SCIENCE: BRAINBOX

ONE OF the greatest mysteries of the brain is how and why it evolved. There is little doubt that it is the supreme achievement of natural selection, the force that drives evolution. The human brain has quite literally transformed life on Earth. How did this occur?

Studies of fossilised human skulls show that the brain of modern humans is very similar to those of our ancestors who lived 50,000 years ago. Natural selection must have been operating on the evolution of the brain hundreds of thousands of years before this, long before the advent of agriculture and the start of civilisation.

Any discussion of the origins of the human brain inevitably begins with a comparison with modern apes, our closest living relatives. It also involves looking at our distant ancestors, the hominids and early humans. Pouring latex rubber into the brain cases of fossil skulls has given scientists some idea of the volume of our early ancestors' brains. The inner surface of the brain case also leaves faint impressions on the outer surface of the latex casts, which match the outermost folds of the ancient brain. This provides important information about its shape and complexity, even though the soft tissue itself has long disappeared.

One of the most telling features of the study of fossil skull cases is the increase in size of the brain of our early ancestors. For instance, the australiopithecines - who lived as long as 4.5m years ago - fall within the range of modern apes in terms of brain size, about 450g. Homo habilis (or "Handy Man"), who lived two million years ago, had a brain weighing 750g. By 1.5m years ago, the brain of Homo erectus weighed 1kg, and continued to grow without the appropriate increase in body size until the appearance of the first Homo sapiens about 400,000 years ago. They had brains equivalent in size to those of modern humans, weighing around 1-2kg.

Brain size alone, however, is not terribly meaningful in terms of measuring intelligence. What distinguishes us most from apes is the degree of "encephalisation", or complexity, within the structure of the brain. Encephalisation is a measure of how many brain cells exist over and above those needed by a body of a certain size. The human brain is as large as might be expected for a far larger primate, whereas an ape's brain is comparatively smaller.

Apparent differences in the mental abilities of men and women, evolving over perhaps millions of years, have been particularly intriguing for evolutionary biologists. In prehistory humans lived in relatively small groups of hunter-gatherers where the division of labour between the sexes was quite marked. It is likely that men were responsible for hunting big game, requiring long-distance travel and good navigation skills. They had little need to be great conversationalists, but had to solve tactical problems together and be good at aiming and throwing.

Women would have been more involved with childrearing, tending the camp and preparing food and clothing - evolutionary survival was never non- sexist. This would put a premium on women with inherent abilities in short- range navigation (remembering landmarks), dexterity, being emotionally sensitive towards others, especially children, and communicating emotion verbally.

This does not mean, however, that we are doomed to our sexual stereotypes as laid down by our evolutionary history. Far from it. The very power of our mental imagination means that our brains have given us the ability to be liberated from our past.