Out of Eden: The Peopling of the World (6 page)

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 unlikely 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 cm
³
. 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.

Symbolic thought and language: purely human abilities?

Deliberate communication of one form or another undoubtedly started a long time ago in animals. Vocal speech is merely the most sophisticated form of animal communication, and has selected for a number of specialized physical changes in humans. Vocal speech has special advantages over simple gesture language apart from its ability to convey complex ideas. We can communicate in the dark, through trees, and without looking at the person we are speaking to. It is much easier to con, deceive, and tell lies, and to hide our communication from strangers speaking other tongues. Children learn to lie at around the age of four. Some have suggested that males’ prowess at telling jokes, and making females laugh, might have been an element in sexual selection. Like all other aspects of culture, however, language was invented and has to be internally reinvented in every child learning to speak.