Science: A race of potato-heads

New research argues that the consumption of cooked vegetables is the source of our large brains. By Steve Connor

A visitor from another planet may wonder how it was that human beings acquired their large brains. The enormous cranial capacity of Homo sapiens, compared to body size, is truly exceptional. It is one of the most obvious physical features that distinguishes us from the rest of the animal kingdom and yet scientists have long been puzzled over why the human brain evolved to be so big over such a relatively short period in history. Now a team of anthropologists believe they have found the answer - big brains came about when early humans learnt how to cook root vegetables.

It may seem far fetched to believe that the brain, with all its stupendous achievements, owes its origins to something as unappetising as a boiled carrot or baked potato, but this is just what Richard Wrangham has proposed. Wrangham, professor of biological anthropology at Harvard University, has devised a plausible argument for why human brain growth went into overdrive. It was, he suggests, powered by the high-energy fuel locked in plant tubers which suddenly became available when humans learnt how to cook.

"The suggestion is that cooking is what makes humans human," Professor Wrangham explains. It was cooking that allowed the transition from the Australopithecines - a human-like ape - to a true human ancestor, in the form of Homo erectus, nearly two million years ago. It was cooking that enabled the human anatomy to undergo a major reconstruction, and it was cooking that provided the energy needed to feed the enormous appetite required to build and maintain a large brain.

There have been many theories to explain the rapid evolution of the human brain, which took place over a few hundred thousand years, an eye-blink in evolutionary history. Some scientists have suggested that a large brain initially evolved as a way of cooling the head when early humans began to walk on two legs. Others have proposed that bigger brains became sexually advantageous when courtship rituals began to involve ever more intricate displays of linguistic and artistic prowess. But the most plausible idea was that large brains came about as a result of some major change in diet.

Anthropologists thought this must have been when early humans began to scavenge or hunt for meat, which is nutritionally rich and could easily provide the extra calories necessary for brain development. It was a meat- eater's answer to the big-brain conundrum, and the evidence of a major change of diet being responsible was convincing.

The difference in anatomy between Australopithecines, who lived in Africa between one and five million years ago, and Homo erectus, perhaps the earliest human ancestor, points to a major transition in the type of food eaten. In addition to an increase in brain capacity, from about 500 cubic centimetres in a typical Australopithecine to about 750ccs in H erectus (which doubled again for later members of the species), teeth became smaller and intestines shrunk. Clearly this must have meant a conversion to a nutritionally richer diet.

A further change in the transition from Australopithecine to H erectus is that the difference in size between females and males became less noticeable. The larger H erectus females must have had a better diet than their smaller Australopithecine sisters. Putting all this together clearly showed that food must have become more nutritious as a result of a radical change in diet.

The traditional explanation is that this must have been caused by the transition from a fundamentally vegetarian diet of nuts and berries to a more carnivorous diet. Professor Wrangham, however, is not convinced. In a scientific paper to be published in the journal Current Anthropology, he cites several problems with the meat theory. Modern tribes today rarely, if ever, eat more meat than plants, and it was almost certainly true two million years ago that meat must have been very much a luxury food, and not something that could be relied on as a staple item of the diet.

Another problem is that when animals, such as chimps, become more carnivorous there is little evidence that they grow any bigger as a result. When Wrangham and his colleagues tried to compare the amount of energy added to the diet by changing to meat and cooked tubers, the tubers won each time. Wrangham says that a diet consisting of 60 per cent cooked tubers - about the proportion eaten by modern African tribes - boosted calorie intake by more than 40 per cent compared with humans who ate nuts, berries and uncooked tubers. Boosting the meat part of the diet to 60 per cent merely added an extra 20 per cent to the total calorie intake.

"Cooking makes food more available and digestible in at least five ways," the researchers say. "It can crack open or otherwise destroy physical barriers such as thick skins or husks. It can burst cells, thereby making cell contents more easily available for digestion and absorption. It can modify the three-dimensional structure of molecules such as proteins and starches into forms more accessible for digestion. It can reduce the chemical structure of indigestible molecules into smaller forms, and it can denature toxins."

Of course, a crucial aspect of the theory is determining exactly when early man was first able to control fire. The subject has vexed many anthropologists who have suggested several possible dates, ranging from two and a half million years ago to as recently as 200,000 years ago, with the first appearance in the archaeological record of burnt animal bones, hearths and the earliest examples of earthen ovens.

Critics of the Wrangham hypothesis have pointed out that there has never been any direct evidence of controlled fires at around the time of his proposed invention of cooking - roughly 1.8 million years ago. Wrangham, however, retorts that more ephemeral cooking fires - rather than cooking hearths and ovens - are less likely to have left traces behind for scientists to discover. "Absence of evidence does not mean evidence of absence. We need better ways of looking for early examples of controlled fires," he says.

The African climate of two million years ago would have created a tinder box environment for natural fires to occur frequently, leading to many charred roots being left behind in their wake. "Whether or not they were controlled, however, fires could have cooked plant or animal foods without hominid effort, providing an easy initial entree for the use of cooking technology. Once hominids had detected the merits of cooking, we suggest the idea would spread rapidly," the researchers say.

One of the more intriguing aspects of the Wrangham idea is that tuber cooking could easily have been controlled by females, giving early women a powerful influence over their menfolk. Wrangham suggests this may have led to a tendency for males to scrounge food from females, leading women to form sexual liaisons with other men who would act as guards - a sort of quid pro quo. In effect, this would have led to the sexual strategy of semi-monogamy common in human societies today. "It is striking that among all modern humans, women tend to be the cooks," the scientists say. "The critical logic is that whenever cooking evolved, it led rapidly to the evolution of males scrounging from females (especially mothers) and thence to sexual alliances."

Who said baked spuds could never be sexy?