Human Evolution and Migration: Out of Africa, Textbook

A family tree with many branches

The map opens on Africa. Between about seven and six million years ago, the line that leads to us parted from the line that leads to chimpanzees. Scientists know this from fossils and from DNA: we are closer to chimpanzees than to any other living animal. A skull from Chad, named Sahelanthropus, may be among the first of our line. Lucy, found in Ethiopia in 1974, lived 3.2 million years ago and walked upright, though she still climbed trees. Walking on two legs came long before big brains. Our family tree is a bush, with several species often living at the same time.

The first travellers

Homo erectus, “upright man”, had a body much like ours and made stone tools. About 1.9 to 1.8 million years ago, groups of them left Africa and spread into Asia. The proof is at Dmanisi in Georgia, where skulls about 1.8 million years old were found together with stone tools. Later, Homo erectus reached Java and China. Nobody set out on a great journey. Each generation moved a few kilometres to follow animals and plants, and over thousands of years that added up to whole continents.

Homo sapiens appears

Now our own species arrives. The oldest fossils attributed to Homo sapiens come from Jebel Irhoud in Morocco, and a new dating in 2017 put them at about 300,000 years old. The faces look modern, though the skulls are still long and low. Other early fossils come from Ethiopia. Some scientists think this means that our species did not arise in one small place, but in a network of African groups that kept meeting and mixing. The picture is still changing with every new find.

The great journey

Around 70,000 to 60,000 years ago, say genetic studies, a group of modern humans left Africa for good. Most non-Africans alive today descend mainly from them. They followed the coasts into Asia and reached Australia by about 50,000 years ago. That needed a sea crossing, so they must have had boats or rafts. A site called Madjedbebe suggests people were in Australia as early as 65,000 years ago, but this date is debated. Along the way the groups were small, and they kept finding new lands with animals that had never seen humans.

Two theories, one answer

How did the world get populated? One theory, “multiregional”, says that people evolved from Homo erectus in several places at once and kept mixing. The other, “Out of Africa”, says that modern humans arose in Africa and then spread, replacing or mixing with older groups. In 1987 studies of mitochondrial DNA pointed to a recent African origin, and many whole genomes have followed. Today most scientists favour Out of Africa, with an important detail: the newcomers did not simply replace everyone. They also had children with the people they met.

Meeting the Neanderthals

By about 45,000 years ago, Homo sapiens lived in Europe. Caves in Bulgaria and Germany hold their bones and tools. Europe already had inhabitants: the Neanderthals, stocky, strong people who had lived there for hundreds of thousands of years, made tools and buried their dead. In 2010 a team led by Svante Pääbo published the Neanderthal genome. It showed that people outside Africa carry about one to two percent Neanderthal DNA, so the two groups had children together. DNA from a finger bone in a Siberian cave revealed yet another group, the Denisovans. Neanderthals disappeared about 40,000 years ago.

Into the Americas

At the end of the Ice Age so much water was frozen in glaciers that sea levels fell, and a wide plain called Beringia joined Siberia and Alaska. People crossed it, probably along the coast, and walked into a continent with no humans. A site in Idaho shows them there about 16,000 years ago. Footprints in New Mexico may be 21,000 to 23,000 years old, but that dating is debated. Within a few thousand years people reached the far south. Humans now lived on every continent except Antarctica, and the next big change was waiting in a warming world.

Out of Africa