Out of Eden: The Peopling of the World (5 page)

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 apes, 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 cm
³
, 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

Then, 350,000 years ago, another severe ice age struck, perhaps forcing yet another large-brained human onto the African stage around 300,000 years ago. They are known to some as archaic
Homo sapiens
, and to others as
Homo helmei
. To avoid confusion I shall use the latter name. Beetle-browed, the same size as us, and with an average brain volume slightly larger than ours at 1,400 cm
³
, they represented the plateau as far as dramatic brain growth was concerned. As we shall see in
Chapter 2
, they were also associated with the start of one of the most important revolutions in human technology, known as the Middle Palaeolithic. Some have gone so far as to suggest that if brought up in a modern family, these heavy-browed creatures might fit into our society.
¹³

A larger and longer out-of-Africa movement, during a warm period, saw
Homo helmei
spreading throughout Eurasia 250,000
years ago.
Homo helmei
may have given rise to
Homo neanderthalensis
in Europe and Asia (see
Plate 4
) and had several possible relatives in India and China from the same period. The source human family containing our own ancestors remained in Africa, for the time being, physically separate from their Neanderthal cousins in Europe.
¹⁴

Our own species,
Homo sapiens
, was born over 170,000 years ago, out of what was nearly a human extinction in which the total population fell to an estimated 10,000 in a mother of all ice ages.
¹⁵
Although
Homo sapiens
duly made it out of Africa to the Levant at the next interglacial, 120,000 years ago, the genetic and archaeological evidence indicates that their descendants died out there without issue in the ice age after that. (The Levant – an old-fashioned label, but useful in this context – comprises modern Syria, Lebanon, Israel, Palestine, and Jordan: the Mediterranean Near East minus Egypt.) When modern humans finally spread out of Africa to the rest of the world around 70,000–80,000 years ago, Eurasia was still inhabited by several other human species. The European Neanderthals, and possibly the Southeast Asian
Homo erectus
, persisted until less than 30,000 years ago, but no genetic traces of them remain in living humans.

Significantly, both Neanderthals and those modern humans living before the last ice age 20,000–30,000 years ago had rather bigger brains than do people living today.
¹⁶
It seems that the magic brain-enlarging effect of ice ages had played itself out before the time of our birth as a subspecies of
Homo helmei
(
Figure 0.2
). Maybe the obstetric risks of large heads were limiting. Either that, or brain size was no longer the most important determinant of success, and something new that we were doing with our brains – some other behavioural or cultural innovation – had taken over.

Figure 0.2
   Brain size and cultural evolution. A graph of brain growth reveals three phases over the last 2.5 million years, as separated by vertical dashed lines. The curve below shows how rapid cultural acceleration occurs during brain size reduction. (Recognized cultural milestones given equal weighting. Log-log Regression lines 1–6 relate to closely related contemporary regional human types as shown by symbols.)

Once we had left Africa, although our brains had stopped growing, the climate continued to dominate human expansions and inventions right up to the modern age. It may be no exaggeration to say that the forces driving the waves of human technical innovation advancing across Eurasia from 80,000 years ago were more a result of stress and relief than of any biological improvement in the human computer. For example, the spreads of new technologies labelled by archaeologists as Early, Middle, and Late Upper Palaeolithic, Mesolithic, and Neolithic all coincided with dramatic ameliorations of Europe’s climate and population expansions into new territories. These events were mirrored in Southeast Asia with expansions and advances of boat-building and sailing in response to the flooding of continental shelf as the sea level rose and fell.

In summary, then, rapidly increasing brain size was a key feature that set humans apart from the walking apes that lived before 2.5 million years ago. Since then our brains have trebled in volume. This increase was not gradual and steady: most of it came as a doubling of volume in
Homo erectus
2 million years ago. In other words, the greatest acceleration in relative brain size occurred before 1.5 million years ago, rather early in our genus, and then gradually slowed down. The paradox is that our apparent behavioural explosion is mostly recent and is accelerating.

Baldwin’s idea

The resolution of the paradox of ancient brain growth versus the recent human cultural explosion is that human culture feeds into itself, thus generating its own, exponentially accelerating tempo. As will become apparent, the history of human cultural evolution is not a virtual copy of the biological tree, with each successive human species leaping in intelligence and immediately using much smarter tools. Far from our biological evolution driving our cultural
innovations, it was always the other way round, and although our brains stopped growing a long time ago our culture continues to evolve. The coevolution of culture and genes underlies recent human revolution. Although a deceptively simple concept, it runs counter to all our ethnic and species prejudices.

The mechanisms by which behavioural innovations or ‘new culture’ drive evolution were first elaborated by American psychologist Mark Baldwin a century ago.
¹⁷
Baldwin gave a behavioural interpretation of Darwin’s view of evolutionary phenomena even as simple as the giraffe growing a long neck to eat the leaves at the tops of bushes and trees. He suggested that behavioural flexibility and learning could amplify and bias the course of natural selection. Once new, invented, or learnt habits had changed the context or habitat of a particular group of animals, natural selection could favour genetically determined behavioural and physical characteristics that best exploited that new environment. Known as ‘coevolution’ or ‘genetic assimilation’, this simple argument avoided the pitfall of Lamarck’s discredited theory of inheritance of acquired characteristics, while retaining one of the forgotten but more prescient of his ideas.