A head start for humans

The papers in this volume record the proceedings of a symposium on brain evolution that took place in St Germaine en Laye at the end of 1990. Luckily for the publisher, in spite of the five-year delay, the controversies remain largely unresolved, and the extremely divergent views of the participants have probably not changed very much.

The questions posed by the symposium's organisers are fascinating. How did our remarkable brains evolve? How long did this evolution take? How different are we from other animals as a result? When did "true" humans first appear? Was our evolution gradual, or did it take place in sudden spurts? How did uniquely human capabilities such as the ability to learn and speak languages evolve? And how is it that, between 25,000 and 50,000 years ago, throughout the Old World, our ancestors began to ornament their bodies, decorate their tools and weapons, and draw pictures on rocks and the walls of caves? Why not earlier? And why did this huge cultural step seem to take place almost simultaneously in regions as separate as southern Africa and the Australian outback?

The book is designed for experts, and will be heavy going for those who have not immersed themselves in some of the literature. It would help readability a good deal if the paleontologists had not thrown about terms like Castelperronian and Mousterian without bothering to define them and to place them in space and time, and if the anatomists had not managed to lose us among the lambdoid sutures and the middle meningeal venous networks. But discussions of each paper are included that help illuminate the arguments, and for the persistent reader there are marvellous nuggets of information scattered throughout.

Pasko Rakic of Yale University shows us in detail the developmental mechanism by which our cerebral cortex has managed to explode to ten times the size of a monkey's. The expansion was accomplished chiefly by adding a few cell divisions to the brain's early developmental period, when the cells slated to become cortical neurons were still able to divide and divide again. Such a small alteration with vast consequences, he suggests, is probably rather easy to accomplish in genetic terms, so that it probably did not take many different genetic mutations to produce our dramatic increase in brain size.

Ralph Holloway of Columbia University argues that as long as three million years ago the brains of some of our most distant ancestors, the Australopithecines, were already distinct from those of the apes. In the process he runs into much criticism from the other symposium members. The consensus seems to be that we cannot yet extract quite enough information from the insides of those ancient and fragmented skulls to be sure of these earliest stages in the evolution of our brains. This is, I am sure, a temporary problem.

Other unrelated observations make us wonder whether we might have traced a separate path from the chimpanzees and gorillas for a long time. Yves Coppens, of the Paris Musee de l'Homme, thinks that our ancestors split off from those of the apes as much as eight million years ago, as Africa's Great Rift Valley widened and the two sides of the valley became more different ecologically. He points out that over the last 25 years some 200,000 animal fossils of all kinds have been recovered from that region, and an astonishing one per cent of them belonged to our ancestors. Yet not a single fossil of ancestors of chimpanzees or gorillas has ever been found. Even if our ancestors were much more likely to be preserved in the fossil record than the ancestors of chimpanzees or gorillas, such a superabundance of hominid fossils makes it likely that in those early East African days there were many more of us than there were of them. This suggests that we have been an extremely successful species for a long time, and it seems this was due in some measure to our superior brains.

Much of the symposium is involved with the question of what makes us uniquely human. Language, for example, certainly appears to be a uniquely human attribute. Steven Pinker of the Massachusetts Institute of Technology expands the arguments of his colleague Noam Chomsky, that the ability to learn and speak language is built into our brains and uniquely human. He contrasts this with a view from the other end of the evolutionary spectrum that has been popularised by Stephen Jay Gould, which is that the ability to speak was just something that happened to us when our brains got big enough.

But there is surely a middle ground here - sheer size probably has something to do with it, but so does our evolutionary ancestry. Some chimpanzees are awfully good at learning words. The astonishing pygmy chimpanzee Kanzi, whose abilities have been studied intensively by Sue Savage-Rumbaugh and her colleagues, is a good example. He learned so many words that the experimenters had to spell out the ones they did not want him to understand. But a chimpanzee's voicebox and palate are woefully unsuited to making the range of sounds that come so easily to us. I suspect that in terms of language the chimpanzee's brain is ahead of the evolutionary curve. Its abilities have outstripped those of its body, just as the brains of our ancestors probably did. If this is right, then the brain of the chimpanzee is driving its evolution, just as I suspect the brains of our ancestors did. Yes, the ability to understand and produce language is built into our genes, but no, it is not uniquely human. It is part of a long and fascinating evolutionary journey.

This volume provides few firm answers to the question of the evolution of our brains, but it does provide some fascinating new insights. But neuroscientists have just announced the decade of the brain, and information is accruing at dizzying speed. Since the symposium, new techniques such as functional magnetic resonance imaging are providing us with tools to look at the brain at work. And the more we examine the brains of our nearest relatives, using both these new technical capabilities and ingenious behavioural studies, the smaller the gap becomes between other animals and ourselves.

Christopher Wills is professor of biology, University of California, San Diego.