Out of Eden: The Peopling of the World

by Oppenheimer, Stephen

  Others have suggested alternative theories of why it was advantageous to adopt a two-legged posture, such as literally keeping a cool head2 or, like the African meerkats, keeping an eye out for predators on the plain. However, our ancestors’ brains, although larger than those of most other land mammals, were no bigger than that of our cousin the chimpanzee, so there was less danger of them overheating. Nor is standing upright – which many mammals do, including monkeys, chimpanzees, bears, and meerkats – the same thing as habitually walking on two legs for long periods. The idea of leaving hands free to do other mischief such as wielding heavy sticks for hunting (or more likely for defence against predators, since our ancestors were mainly vegetarian) is attractive as an evolutionary force. Unfortunately we have no direct proof, since wood is perishable and stone tools are not found from that time.

  Those early walking apes, for whom there is still only fragmentary evidence, were followed by the famous ‘Lucy’ family, Australopithecus afarensis. Lucy’s partial female skeleton was discovered by Donald Johanson in 1974 at Hadar, in Ethiopia. Living between 3 and 4 million years ago, her kind were 1–1.5 metres (40–60 inches) tall, more clearly upright and bipedal, with a pelvis more similar to ours. Above the neck the similarity ceased, for their skulls and brains were like chimpanzees’ (375–500 cm3 in volume), although they had smaller canine teeth (see Plate 1). As with gorillas, their females were much smaller, suggesting a harem society. A different two-legged version (Australopithecus africanus) lived between 2 and 3 million years ago and, although the same size, had a slightly larger average brain size than chimps at between 420 and 500 cm3. Their teeth were also smaller and more like ours. It should be said that these two particular bipeds are not necessarily steps on a direct evolutionary sequence towards ourselves: our ancestor at this time could easily have been a sister species as yet undiscovered. For instance, our nearest ancestor could have been a recently discovered species from about 3.6 million years ago called Kenyanthropus platyops – quite literally, ‘flat-face’ (Figure 0.1). The flat face, a feature of humans, might represent a bridge between the walking apes and us. What is certain, though, is that over the few million years in which the australopithecines (‘southern apes’) and their immediate ancestors walked Africa’s grasslands, we see only a moderate, not a dramatic, increase in brain size.

  Figure 0.1 The ‘untidy tree’. Over the past 8 million years of hominid evolution (including here only our nearest living relatives, chimps), several species usually co-existed at any one time, so drawing a tree of direct descent on the basis of chance fossil finds may be misleading – hence the lack of branches on this tree.

  Growing brains in the big dry

  Things were about to change, because 2.5 million years ago the world started getting colder. Within a million years, the wet and warm Pliocene geological period gave way to the Pleistocene ice epoch. This was a grinding cycle of repeated dry ice ages, with alternating advances and retreats of African grassland lasting right up until the most recent glaciation, which climaxed 18,000 years ago. Soon after the start of this unstable, icy, and dry period, the first humans (the Homo genus) with their stone tools and larger brains made their entrance on the African savannah stage. As had happened a few million years before with the split into four-legged and two-legged locomotion, this was a parting of the ways for the descendants of the walking apes. One branch, known as Paranthropus, developed larger jaws to cope with grinding up tough vegetable matter. The other branch, Homo, made stone tools, developed substantially larger brains, and set off down the road towards becoming better hunter-gatherers and then, finally, us.

  Naturally, we are inclined to think of humans as being special and set apart from the other apes. Many think that it is our especially large brain that makes us what we are. Some even think that an increase in brain size led to tool-making, but this argument seems unlikely. Fashioning stone tools, unlike walking on two legs or manual dexterity, may be unique to humans (and possibly to Paranthropus3), but even chimpanzees make crude but effective tools out of wood, and they have smaller brains and branched off much earlier than the walking apes. Although we do not have the evidence in wood from the last 7 million years, chimps still have roughly the same sized brain as our common ancestors who lived at the beginning of that time. This does not seem to constitute a strong link between simple tool-making and achieving a critical brain size. Nor does it rule out the possibility that the common ancestors of chimps and humans were making tools so long ago.

  One of the earliest human tool-makers, Homo habilis, had an average brain volume of 650 cm3, but among the known habilis skulls is one 1.9 million years old with a chimp-like brain volume of only 500 cm3, which is at the top end of the range for the earlier australopithecines.4 The small body and brain size and the other apelike features of Homo habilis have led some anthropologists to call for their expulsion from the Homo genus or ‘human club’, but in spite of the well-argued case, this seems more like size prejudice than scientific reasoning. Homo habilis were unlikely to have been our direct ancestors, but that can be said for most hominids; and they made stone tools.

  The idea that we somehow grew a big brain first, then decided what it was for, is a negation of Darwinian principles. Any new kind of behaviour always precedes the physical adaptation that evolves to exploit that behaviour. Well before the start of the Pleistocene ice epoch there must already have been some aspect of our behaviour – something to do with the way we faced the climatic challenge, perhaps – that gave large, energy-expensive brains survival value. The problem of finding food in an increasingly dry environment must have taxed our ancestors’ resourcefulness. Larger brains clearly helped them in some way. That behaviour must still be with us today, because over subsequent major glaciations during the past 2.5 million years, new human species with larger brains and more skills appeared in Africa. As the climate briefly warmed after each glacial maximum, the Sahara would become green for a few thousand years and the new human species would venture out to try their luck in Eurasia. By 1 million years ago, brain volumes of various human species living both within and outside Africa had increased from 400 to 1,000 cm3, and even into the modern size range. In other words, human brains had grown to three-quarters of their modern size long before we came on the scene.5

  Why did we grow big brains?

  There have been various suggestions as to what the key behaviour selecting for big brains might have been. The ice ages forced Africa into extreme aridity and would have inspired the dwindling numbers of savannah humans to greater resourcefulness and cooperation. We can see the value of larger brains in such circumstances, but why should our brains have grown, and not those of the other mammals living at the edge of the savannah? 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.6 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 specu
lation. We know that, at least in Africa, stone tools were used by Homo erectus to butcher meat,7 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,8 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 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 for all species increased 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 – see Figure 0.2). 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 that 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.10 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 and dietary strategies 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.

  Ever newer models

  Although other intelligent ape
s, including several Paranthropus species, continued to walk the African savannah from 2.5 million years ago, it is humans – genus Homo – with whom we are concerned. Humans represented a new evolutionary concept in a number of ways, not only with their enlarged brains, mixed diet, and smaller teeth, but in their adaptive behaviours, including the making of the first shaped stone tools by the very earliest human species.

  If we take Homo habilis as the prototype, then Homo erectus was the line-defining human – the Model T Ford of the new genus. Even more successful than the Model T, they dominated the planet for a million and a half years. With a sad, wary face, a flat nose, and, initially, a rapidly growing brain, Homo erectus was just like us from the neck down (see Plate 2). They had stone tools – simple retouched pebbles at first, but later more sophisticated hand-axes. Their African progenitor Homo ergaster was the first human to leave Africa, 1.95 million years ago, to become the Asian Homo erectus. The latter were slightly smaller than ourselves, and rapidly spread to the Middle East, Russia, India, the Far East, and Southeast Asia, carrying with them their so-called ‘pebble-tool’ technology.11 There are controversial claims that the smaller predecessor on the tree Homo habilis also made this leap at the same time. There is better evidence, however, that all subsequent human species made it out of Africa at the first available interglacial warm-up between ice ages.

  Homo erectus types then dominated the world for nearly a million years until another terrible series of ice ages dried up much of Africa over a million years ago and brought about the emergence of a new, more specialized family. The first African representative of this new model was Homo rhodesiensis. The same size as us and with a brain volume of as much as 1,250 cm3, they used a more sophisticated stone tool kit known as Acheulian, named after a village in France near where the style was first found. Acheulian tools included largish flat stones shaped on both sides to form teardrop-shaped pointed hand-axes. This new arrival first made it out of Africa to Europe, and possibly to China, during a brief warm-up about half a million years ago, and carried the Acheulian technology with them.12