What did bring out the brains in our ancestors?

There are now six billion of us on the planet. We inhabit every continent. We can live crowded together in vast cities or as tiny groups in remote deserts. We're a social animal with language which gives our communication infinite flexibility. We dream and reflect on our place in the world, with religion and art, expressing them through our multitude of cultures. We're the animals that place technology at the heart of our lives.

But animals we remain. So how did we get to be the way we are, acquire our big brains and intellect, our language, our domination of almost every corner of the planet? It's a remarkable story - not one of predestination, but of chance circumstances and struggle. We modern humans are far from being an inevitable outcome of our primate past.

The first four million years of human evolution happened only within the African continent. This is where our ancestral family - the hominids - first appeared, branching off from another evolutionary line that ultimately led to our closest relative today - the chimpanzee. Africa is certainly the place to look for our earliest ancestors.

An hour's drive north of Johannesburg in South Africa is a place called Sterkfontein, a labyrinth of underground limestone caves. Ron Clarke, a researcher on human origins from the JW Goethe University in Germany, has made a spectacular discovery. "I found an ape man that fell into the cave and was perhaps still alive at the bottom of the shaft and in trying to find a way out stumbled down deeper and deeper into the back of the cave. This was a creature that was one of our relatives. It's very hard to think of it suffering in that way."

Ron Clarke's unlucky ape man is a new species of a group of creatures known as the australopithecines. There were quite a number of different species and they lived from, we think now, about four and a half million years ago all the way up to almost one million years ago. So, as a group, they were very successful and around for a very long period of time.

So just over four million years ago, this creature stood with a human posture. This is a radically different means of moving around the world from that of a chimpanzee. The common ancestor we share with the chimps is thought to have lived in African forests about six million years ago - a date that comes from comparing the differences between human and chimp DNA. So the question then is, why did the line that lead to us begin walking upright? According to Philip Tobias of the University of Witwatersrand in South Africa, it has something to do with dramatic changes happening across Africa at this time.

"We know there was a time of drying up", says Tobias, "a deterioration of Africa's climate. We're left with an interesting situation where gallery forest might persist along the streams, with patches of sparse woodland and partly open savannah between the patches of forest."

The opening up of the forest and the development of the savannah is a popular explanation for why two footed walking was advantageous for our ancestors. Would it have been more efficient to move on two legs? Standing upright gives less surface for the sun to beam down on - so in hot, open environments it may have been an advantage in keeping cool.

And there's no doubt the australopithecines were roaming the open country - the bones and even the footprints of some of them have been found alongside those of typical savannah dwelling animals. But their remains are also found in former forested areas.

There's certainly no evidence that ape man made stone tools - these don't turn up in the fossil record for another million years - though the configuration of their hands does suggest at least the potential for the dawn of stone age technology. But the australopithecines weren't just biding their time between savannah and forest, waiting to evolve into brighter descendants.

At this point, two and a half million years ago, real drama comes in Africa. Huge numbers of species, that lived before then, disappear from the scene. New species appear for the first time. Mammals changed. Hominids changed. The first brain enlargement of the hominids comes into the picture - and the first stone tools.

And against this background our kind of mammal called homo, which leads ultimately to Homo sapiens, appears for the first time.

"What was happening in this period was probably the most interesting period in the whole history of human evolution because there were a variety of different kinds of species," says Leslie Aiello of University College, London. "There may have been five or six or even seven different kinds of early human at this time period more or less experimenting with adaptation."

We're not dealing with a simple ladder of progression here, but a blurry bush of evolutionary possibilities, and only one of these species founds the line that leads to us modern humans. What we can say is that these first humans were bigger brained than the australopithecines - some by about 50% - and in general they had less ape-like faces. Some researchers even suggest that they were the first with speech - though that is a minority view. But at least one of them made stone tools, and the dawn of the stone age made available new kinds of food such as meat and bone marrow.

Another theory suggests that the brain might have developed for social reasons - for social intelligence. We needed faithful friends on the open savannah. "We know that in human history that as we moved into a more open environment living in groups would be more and more important. We get our protection from groups," says Aiello. "Now if you are living in a larger group you're going to have to keep it together, you're going to have to service your social relationships with the individuals in that group. So the increased need for social intelligence would also raise brain size, as well as the use of tools."

When do early human childhoods become longer and more like ours? It seems to be starting with a kind of early human called homo ergaster. It's the first of our ancestors to be tall in stature with relatively long legs and short arms like us. Although its brain was half the size of ours, it was the biggest around at the time. Larger brains need a longer time to grow and develop: this early human had a more leisurely childhood.

The downside of an extended childhood is that you need to be looked after for longer and that's a heavy burden on your mother - especially if your kind is now dedicated to a difficult life on the savannah.

So it's perhaps at this point in human evolution that a radical change in social roles takes place. The classic view has always been that this is where males come in. As the mother's costs of reproduction go up, it pays the father to invest more in their offspring - just to make sure they survive.

Aubrey Manning is Professor of Natural History at Edinburgh University. This article is adapted from the first episode of his new series Origins - the Human Factor, beginning on BBC Radio 4, January 26 at 9pm

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