In 2001, scientists announced an amazing discovery: the oldest skull of a human ancestor ever found. The 3½ million year old fossil was remarkably complete, and unlike any previous fossil find. Its discovery – by a team led by Meave Leakey of the famous Leakey fossil-hunting family – has revolutionised our understanding of how humans evolved.

The great mystery of our evolution is how an ape could have evolved into the extraordinary creature that is a human being. There has never been another animal like us on the planet. And yet ten million years ago there was no sign that humans would take over the world. Instead the Earth was dominated by the apes. More than 50 different species of ape roamed the world – ten million years ago Earth really was the planet of the apes. Three million years later, most had vanished. In their place came something clearly related to the apes, but also completely different: human beings!

Brainy or bipedal?

For years scientists searched for the first key characteristic which had allowed us to make the huge leap from ape to amazing human. At first they thought the development of our big brains was decisive. They even found the fossil that seemed to prove it, until along came the famous three million year old fossilised skeleton Lucy. This quashed the big brain theory, because here was a human ancestor which clearly walked on two legs, just as we do, but had the tiny brain of an ape. It seemed that the development of walking on two legs (bipedalism) was the first key human characteristic, the thing that set us on the road to becoming human.

Lucy soon became even more important. She seemed to defy the laws of evolution. Normally a major evolutionary adaptation like walking on two legs is followed by what scientists call an adaptive radiation. Many related species quickly evolve from an initial evolutionary innovation. It gives a very bushy evolutionary family tree, with many different but related species. Scientists knew that the human branch of the family tree had begun about six or seven million years ago, when the planet of the apes ended. And yet there was no sign of an adaptive radiation. The family tree showed just a straight line leading from the planet of the apes through to Lucy.

Evidence for evolution

All that has changed with Meave Leakey’s spectacular new discovery, named Kenyanthropus platyops or, less formally, Flat-faced Man. Her find is the same age as Lucy’s species, but also completely different. It’s proof that there were two different bipedal human ancestors living at the same time, more than three million years ago. And it’s the first sign of the adaptive radiation that the theory of evolution says should have followed the planet of the apes.