Christopher Wills

THE RUNAWAY BRAIN

The evolution of human uniqueness

Is there a direction in evolution? Some have thought so, beginning with Alfred Russel Wallace, the co-discoverer with Darwin of natural selection. Wallace came to believe that there must be an underlying spiritual principle to explain the striking differences between human beings and other organisms, and the same idea came to seem inescapable to others. In our own time it was most notably espoused by Teilhard de Chardin. Wills confronts this question in his stimulating book. While not seeking to minimize the astonishing uniqueness of the human mind, he insists that there is "a good, consistent, even conventional neo-Darwinian explanation for human evolution … There is nothing mystical, magical, or theological about this explanation, but it tells a remarkable and exciting story nevertheless." This a wide-ranging book, taking in palaeontology, genetics, and neurophysiology. Extra depth is provided by the inclusion of biographical sketches of many of the scientists whose discoveries are described; some of these were known to Wills personally.

The book begins with a discussion of the famous mitochondrial Eve. Wills points out that popular accounts of this lady are potentially misleading. Although the frequently reproduced horseshoe-shaped diagram of mitochondrial DNA from different parts of the world appears to indicate an African origin for modern humans, the same data can be arranged in many different ways, some more parsimonious, in which it is much more difficult to decide where the "root" of the tree is to be found. Wills suggests that there has been an element of political correctness in the enthusiasm with which the original family tree has been greeted.

Mitochondrial Eve, Wills believes, may have lived as much as a million years ago, far earlier than the 200,000 or less often cited. In that case, the ancestors of peoples in Africa, Asia, and Europe might have made the transition from Homo erectus to Homo sapiens independently of one another. The politically incorrect multiple-origins model would then become more plausible. What would help to decide this question would be to isolate Neanderthal DNA. This will be technically difficult, unless we happen to come across a Neanderthal who fell into a glacial crevasse and has been frozen ever since. Wills's guess is that, if this happens, the Neanderthals will turn out to be members of our own species.

The section of the book that deals with genetics centres on the geneticists' favourite subject, the fruit fly, and might seem to be something of a digression; however, it is certainly fascinating in its own right. Critics of Darwinism sometimes claim that speciation (the production of new species in evolution) is not observed in practice and is therefore open to question. But this is not true, as Wills explains. Work by Theodore Dobzhansky back in the 1940s on two species of fruit fly (D. persimilis and D. pseudoobscura) showed that they are in the early stages of diverging from each other. They do not interbreed in the wild but do so when confined together in the laboratory. Studies of the large chromosomes from their salivary glands allowed the gene relationship between the two species to be analysed in detail. Wills tells this story very well.

The final section of the book deals with the brain itself. In passing, Wills makes the very interesting observation that the forebrain, which is so hugely developed (overdeveloped?) in us, appears to be phylogenetically older than the midbrain and hindbrain. The early vertebrate brain seems to have evolved, not through the addition of a forebrain onto the head end of the neural tube, but through a lengthening of the neural tube just behind the head end.

Brain size is important but is not the only thing that matters; organization is also important, particularly in relation to our ability to speak. But Wills sees no sharp discontinuity between humans and other organisms, especially the great apes. He says that the physical and mental machinery needed for speech must have been largely in place before language developed; too many parts of the brain are involved for all these pathways to have appeared at once, and we share a surprising amount of our abilities with chimpanzees.

There has been a positive gene-environment feedback loop, Wills believes, acting on our species to produce rapid brain enlargement. This is a runaway process like that which gave rise to the peacock's tail. Dramatic though the process has been, however, there is nothing inexplicable about it. And though it has been rapid in evolutionary terms, the antiquity of much of our nature and abilities is a recurrent theme in this book. Most of our evolution had already taken place by the time Homo erectus appeared. "We are not the result of some dramatic change in the direction of evolution but of a gradual and continuing process that would still have taken place among the Homo erectus of Zhoukoudian or the Homo erectus of Java or the people of southernmost Africa even if all the hominids living elsewhere had disappeared."

The comparison of the human brain to the peacock's tail as a feature produced by a runaway feedback loop has been put forward independently by a number of writers on evolution, but Wills is unusual in his advocacy of the currently unfashionable multifocal origin of modern humans and in his identification of Homo erectus as marking the largest apparent discontinuity in the hominid fossil record. Whether or not he proves to be right, his book is well worth reading as a counterpoise to the prevailing orthodoxy.

4 June 2003