Out of Eden: The Peopling of the World

by Oppenheimer, Stephen

  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 cm3, 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.

  Coevolution is not relevant only to our own species’ history. Far back on the tree of life, new, invented, or perhaps randomly adopted but adaptive behavioural skills drove the genetic changes that determined the subsequent development of special physical traits to exploit those habits. All Darwin’s finches were descended from a single ordinary Central American finch species that had to try different solutions in order to survive in the challenging new environment of the Galapagos Islands. Later, multiple new species of finches evolved physically, the better to exploit those different skills.

  Just as far back on the vertebrate tree, at the start of each generation, the young of many species imitated and re-learnt the ‘innate’ skills of their parents. We know of many instances among higher vertebrates where the parents actively participate in teaching their young. So at first these new ‘invented’ behaviours were transmitted not primarily by genes, but by parents and others teaching – and by the young learning. Subsequently, genes favourable to the new behaviour would begin to be selected by biological evolution, thus equipping new species to better exploit the new behaviours. In other words, genes and culture coevolved.

  The development of culture need not necessarily be so tightly bound to genetic inheritance. Throughout most of mammalian evolution, such teaching of culture was strictly confined to members of the immediate family or group; as a result, behaviour was bound to genes. Among social mammals, however, survival skills are transmitted among members of a social group that are not always related. Thus, at some time over the last few million years of primate biological evolution, the evolution of culture gained a degree of independence from the genes coding for the animals that carried it. By analogy, the evolution of the violin family could equally have been achieved by a guild of viol-makers as by a family that passed the skill from father to son.

  What is the evidence for this? Some purely learnt rather than innate cultural traits are geographically localized in a way which may be independent of genetic relationships. We know of Japanese macaques that wash sweet potatoes in the sea – a local cultural trait, with a recorded historical and geographical origin, which was subsequently passed on from generation to generation. It is extremely un
likely that this new behaviour depended on any new genetic trait; but, to follow this trivial example through, if there was a special survival advantage to washing sweet potatoes and they became the main dietary support for this local race of macaques over many generations, natural selection of random genetic alterations in those future generations could enhance the practice of sweet potato washing in some way. That would be coevolution.18

  The geographical localization of invented culture in higher nonhuman primates is seen particularly clearly among chimps. In chimp tribes, specific tool-making techniques are possessed by members of a particular group and by other nearby but unrelated groups. These techniques are culturally acquired and not genetically determined and are therefore not necessarily found farther afield. At some point, perhaps even before the appearance of hominids, culture jumped the species barrier and was shared between different apes. Long before this time, cultural evolution can be said to have entered its teens and to possess its own prehistory in parallel with genetic evolution.

  From this Baldwinian perspective, we can make one prediction and one observation. The prediction is that if complex deliberate communication requires a developed brain, then simple deliberate communication of some sort must have preceded the evolution of big brains. The observation is that the extraordinary invention and sophisticated flowering of writing happened some 5,000 years ago, and the invention of musical notation much more recently. These two coded non-oral systems of communication unleashed, arguably, the highest peaks of human achievement, yet we do not invoke a new species of human with special genes and a new brain to account for each of them.

  How did our brain grow, and why does size matter?

  Much of the perceived difference between modern humans and other animals has been related to a large brain. Several things, however, need to be pointed out. Size is very important but it is not everything. Bigger may not necessarily be smarter. For instance, pigs, being big, have much larger brains than small, expert, hunters such as wild cats. Humans who for medical reasons have had half their brain removed in childhood can enjoy near-normal human intellect and skills with the remaining 700 cm3. Clearly, connections do count for something, and we definitely have more interconnections inside our brains than do other mammals; but how did this come about?

  In general, larger bodies require larger brains. To put it crudely, this is because the larger organs and muscles of larger bodies need more brain to control them, or at least a minimum share of the attention the brain pays to the larger bulk of the body. This relationship between body and brain size, although predictable in most mammals, is not a simple ratio – if it were, then mice, for example, would have much smaller brains than they actually possess. The relationship becomes even less straightforward in the higher mammals since the body/brain size ratio has been distorted in several profound ways. Primates, for instance, have proportionately larger adult brains than do other mammals, because they have bodies that, from early life, grow more slowly for the same absolute rate of brain growth.

  Humans also have a slower clock for brain maturation than do other apes. In all mammals, brain growth switches off before body growth in a way that matches the functional needs of the adult body size. Humans, however, differ from other primates in that their internal clock keeps their brains growing for longer than would be expected for their final body size as primates. The result of the prolongation of foetal and infant development stages is a brain size more appropriate for a 1,000 kg ape such as the extinct Gigantopithecus.19

  Another simple gene-controlled difference in humans is that the parts of the brain originally sited on the back of the early developing embryo grow relatively larger than in other primates.20 In the adult human, this means that the cerebellum and the cerebral cortex end up disproportionately large. These two parts of the brain are essential for coordination and higher thought. The genetic changes that brought about these dramatic effects were probably simple and involved rather few developmental genes. The resulting relative changes in the sizes of different parts of the brain have profound effects.

  All these distorting size effects are genetically programmed to start in the embryo at a very early stage, before most brain cells develop their connections. The ballooning of the cerebral cortex endows it with far more neural tissue than is required for the mundane tasks of keeping the rest of the body running. In other words, in humans (and to a lesser extent in modern apes) there is a huge volume of apparently redundant cortex without a civil service role.

  If the overexpansion of the cortex happens in the embryo long before the different parts of the brain start connecting up with one another, how might this affect the quality of the final connections? The answer is that when nerve cells in distant parts of the brain do start connecting up with one another, later on in the embryo’s development, size plays a strong role in determining the strength and number of connections that the cortex makes internally with itself and externally with the rest of the brain and spinal cord. The resulting overgrowth in cortical connections may be described as a powerful ‘ministry without portfolio’ that is truly well-connected and has its fingers in every executive pie. The increased internal cortical connections may, in particular, make us humans hard-wired for mischief, creativity, and associative symbolic thought. The increased external connective power of the cortex has also given us direct control of motor nuclei in the brain stem which govern speech production. Those nuclei were previously under a subcortical autopilot control. All this, merely as a result of the crude resetting of perhaps half a dozen controller genes.21

  Most of this ‘upsizing’ happened long before we came along. Simple comparison of brain and body size in earlier humans shows that the these changes moved into overdrive with the evolution of Homo erectus. So, with the knowledge that just a few genetic alterations brought about a huge growth of functional potential in the human brain, we come back again to the question of what new behaviour drove that rapid growth 2.5 million years ago.

  Food for thought or just talking about food?

  Evolutionary psychologist Robin Dunbar, from the University of Liverpool, has argued that animals with relatively large brains can remember, and interact closely with, a larger social network. In theory, he argues, those with the greatest ‘social capacity’ are humans. From comparison with other animals we could extrapolate a group size of over 300 for both modern humans and Neanderthals. From a personal point of view I have to say that, although I could probably recognize over a thousand individuals when I was at school, this does not fit with the number of people I am personally familiar with on a regular basis today. In a more relevant context, there is also a limit to the density of population a given area of dry savannah can support. Studies of the !Kung hunter-gatherers of southern Africa show average extended family group sizes in the teens and a maximum, dry-season, extended family camp of forty. Clearly, in the larger groups social interaction may be more superficial than in the smaller ones. Palaeolithic expert at the University of Southampton, Clive Gamble, has argued that our ancestors (and, more recently, our own societies) shared different sized networks with different functions. The immediate intimate group or network size, mainly consisting of the nuclear family, may have been only around five; a larger, effective network might have been around twenty, and an extended network, with less frequent face-to-face contacts, could have been 100–400. The opportunities for sharing or exchanging material goods would arise only in the first two of these networks, while exchange would have more of an element of calculated self-interest in the third. It does not add up to a strong case for sociability, in itself, driving brain growth.22

  While the ability to recognize large numbers of colleagues may be associated with a large brain, it is difficult to see such a networking effect fuelling each jump in human brain size over the past 2.5 million years – especially if the network interaction was little more than grooming for lice and fleas and being nice to one another. Time left over for the serious business of finding food
could well be diminished by too many such contacts.

  Robin Dunbar and Leslie Aiello have suggested that language might originally have been an energetically cheap means of social grooming in this context,23 although it also serves as a means of exchanging information. Most of us spend much of our time in social talking. I find it difficult, however, to conceive that complex spoken language – our own unique skill – evolved more as a form of reciprocal grooming and gossip than as a means to extend our cooperation productively and to teach our offspring by transmitting practical information. The human family moved from lowly scavenger-gatherers to one of the top predators on the African plain in the period before our fully modern ancestors left Africa. Surely this was not by dint of gossip and social point-scoring. Chimps that have been taught to communicate by sign language certainly concentrate much more on food issues in their communications than on social chit-chat.

  In fact, I would turn it the other way round. I argue that language was that unique behaviour shared between the sister genera Homo and Paranthropus 2.5 million years ago which enabled them, cooperatively and flexibly, to survive the barren cycles of the Pleistocene ice epoch and thus drove their brain growth. According to Baldwin’s ‘new behaviour before adaptive physical change’ coevolution theory, they must have had some form of language to start with. It would be hard to argue that the symbolic coded lexicon and syntax of complex language and the productive cooperation it unlocks should not benefit in a graded way from an increase in computing power. Put simply, it is much more likely that we were already communicating usefully and deliberately 2.5 million years ago, and that this drove our brain growth, than that our brain grew until some threshold size was reached and, like Kipling’s Elephant’s Child with its new trunk, we suddenly discovered we could talk.