Brain versus brawn

about 65 million years ago the dinosaurs, at that time the dominant animals, became extinct probably because of the secondary consequences of a meteorite impact suffered by the earth. The mammals, an inconspicuous group of rodent-like creatures, seized the opportunity to diversify into an enormous variety of forms within a relatively short time. Among them were primitive primates the ancestors of modern monkeys, apes and humans. The lineage leading to the orangutan broke away about 14 million years ago, the gorilla lineage branched off seven million years later and the ancestors of humans and chimpanzees went their different ways 4.5 million years ago. This book is about the exciting story of what has happened since then and how it might have happened. It is a marvellous read.

As an introduction to the subject, let me answer a few commonly asked questions. Have humans evolved from monkeys? No, but humans and monkeys have both evolved from a comparatively recent common ancestor. If we wait long enough, will a chimpanzee turn into a human? No. We are on two distinct evolutionary pathways. Has the 'missing link' between humans and apes been identified? Not yet. Are human beings the most advanced form of life? Do they stand at the peak of the evolutionary tree? Yes, in the sense that the human brain is probably the most complex structure in the universe; no, in the sense that there is no 'peak' in the tree of life. Has evolution come to a halt in the case of humans? No, but biological evolution has slowed down compared to cultural evolution. What will we evolve into? Impossible to predict with certainty. My guess is that if we do not become extinct, the importance of musculature will diminish and that of mental activity will increase.
Racing ahead Though there are many sub-themes in this book, like fossil hunting, comparative anatomy and molecular biology, the grand theme is the idea that the accelerating evolution of the brain sets humans apart from all other animals. Wills' original contribution to this well-known theory is to suggest a possible cause for the acceleration. According to him, the demands of functioning in a highly complex and stressful environment made it essential for the human brain to keep a pace ahead of the others. An explosive feedback cycle was set up because, to a large extent, the environment within which brains had to compete was defined by the brains themselves. Runaway brains were the outcome.

Compromises had to be made along the way. The evolution of an upright stance, which freed our forelimbs for grasping and shaping tools, made it necessary for the pubic bones to move up and forward and alter the angle at which the femur and the pelvis join. Because of this, the opening of the birth canal became smaller so that neither a further reduction in size nor a larger brain at birth is possible. As a result, and in contrast to our closest relatives, the chimpanzees, our brains develop largely outside the womb, moulded by the sensory stimuli that bombard us during our early years.

Like everything else in the living world, the human brain too is a product of evolution. Our picture of evolution remains largely as sketched by Darwin (survival of the fittest), but there are two paradoxes. First, the work of the Japanese geneticist, Motoo Kimura and his collaborators has made it evident that evolution operates at two levels. The first level concerns the molecular hardware (proteins) that specifies us and the molecular software (dna) that encodes the information needed to build the hardware. At this level, most of the evolutionary changes are due to pure chance and have nothing to do with fitness. The other level concerns the largescale modifications in form and behaviour which are fuelled by that tiny fraction of molecular change that is not neutral. The second paradox, as Wills points out, is that the enormous difference in mental capacity between chimpanzees and humans does not imply a comparable degree of genetic differentiation (genetically speaking, chimpanzees and humans are 99 per cent identical). Therefore, the rapid evolution of our brain must have been driven by genetic changes whose number was small but whose effects were large.

The most interesting sections of the book are where Wills discusses the major findings of palaeontology interspersed with fascinating asides about the dedicated and often eccentric people who made them. This aspect of the subject is full of jockey ing for personal fame (all too often by identifying every new find as a new species). It is also very confusing, especially when trying to arrange the fossils into a coherent lineage.

As discussed in the book, there are two striking aspects to recent human evolution. First, based primarily on our ability to manipulate matter, we have been astonishingly successful in freeing ourselves from the demands of the (non-human) environment. Second, the most dramatic changes in our lifestyles have come from cultural evolution changes in the way we feed, dress, organise and entertain ourselves and communicate with each other. Biological evolution depends on genetic differences; more precisely, on the ability of individuals carrying some combinations of genes to have more children than others. Therefore, 'selfish genes' that endow their carriers with reproductive success are the ones that become widespread in a population. In contrast, cultural evolution depends on the ability of behavioural variants to spread, not necessarily because they reflect underlying genetic differences, but because they are easy to copy. As a bonus, indulging in them may please the senses.

A pithy way of putting it would be to say that the evolution of hardware has been outpaced by the adaptability of the software. Because of our runaway brains, we attempt to maximise the amount of sensory gratification that we get. This opens up the way for sufficiently skilled individuals to manipulate our behaviour by doling out a suitably engineered mix of sensory stimuli.