1. From the Big Bang to the first humans

From the Big Bang to the first humans

The universe took 13.8 billion years to build everything a city needs: matter, stars to make the elements, a planet, oceans, life, minds. This page follows that story in order, from the Big Bang to the point where our own species starts behaving like us. The oldest dates are the best estimates science has now, and they get more exact closer to the present. Where the evidence gives a range of years instead of one year, this page gives the range.

• 13.8 billion years ago — Big Bang. Space, time, matter, and energy begin. The universe starts as a hot dense point and expands. Only hydrogen and helium exist at first.
• 13.6 billion years ago — First stars and galaxies. Gravity pulls gas into stars. Inside those stars, heavier elements form: carbon, oxygen, iron, everything a planet and a body need. When stars die, they scatter these elements.
• 4.6 billion years ago — The Sun and Earth form. A cloud of gas and dust collapses into our Sun. Leftover material forms the planets. Earth forms as a ball of molten rock.
• 4.5 billion years ago — The Moon forms. An object the size of Mars hits Earth. The broken rock thrown out forms the Moon. That impact gave Earth its tilt and its spin.
• 4.4 billion years ago — First oceans. Earth cools enough for water to stay liquid. Comets and volcanic gas add more water. The planet gets its seas.
• 3.8 billion years ago — First life. Single cells appear in the ocean, simpler than any bacteria today. Life begins too small to see, one cell at a time.
• 2.7 billion years ago — Photosynthesis begins. Cyanobacteria start to turn sunlight into energy, and release oxygen they do not use. That oxygen later changes the whole planet.
• 2.4 billion years ago — The air fills with oxygen. That oxygen builds up in the atmosphere over a billion years. It poisons most existing life, and it makes complex life possible. Scientists call this the Great Oxidation Event.
• 2 billion years ago — Complex cells. Cells with a nucleus appear. One cell swallows another and keeps it alive inside to produce energy. Those swallowed cells became mitochondria, and human cells still carry them.
• 600 million years ago — First animals. Cells start living together as one body. The first soft-bodied animals appear in the sea, such as jellyfish and worms.
• 540 million years ago — The Cambrian explosion. In a short period, almost every animal body shape appears: eyes, shells, legs, backbones. The ancestors of nearly all animals alive today appear here.
• 470 million years ago — Life moves onto land. Plants spread over bare rock first and cover it in green. Land becomes a place where living things can survive.
• 375 million years ago — First animals walk on land. Fish with strong fins crawl out of shallow water and become the first four-legged land animals. Every land animal with a backbone descends from them, humans included.
• 320 million years ago — The hard-shelled egg. Reptiles evolve an egg with a hard shell. Animals no longer need to return to water to reproduce. Life spreads far inland.
• 230 million years ago — Dinosaurs and first mammals. Dinosaurs appear and dominate the land. The first mammals appear beside them, small and active at night, and stay small for 160 million years.
• 66 million years ago — The asteroid. A rock the size of a city hits Earth. Dinosaurs die out. Mammals survive and spread across the empty world.
• 55 million years ago — First primates. Small mammals living in trees evolve hands that grip and eyes that face forward. The first primates appear.
• 34 to 25 million years ago — Apes split from monkeys. Apes separate from the Old World monkeys. Apes lose the tail, grow heavier bodies and stronger shoulders, and start living longer with fewer young.
• 20 to 15 million years ago — Gibbons split off. Gibbons are the first group to separate from the other apes. They stay small and stay in the trees, in Southeast Asia.
• 16 to 12 million years ago — Orangutans split off. The Asian great apes separate. Every great ape after this point comes from Africa.
• 12 to 8.5 million years ago — Gorillas split off. Gorillas separate. One group remains, and chimpanzees, bonobos, and humans all come from it.
• 8 to 4 million years ago — Humans split from chimpanzees. Humans and chimpanzees separate. The date is uncertain by millions of years, because almost no chimpanzee fossils exist and the genetic dating methods disagree. Most estimates give 5 to 8 million years, and that range agrees with the fossils. Some genetic methods give 12 to 13 million years, and those methods measure a different thing. They date when the two sets of DNA started to differ, not when the two populations stopped breeding with each other, and a million years or more can separate those two events. A 2018 study measured mutation rates directly in chimpanzees, gorillas, and orangutans, found that all three change faster per year than humans do, and concluded that the human rate slowed down recently. Applying the ape rates instead of the human rate brings the genetic dates back in line with the fossils. Three fossils are the oldest candidates on the human side, and every one of them falls inside the 8 to 4 million year range, so none of them fixes the date.
  • Sahelanthropus tchadensis. Found in 2001 at Toros-Menalla in the Djurab Desert of Chad, and nicknamed Toumaï. Dated 7.2 to 6.8 million years ago. Its braincase holds about 360 cubic centimetres, the size of a chimpanzee's. The hole where the spine enters the skull sits in a position that suggests the head rested on top of an upright body. A thigh bone found at the same time was described in 2022, and the two teams that examined it disagree. One team read it as an animal that walked upright, the other as an animal that moved on four legs.
  • Orrorin tugenensis. Found in 2000 in the Tugen Hills of Baringo County, Kenya, and nicknamed Millennium Man. Dated 6.1 to 5.7 million years ago. It is known from thigh bones, teeth, and arm fragments, with no skull. The structure of the thigh bone is the main argument that it walked upright. Researchers dispute whether it belongs on the human line at all, rather than on the line to gorillas or chimpanzees.
  • Ardipithecus kadabba. Found between 1997 and 2004 in the Middle Awash of the Afar region, Ethiopia. Dated 5.8 to 5.2 million years ago. It is known from teeth, jaw fragments, and a toe bone, and the toe bone is the argument for upright walking. Critics point out that the toe bone came from a different spot inside the study area and may be younger than the teeth. It was first classified as a subspecies of Ardipithecus ramidus in 2001 and raised to its own species in 2004.
• 3.3 million years ago — First stone tools. At Lomekwi in Kenya, stone was broken on purpose to make sharp edges. These tools are 400,000 years older than the next known toolmaking, and half a million years older than the first Homo fossils. The makers had small brains, so toolmaking came before big brains and before our own genus. The date is disputed. Critics argue that the team has not proved the tools came from the sediment layer that was dated, and that the tools found on the surface could be much younger.
• 3.2 million years ago — Walking upright. Australopithecus walks on two legs and still has a small brain the size of an ape's. The famous fossil Lucy belongs to this group. Walking upright came before big brains.
• 2.9 million years ago — The Oldowan tools. Sharp flakes are struck off stone to cut meat. These tools are called Oldowan, and until 2015 they were the oldest known. The oldest Oldowan site is Nyayanga in Kenya, dated in 2023. The teeth found beside those tools belong to Paranthropus, a different branch, so we cannot say our own genus made them. Homo habilis was called the first toolmaker for a long time, and that is no longer supported.
• 2 to 1 million years ago — Chimpanzees and bonobos split. The Congo River divides one population in two. Chimpanzees stay north of the river, bonobos stay south. Our two closest living relatives are younger than the first stone tools.
• 1.9 million years ago — Homo erectus. A tall human species with long legs appears, suited to walking and running long distances. It is the first human species to leave Africa.
• 1.8 million years ago — Humans leave Africa. The fossils at Dmanisi in Georgia are the oldest human remains found outside Africa. This first migration happens long before our own species exists.
• 1.8 to 1.07 million years ago — Fire is carried, not made. Burnt bone lies 30 metres inside Wonderwerk Cave in South Africa, too far in for a wildfire to reach. Early humans carried natural fire into the cave and kept it burning. They did not yet know how to start a fire. Many books link fire, cooking, and the growth of the human brain, but that link is still only a theory, because about a million years separate the two events in the evidence.
• 400,000 years ago — Making fire. At Barnham in England, the ground shows signs of heating, the stone hand axes are cracked by fire, and the site holds pyrite carried in from elsewhere. Pyrite is the stone used to strike sparks. Humans were making fire here, not finding it. Everyday use of fire across Europe and Asia also starts around this date, which means humans had already settled cold Europe without it.
• 300,000 years ago — Homo sapiens. Our own species appears in Africa. The oldest fossils come from Jebel Irhoud in Morocco. Their location shows our species began across the whole continent, not in one region. Their bodies and skulls are the same as ours.
• 100,000 to 50,000 years ago — Modern behaviour appears. Humans start using colour pigment, carving patterns, making shell beads, and building more complex tools. These appear across Africa and beyond. Older books describe one sudden change around 70,000 years ago, but the evidence no longer supports that, because the different signs appear earlier, in different places, and at different times. Neanderthals also made marks on cave walls during this period. Modern behaviour arrived slowly, not all at once.

Related: the other human species, what happened after farming began.

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