high in the Himalaya are its floor. From Plate tectonics and the Himalaya

Tetravalent

Able to form four bonds. Carbon's four outer electrons and four vacancies let it bond to four neighbours at once, in three dimensions. From Why carbon?

The Great Acceleration

The name given to the simultaneous steepening, around the mid-twentieth century, of twenty-four indicators of human activity and Earth system change. From The Great Acceleration

The Great Bath

A watertight brick tank at Mohenjo-daro, about twelve metres by seven, with a bitumen lining and steps at both ends. Its purpose is inferred rather than known. From The Indus Valley civilisation

The Great Divergence

The gap in income and industrial capacity that opened between parts of Europe and the rest of the world after about 1800. Its causes are among the most contested questions in economic history. From Fossil Fuels and Industry

The Great Dying

The demographic collapse of indigenous American populations after 1492, estimated at around 90 percent, overwhelmingly from disease rather than warfare. From The Columbian Exchange

The Needham question

Joseph Needham's question of why modern science arose in Europe rather than in China, given China's long technological lead. It has no agreed answer and the framing is itself disputed. From The Scientific Revolution

The ratchet effect

Michael Tomasello's term for improvements being retained rather than lost, so that the baseline only moves one way. A ratchet needs a pawl, and here the pawl is faithful teaching. From Collective learning

The Tasmanian case

After sea level cut Tasmania off about 10,000 years ago, its population lost technologies its mainland relatives kept. The most-discussed evidence that isolation causes technological loss. From Collective learning

Theia

The name given to the impactor, after the Greek titan who was mother of the Moon goddess. Whether any of it survives as a distinct signature is debated. From Earth's violent birth

Thermostat

A star's self-correcting balance: if fusion speeds up, the core expands and cools, slowing it; if it slows, the core contracts and heats, speeding it. This is why stars are stable for billions of years. From What a star is

Tidal recession

The Moon's slow retreat, currently 3.8 centimetres a year, driven by the tides it raises on Earth. It means the Moon was once far closer and the day was far shorter. From Earth's violent birth

Time dilation

Clocks in the distant universe appear to run slow by a factor of (1+z). Distant supernovae brighten and fade more slowly by exactly that factor. From How we know

Tipping element

A part of the Earth system that may shift into a qualitatively different state once a threshold is crossed, and not shift back when the forcing is removed. From The next century: climate

Tired light

The idea that redshift is caused by light losing energy on its journey rather than by expansion. It predicts no time dilation and fails the supernova and surface-brightness tests. From How we know

Toba

A supervolcanic eruption in Sumatra about 74,000 years ago, the largest of the last two million years. Its effect on human populations has been substantially downgraded. From Out of Africa, into India

Trace fossil

A burrow, track or boring rather than a body. Traces record behaviour and get markedly more complex across the boundary, which dates the arrival of active movement. From The Cambrian explosion

Training data

The collection a system learns statistical structure from. What is in it, who produced it, and who was paid determine what the system can do and whom it works for. From Machines that learn

Transuranic element

Any element beyond uranium, atomic number 92. None occurs on Earth in more than traces; all were made in reactors or accelerators. From The periodic table as a history book

Triangular trade

The Atlantic system moving manufactures to Africa, enslaved people to the Americas, and American commodities to Europe. Roughly 12.5 million people were embarked over about four centuries. From The Columbian Exchange

Triple-alpha process

Three helium nuclei combining into one carbon nucleus. The bottleneck of the whole periodic table, because it has to happen almost simultaneously and beryllium-8, the intermediate step, lives for a ten-millionth of a billionth of a second. From Where the elements come from

Turbulence

Chaotic motion within a cloud, which supports it against gravity and also stirs up the dense knots where collapse begins. From Gravity versus pressure

Turnover

The continuous replacement of the atoms in a living body. Most of your atoms have been part of you for a short time, though some, such as those in tooth enamel, stay for life. From Are you really made of stardust?

Type Ia supernova

The explosion of a white dwarf pushed over the Chandrasekhar limit by a companion. Almost identical every time, which makes it the standard candle of cosmology. From The lives of stars, sorted by mass

Type Ia supernova

A white dwarf detonating completely. It makes about half the iron in the universe and leaves no remnant. From Making gold

Unconscious selection

Selection produced by a practice without anyone intending its effect. Harvesting with a sickle selects for non-shattering automatically, because only attached seeds get taken home. From Domestication

Uniformitarianism

The principle that the processes shaping Earth today are the same ones that shaped it in the past. It lets present rates be used to interpret ancient rock. From Deep time

Uranium-lead dating

The most precise method for very old rocks, using two independent uranium decay chains that must agree. Zircon is its favourite mineral. From Deep time

Urban revolution

V. Gordon Childe's 1950 term for the emergence of cities, defined by ten criteria including size, craft specialisation, monumental buildings, writing and social stratification. From Villages to cities

Uruk

In southern Mesopotamia, generally considered the first true city, reaching perhaps 40,000 to 50,000 people by about 5,000 years ago. From Villages to cities

Varve

An annual sediment layer. Lakes that lay down undisturbed varves give year-by-year resolution, which is why Crawford Lake was chosen as the candidate site. From The Anthropocene

Virgin soil epidemic

An outbreak in a population with no prior exposure and no acquired immunity. Mortality is far higher than in populations where the disease is endemic, and all ages are affected at once. From The Columbian Exchange

Virus

A package of genetic material that replicates and evolves but runs no metabolism of its own. Whether it is alive depends entirely on which definition you pick. From What is life?

Volatile

A substance that vaporises easily: water, carbon dioxide, nitrogen, methane. The inner solar system was too hot for these to condense, so Earth's had to be delivered or retained in minerals. From Oceans and air

Water tower

A mountain system that stores and supplies water to the lowlands below it. The Indus basin depends on one of the most important and most vulnerable in the world. From The next century: climate

Weak force

The force that changes one kind of quark into another, allowing radioactive decay and the first step of the Sun's fusion. Without it, no star would shine. From From quarks to the Higgs

Weathering thermostat

The negative feedback at the heart of that cycle: a warmer, wetter world weathers rock faster, removing carbon dioxide and cooling; a colder world weathers slowly and lets it build back up. From Earth as a system

Weathering thermostat

From lesson 4.7: silicate weathering removes carbon dioxide faster when it is warmer, stabilising temperature over geological time. It is also what will eventually end the biosphere, because it has a floor. From The deep future

Wet-bulb temperature

The temperature a thermometer reaches when wrapped in a wet cloth and ventilated. It is the limit of what evaporative cooling can achieve, which makes it the number that matters for human survival in heat. From The next century: climate

White dwarf

The dead core of a Sun-like star: about half the Sun's mass packed into the size of the Earth, no longer fusing, cooling for ever. From The lives of stars, sorted by mass

White dwarf region

The lower left of the diagram: hot but faint, because these dead cores are tiny, the size of Earth. From One chart for every star

Wien's law

Hotter things glow bluer. The wavelength at which a star shines brightest is inversely proportional to its temperature, so colour is a thermometer. From Reading starlight

WIMP

Weakly interacting massive particle, the long-favoured candidate for dark matter. Decades of increasingly sensitive searches have not found one. From Open questions

Wonderwerk Cave

A South African site with microscopic evidence of burning inside a cave about a million years ago, among the strongest early evidence for controlled fire. From Fire, tools and cooking

Wright's law

The observation that the unit cost of a manufactured technology falls by a roughly constant percentage for each doubling of cumulative production. It is an empirical regularity, not a law of nature, and it fails for some technologies. From Energy after fossil fuels

Younger Dryas

A sharp return to near-glacial conditions about 12,900 to 11,700 years ago, ending within decades. It interrupted a warming trend and coincides with early cultivation in the Near East. From Why farm?

Zircon

An extremely durable crystal that traps uranium and excludes lead when it forms, making it the best natural clock. Grains from Western Australia are 4.4 billion years old. From Earth's violent birth

Zoonosis

A disease that jumps from animals to humans. Living with herds in dense settlements is the origin of a large share of human infectious disease. From Domestication

Zoonotic spillover

A pathogen crossing from animals to humans. Domestication and dense settlement created the conditions, and they still do. From The price of farming