One behavioural characteristic that is still very much with us today is our fondness for meat with our vegetables. Indeed, London-based anthropologist Leslie Aiello and her colleague Peter Wheeler (the originator of the cool heads theory) suggested that we needed to eat meat in order to facilitate our brain growth. Brains need lots of calories to fuel them, and require high-grade nutrients in order to grow. Yet, as Aiello and Wheeler acknowledged, meat-eating is more a means than a motive for brain enlargement. They also argue that the parallel reduction in size of human intestines, as shown by an alteration of the shape of the ribcage in Homo ergaster, is evidence for a change to eating more meat than vegetables. In other words, the oldest true humans had lost the vegetarian pot-belly so characteristic of australopithecines. But this change in the ribcage happened before the dramatic brain growth.
Early humans such as Homo habilis, Homo rudolphensis, and Homo ergaster may have been more scavengers than hunters. Perhaps they learnt that by arming themselves with sticks and stones they could drive larger predators away from kills, and as their tool-making and cooperative hunting abilities improved they became more confident. But without evidence, all such arguments for the role of climate and meat-eating in the enhanced brain growth of early humans remain largely armchair speculation. We know that, at least in Africa, stone tools were used by Homo erectus to butcher meat, but to establish a link between meat-eating, worsening weather, and brain growth we would need a comparison with purely vegetarian primates in the same environment over the same period. Recently, Sarah Elton, an anthropologist based at the University of Kent at Canterbury, has provided just that - but her results rather shake our sense of the uniqueness of the human line. She measured brain size in a number of fossil skulls from primate species over the period roughly from 2.5 million years (the start of the cooling) to 1.5 million years ago. She studied the two main branches of hominids that diverged during that period, Homo and Paranthropus, covering a total of six species. As a comparison primate group she chose several prehistoric species of large, grass-eating, baboon-like Theropithecus monkeys that lived in the same environment over the same period. The results were startling. The large vegetarian monkey species showed no trend of increasing brain size over that time period, but hominids from both the Homo ergaster and Homo habilis and Paranthropus boisei branches did. So, not only were several new Homo and Paranthropus species appearing with successively larger brains, but brain size was increasing specifically within each species of each genus. The latter observation is even stronger evidence for a shared new behaviour selecting for larger brains held by the common ancestor of the Homo and Paranthropus branches, but not shared with other contemporary primates. The relative increase in size in both hominid branches is also surprising since the Paranthropus branch, with their huge jaws, were supposed to be specialist vegetarian grinders. Over the million-year period that Elton studied, the average hominid brain size more than doubled, from 400 cm3 to 900 cm3.
If we compare this early era of phenomenal brain growth with more recent times in the human line, there is a clear discontinuity between ancient and modern. Between the earliest Homo habilis just under 2 million years ago and the first so-called Homo rhodesiensis fossils of 1.07-1.3 million years ago,9 a period of roughly 700,000 years, brain volume increased by two and a half times. In the subsequent 1.2 million years, although there were modest trends in brain size increase in individual human types outside Africa, such as Asian Homo erectus and European Neanderthals, a net increase of only 6 per cent was required to reach the brain size of today’s humans. (In fact there has been an overall decline in brain volume in modern humans over the past 150,000 years). So, from a physical point of view, the earliest period of the human tree was the most dramatic.
These results suggest that the earliest period of increased climatic adversity at the end of the Pliocene, and over the Pliocene-Pleistocene climatic changeover, selectively favoured brain growth in the various new hominid species, but not in other primates sharing the same environment. What does this mean? First, it supports the view that all these hominids belonging to the Paranthropus and Homo branches, and by implication their common ancestor, possessed some new behaviour selectively favouring brain growth which they had shared from at least the beginning of the cool period. In other words, the behavioural seeds of our extraordinarily rapid brain development may already have been in place in walking apes 2.5 million years ago. Second, it puts the meat theory under some strain, although in her defence of the theory Elton argues that Paranthropus were neither strict vegetarians, nor were they incapable of making tools to assist in extracting food from a variety of sources. Third, the selection for brain size seemed to have its greatest acceleration at the beginning of our genus, over 2 million years ago.
There is a further problem with the 'meat makes brainy hunters' theory. Higher primates were not the only cooperative hunters on the African savannah. Yet we do not see lions, hyenas, or the African hunting dog wandering the veldt with huge craniums. To be sure, these committed carnivores do have relatively larger brains and appear more calculating than their prey, but they do not compare to humans, or even chimps. They are true, blinkered specialists in tooth, claw, and muscle. Hominids, by contrast, have always retained their physical and mental flexibility in exploiting food resources. We still eat vegetables - lots of them, including fruits, roots, leaves, seeds, nuts, and berries. Our hands and teeth have become more generalist and flexible rather than specialist. The only physical feature that has developed in relation to our hands, apart from the opposable thumb, is the part of the brain devoted to their manipulation.
A remarkable number of the behavioural differences in dietary strategy that set us apart from the carnivores are in fact characteristics that we share with our nearest living primate relative, the chimpanzee. We even share the social significance of cooperative hunting with them. Astonishing film sequences of chimps hunting colobus monkeys in Africa reveal the differences between primate hunters and specialist carnivores. We are told that those smart primate hunters have much higher success rates than lions. Their quarry meat, although highly prized, is not an essential part of their diet. Not all chimp troops hunt, nor do all chimps in a hunting troop get to eat the meat. Those that do partake may be trusted lieutenants or females with whom the dominant hunting male may wish to copulate. So, hunting among chimps could be more a prestige than a survival strategy, as it is among some humans. Sexual favours would ensure that hunting males passed on their genes more successfully. We all know where runaway sexual selection leads: to peacock tails-or, just maybe, to big brains.
Sexual speculation aside, the point I wish to make is that we should be looking much more closely at the behaviour of our closest living relatives for the seeds of our success. The history of primates over the past 10 million years has not been of specialist ruminants who decided to stop eating vegetables and start eating vegetarians instead, and who in the process became much smarter. It is the history of an already intelligent, large-brained order of forest-based generalists who made a virtue of their flexibility, even when they changed habitat. They all preserved the dexterity of their five fingered hand and in most cases their teeth got smaller rather than larger.
One group, the ape-like ancestor of chimps and ourselves, became larger. As masters of their environment they exploited a wide range of forest vegetable food. In their trend towards omnivory they experimented with a diet of animals smaller than themselves. In their intense competition for mates, hunting may have stuck as a self-perpetuating prestige cultural practice. Being smart and cooperative, they became good at it; but neither chimps nor our ancestors ever gave up the flexibility and survival value of a diverse diet, nor the flexible social cooperation that they used to exploit their environment so well.
The single most important physical specialization that our ancestors the australopithecines evolved was the ability, unique among mammals, to habitually walk on two legs. Whether this adaptation was in response to the encroaching savannah, the need keep a cool head, or - more likely - to free up their hands, it happened millions of years before the sudden acceleration of our brain growth. When the weather became seriously worse 2.5 million years ago, their behaviour and physical form were appropriate for the next step. Their hands were free, their head was smart and cool, and their intelligent, cooperative exploitation of a wide range of foods, including meat, was still the rule. The dry climate merely turned up the selective pressure on the savannah primates to make the best of diminishing vegetable resources. Instead of aping the big cats and growing their canines into sabres and their claws even longer, and becoming true carnivores, they did what they had always done in the past: they used their brains and hands. It was against this long-established background of flexibility and social cooperation that some unique new behaviour associated with rapid brain growth kicked in 2.5 million years ago with the start of the Pleistocene ice epoch. This new behavioural trait offered the potential to cope with climatic adversity. That it was present 2.5 million years ago, before the first humans, is evidenced by the rapid brain growth shared by humans’ sister genus Paranthropus.
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