Human evolution

About 3.6 million years ago three early humans walked across wet volcanic ash at Laetoli in Tanzania, and their trail of about 70 footprints is still there.

Humans are primates, one of more than 200 primate species. DNA and anatomy place us inside the great apes, with chimpanzees and bonobos as our closest living relatives. That does not mean humans evolved from chimpanzees. We did not evolve from any ape alive today. Humans and chimpanzees both descend from a common ancestor that, according to the Smithsonian's Human Origins Program, lived between 8 and 6 million years ago, and both lineages have been changing ever since.

What the DNA says

Two people today differ in their DNA by only about 0.1 percent on average. Comparing the same stretches of DNA, the Smithsonian gives a difference of about 1.2 percent between humans and chimpanzees, the same for bonobos, about 1.6 percent for gorillas, about 3.1 percent for orangutans and about 7 percent for rhesus monkeys. The exact figures depend on how the counting is done; if deleted, duplicated and moved pieces of DNA are counted as well, humans and chimpanzees differ by another 4 to 5 percent. The ranking does not change, though. Humans, chimpanzees and bonobos are closer to one another than any of them is to gorillas. In 1871 Darwin predicted that the human line began in Africa, where our closest relatives live, and the fossils have borne that out: the first 4 million years or so of human evolution took place only in Africa.

Walking first, big brains later

Walking on two legs is one of the earliest human traits, and it evolved more than 4 million years ago. Large, complex brains, tool making and language came later, and art and elaborate culture mostly within the last 100,000 years. Australopithecus afarensis shows the mix well. It lived in eastern Africa between about 3.85 and 2.95 million years ago, more than 900,000 years in all, about three times as long as our own species has existed so far. Remains of more than 300 individuals are known, including the skeleton nicknamed Lucy, an adult female about 3.2 million years old found at Hadar in Ethiopia in 1974. Its brain was usually under 500 cubic centimetres, about a third of ours, and it had long arms and curved fingers for climbing, yet it walked upright on the ground. The Laetoli footprints, found in 1978 by Mary Leakey's team with Paul Abell, are a trail almost 27 metres long. They show a big toe in line with the other toes and a heel-first step, much like ours.

Out of Africa

Homo erectus lived from about 1.89 million to 110,000 years ago. Its African fossils are the oldest known with body proportions like ours, with long legs and shorter arms, and it is generally thought to be the first human species to spread beyond Africa. Early humans reached Asia probably between 2 million and 1.8 million years ago and Europe between 1.5 million and 1 million years ago. The first fossil of the species was found on Java, Indonesia, by Eugène Dubois in 1891. Neanderthals lived in Europe and southwestern to central Asia from about 400,000 to 40,000 years ago. They made sophisticated tools, controlled fire, wore clothing and buried their dead. Their bones were first recognised as an early human in 1856 in the Neander Valley of Germany, and the species was named in 1864. Neanderthals and our species share a common ancestor that lived between 700,000 and 300,000 years ago, and genetic evidence shows they interbred with modern humans outside Africa.

Our own species, Homo sapiens, evolved in Africa about 300,000 years ago, during a time of large swings in climate. Modern brains average about 1,300 cubic centimetres. People reached Australia probably within the last 60,000 years and the Americas within roughly the last 30,000, and farming and the first cities began within the last 12,000 years. Most scientists recognise some 15 to 20 species of early humans, and they still debate how many there were and how they are related. Most of those species left no living descendants. The tree of human evolution is a bush with many dead-end branches, not a single line.