The Recipe For Uniqueness - Was Cooking The Secret To Our Success?

Since time immemorial, we have recognised that humans differ hugely from all other animals. Despite the increasing evidence in the last few decades that other primates not only use tools but even have rich 'cultural' traditions, the human capacities for language, symbolic thought and cumulative culture are qualitatively different from anything found elsewhere in the animal kingdom. These abilities are all made possible by our unusually large brains, which are around five times larger than would be predicted for a mammal of the same mass. Although we do differ from our nearest relatives in some other interesting ways (we are hairless, and bipedal), if we are to explain how and why we became 'human' we must explain how and why we came to have such extraordinarily large brains.

Many hypotheses have been put forward regarding the selection pressures which would have favoured this encephalisation (development of larger-than-expected brains) in the human lineage, and in primates more generally. Obviously the associated cognitive abilities would have provided many advantages during our evolutionary history, but the big question has been which of these adaptive benefits has been the real driving factor.

Many authors have suggested that the key benefits lie in the ability to solve ecological problems - the foods exploited by primates are patchily distributed in both time and space, and are often difficult to process, so large brains would have been useful in terms of resource acquisition. Still, the current consensus is that encephalisation was actually driven by the need to solve social problems. Primates are on the whole very social animals, and tend to live in groups. This brings with it issues of social group conflict and competition, and also opportunities for reciprocity and cooperation. The individuals best able to navigate these complex social environments would be most liable to pass on their genes to future generations. Several indices of social complexity or skill including social group size, grooming clique size, the frequency of tactical deception and the frequency of social play have all been shown to correlate with the size of the neocortex (the area of the brain where the majority of human encephalisation has taken place) in extant primates. It appears that the reason we have such big brains is that they enabled our ancestors to better deal with the challenges that arise from living in social groups.

Still, this can only be part of the answer. Rather than asking "why do humans have such big brains?", perhaps we would be better off asking "why do gorillas NOT have big brains?"After all, they also live in social groups, and face the same problems. In 1995, Leslie Aiello and Peter Wheeler2 pointed out that brains are hugely metabolically expensive organs, yet our overall basal metabolic rate (BMR) is no higher than would be expected for a mammal of our size. It turned out that in humans, increases in brain size have been accompanied by corresponding reductions in gut size, so that our BMR overall stayed more or less constant. If an organism is to have such small digestive apparatus and still obtain enough energy and nutrients to survive, it requires an extremely high quality diet. The reason our cousins the Gorillas didn't follow us down the path to encephalisation is that they already spend a huge amount of time every day grinding their way through foliage in order to meet their metabolic demands, and they simply can't afford to pay for larger brains with such a low quality diet.

If during the course of human evolution, a large dietary shift occurred that facilitated our encephalisation, then perhaps we can see signs of it in the archaeological record. The traditional view among palaeoanthropologists is that our increasingly complex stone tool use enabled us to scavenge and hunt large game, and an increasing reliance on meat was the key. However, Richard Wrangham has suggested that this transition is not the most important one - plenty of animals eat meat, but humans are unique in that they cook their food. The cooking of food is a human universal, and it seems that modern humans are biologically adapted to a diet of cooked food, struggling to survive on solely raw food for any prolonged period of time. By cooking our food we externalise part of the digestion process, allowing us to obtain more energy from it, and also breaking down harmful toxins. We also increase the range of edible foods, which could feasibly have allowed our ancestors to expand their ranges out of Africa and into Asia and Europe. Although cooking very rarely preserves in the archaeological record, theoretically we could infer the identity of the first 'chefs' by examining their fossil remains for evidence of adaptation to a cooked diet.

The aim of my own research is to identify these clues of a cooked diet in a number of fossil human ancestor species dating from between ~3million years ago, before the first known use of stone tools, to around 500k years ago when brains had more or less reached modern sizes. By looking at the masticatory apparatus and inferred gut mass (estimated from the size and shape of the lower ribs) of our ancestors, we should hopefully be able to identify the point in time when they began to regularly cook their food. If we know both when these dietary shifts took place and when brain size increased, we can look at how the two map onto each other in order to infer whether the shift to cooked food was indeed the driving factor. In doing so, we will be throwing light on what made us human, and how we became the world dominating species that we are today