How did we get here from a tiny particle?

The Big Bang

The universe began expanding from an extremely hot, dense state. It was not an explosion at one point in space; space itself began to stretch. There were no atoms yet, only energy and elementary particles.

Quarks combine

As the universe cooled slightly, quarks joined in threes to form protons and neutrons. A proton is two “up” quarks and one “down” quark, held together by particles called gluons.

Your atomsThe nucleus of every hydrogen atom in your body is a single proton. Those protons formed right here: part of you is 13.8 billion years old.

The first atomic nuclei

Protons and neutrons fused into the first nuclei: about 75% hydrogen and 25% helium by mass, with a trace of lithium. Because the universe cooled quickly, this lasted only about 20 minutes. Heavier elements would have to wait for stars.

The first atoms and the first light

When the universe cooled to about 3,000 kelvin (2,700 °C), electrons could finally bind to nuclei and the first true atoms formed. Light could now travel freely. We still measure that first light today as the “cosmic microwave background”, and the age of the universe is calculated from it.

The first stars ignite

Gravity squeezed clouds of hydrogen and helium. When their centres reached millions of degrees, nuclear fusion began and the first stars lit up. Stars are element factories: they turn hydrogen into helium, helium into carbon and oxygen, and the biggest ones go all the way to iron.

The Milky Way takes shape

Gas clouds and small galaxies merged to build our galaxy. The Milky Way's oldest stars are almost as old as the universe. Today it holds more than 100 billion stars, and the Sun is just one of them.

Stars die and scatter elements

A massive star's core eventually fills with iron. Fusing iron releases no energy, so the core collapses and the star explodes as a supernova, scattering the carbon, oxygen, calcium and iron it made into space. Most of the very heavy elements, such as gold and platinum, were probably forged when neutron stars collide.

Your atomsThe iron in your blood, the calcium in your bones and the oxygen you breathe were made inside stars that lived and died before the Sun was born. “We are stardust” is not a metaphor.

The Sun is born

A cloud of gas and dust, enriched by the remains of earlier generations of stars, collapsed under its own weight. The Sun ignited at the centre, and in the spinning disc around it dust grains stuck together into the seeds of planets. Traces in meteorites suggest a nearby supernova may have triggered the collapse.

Earth forms

Rocky and metallic fragments collided and merged over tens of millions of years. The young Earth was largely molten: heavy iron and nickel sank to the centre to form the core, while lighter rock formed the mantle and crust. About a third of Earth's mass is iron, but most of it lies in the core, which begins about 2,900 km down.

The birth of the Moon

According to the leading explanation, a Mars-sized body (Theia) struck the young Earth. The debris gathered in orbit around Earth and formed the Moon.

The first oceans

As Earth cooled, water vapour condensed and fell as rain, filling the oceans. Zircon crystals from Australia, 4.4 billion years old, suggest liquid water already existed. Some water may have come from the rocks that built Earth and some from meteorites; the proportions are still debated.

The first living things

In the oceans, self-copying molecules gave rise to the first cells wrapped in membranes. Exactly how this happened is one of the biggest open questions in science. Clear evidence dates to 3.5 billion years ago, with possible older traces. What we do know: every living thing today, humans, trees, fungi and bacteria, descends from a common ancestor.

Your atomsThe atoms of living things are not special: carbon, hydrogen, oxygen, nitrogen... What is special is that they fell into an arrangement that copies itself.

The Great Oxidation

Cyanobacteria used sunlight to split water and released oxygen as waste (photosynthesis). About 2.4 billion years ago oxygen began to build up in the air. It was poison to many organisms, but it prepared the atmosphere animals would one day breathe.

Iron ore deposits form

The ancient oceans held lots of dissolved iron. When oxygen made by living things met that iron, it formed iron oxide, rust, which sank to the sea floor. These layer-upon-layer deposits, called “banded iron formations”, are the world's most important source of iron ore today. So living things helped make iron ore.

Complex cells

One cell engulfed a bacterium and, instead of digesting it, the two began living together. The inner bacterium became the mitochondrion, the cell's power plant. Fungi, plants, animals and you are made of this kind of cell.

Snowball Earth

At least twice, ice covered Earth almost to the equator. Carbon dioxide from volcanoes slowly warmed the planet again. Soon after the ice retreated, the first large multicellular organisms appeared.

The first large organisms

Soft-bodied creatures shaped like fronds and discs, the first large multicellular organisms, lived in the seas of the Ediacaran period. Among them were small worm-like animals whose left and right sides mirrored each other, the kind your own lineage traces back to.

The Cambrian explosion

In a short span by geological standards, a few tens of millions of years, the ancestors of most of today's animal groups appeared: eyes, shells, legs... The first relatives of vertebrates, your group, swam in these seas too.

Plants move onto land

The first land plants spread over rocks like mosses. Roots broke the rock apart, decaying remains built up soil, and the land became a place animals could live.

From water to land

In some fish, fleshy, sturdy fins slowly became legs. Transitional animals such as Tiktaalik could prop themselves up in shallow water. The layout of their fin bones (one bone, then two, then many small ones) is the same in your arm today.

The coal forests

Swamp forests of giant tree ferns and tree-sized clubmosses soaked up carbon dioxide from the air. Dead trees were buried underwater before they could fully rot, and millions of years of heat and pressure turned them into coal. The period is called the “Carboniferous”: the coal-bearing period.

The Great Dying

Enormous volcanic eruptions in Siberia threw the climate into chaos. More than 80% of marine species and most land species vanished: the largest extinction in Earth's history. New groups spread into the emptied world, among them the archosaurs, ancestors of the dinosaurs.

Dinosaurs appear

Dinosaurs evolved from small, nimble archosaur reptiles that ran on two legs. The first dinosaurs were small and lightly built. Because their legs stood upright beneath the body, they moved quickly and efficiently, and for more than 160 million years they ruled the land.

Your atomsYour ancestors and the dinosaurs' ancestors were the same animal about 310 million years ago. Dinosaurs are your distant cousins.

The first mammals

Alongside the dinosaurs, the first mammals appeared: mouse-sized, furry and probably active at night. Your ancestor was one of these little creatures living at the dinosaurs' feet.

Birds: living dinosaurs

One branch of small, feathered meat-eating dinosaurs took to the air. Archaeopteryx is the most famous fossil of this transition. Birds are the living descendants of dinosaurs, so dinosaurs never fully disappeared: the sparrow at your window is one.

The asteroid strikes

An asteroid about 10 km across struck what is now Mexico's Yucatán Peninsula (the Chicxulub crater). Dust and soot darkened the sky for a long time. All dinosaurs except birds, and about three quarters of all species, died out. Small mammals survived and spread into the emptied world.

The first primates

The first primates appeared: tree-dwellers with grasping hands and forward-facing eyes. You still have both: your thumb and your depth-perceiving eyes are their legacy.

Our path splits from chimpanzees

In Africa, two lineages split from the common ancestor of humans and chimpanzees. Chimpanzees are not our ancestors but our cousins: two branches of a family descended from the same great-great-grandmother.

On two feet: Lucy

“Lucy” (Australopithecus afarensis), found in Ethiopia, walked upright but still had a chimpanzee-sized brain. Walking on two legs began long before our brains grew large.

Fire and travel: Homo erectus

Homo erectus was the first human species to leave Africa, spreading as far as Asia. It made carefully shaped stone tools and, over time, learned to use fire. Fire lit the night, kept predators away and made cooking possible.

Homo sapiens: us

Fossils about 300,000 years old from Jebel Irhoud in Morocco are the oldest known Homo sapiens remains. Over the following tens of thousands of years, our ancestors spread from Africa across the whole world.

Göbekli Tepe

Near Şanlıurfa in Türkiye, hunter-gatherers raised one of the oldest known monumental sites, famous for its giant T-shaped pillars. It happened in Anatolia, right on the threshold of farming and villages.

Farming begins

In the Fertile Crescent, including southeastern Anatolia, people began growing wheat, barley and lentils and domesticating sheep and goats. Einkorn wheat is thought to have first been domesticated around Karacadağ. Settled life, villages and later cities followed.

The first metal: copper

People had been hammering naturally pure copper for much longer; about 7,000 years ago they learned to extract copper from ore with heat (smelting). This was the first step of the metal ages. Later, adding tin to copper produced harder bronze.

Writing

In Mesopotamia, the Sumerians began writing in cuneiform on clay tablets. Knowledge could now outlive the people who held it; history begins with writing.

The Iron Age

Smelting iron from ore needed far higher temperatures and more skill than copper. Anatolia is one of the earliest known centres of ironworking; the Hittites worked iron as a precious metal in the 2nd millennium BCE. After about 1200 BCE, iron tools and weapons became common.

Mass-produced steel

Henry Bessemer patented a method of blowing air through molten iron to burn off excess carbon. Steel became cheap, making railways, bridges, skyscrapers and machines possible.

Inside the atom

Ernest Rutherford showed that almost all of an atom's mass sits in a tiny nucleus and the rest is mostly empty space. In the 1960s scientists found that protons and neutrons are made of quarks.

Today: you

You are made of about 7 × 10²⁷ atoms: seven billion billion billion. Your hydrogen comes from the Big Bang; your carbon, oxygen and iron come from stars. Over billions of years these atoms fell into an arrangement that copies itself, thinks, and is reading this sentence right now.