SURVEY: HUMAN EVOLUTION
If this is a man
Dec 20th 2005
From The Economist print edition
Why it pays to be brainy
THANKS to Dr Cann and her successors, the story of how Homo sapiens spread throughout the world is getting clearer by the day. But why did it happen? What was it that gave the species its edge, and where did it come from? Here, the picture blurs.
Until recently, it was common to speak of an Upper Palaeolithic revolution in human affairs—what Jared Diamond, of the University of California at Los Angeles, called the Great Leap Forward. Around 40,000 years ago, so the argument ran, humanity underwent a mental step-change. The main evidence for this was the luxuriant cave art that appeared in Europe shortly after this time. Palaeopsychologists see this art as evidence that the artists could manipulate abstract mental symbols—and so they surely could. But it is a false conclusion (though it was widely drawn before Dr Cann's work) that this mental power actually evolved in Europe. Since all humans can paint (some, admittedly, better than others), the mental ability to do so, if not the actual technique, must have emerged in Africa before the first emigrants left. Indeed, evidence of early artistic leanings in that continent has now turned up in the form of drilled beads made of shells and coral, and—more controversially—of stones that have abstract patterns scratched on to them and bear traces of pigment.
That certainly pushes the revolution back a few tens of millennia. The oldest beads seem to date from 75,000 years ago, and an inspired piece of lateral thinking suggests that clothing appeared at about the same time. Mark Stoneking and his colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, applied the molecular-clock technique to human lice. They showed that head lice and body lice diverged 75,000 years ago. Since body lice live in clothing, and most other species of mammal support only one species of louse, the inference is that body lice evolved at the same time as clothes.
That is an interesting coincidence, and some think it doubly interesting that it coincides with the eruption of Toba. It may be evidence of a shift of thought patterns of the sort that the Upper Palaeolithic revolutionaries propose. On the other hand, there are also signs of intellectual shifts predating this period. Sally McBrearty, of the University of Connecticut, and Alison Brooks, of George Washington University, have identified 14 traits, from making stone blades to painting images, which they think represent important conceptual advances. Ten of them, including fishing, mining, engaging in long-distance trade and making bone tools, as well as painting and making beads, seem to be unique to modern Homo sapiens. However, four, including grinding pigments (for what purpose remains unknown, but probably body painting), stretch back into the debatable past of Homo helmei.
Given the fragmentary nature of the evidence from Africa, which has not been explored with the same sort of archaeological fine-tooth comb as Europe, the speed of the emergence of modern behaviour is still debatable. One thing, however, that clearly played no part in distinguishing Homo sapiens from his hominid contemporaries was a bigger brain.
Modern people do, indeed, have exceedingly large brains, measuring about 1,300 cm³. Other mammals that weigh roughly the same as human beings—sheep, for example—have brains with an average volume of 180cm³. In general, there is a well-established relationship between body size and brain size that people very much do not fit. But as Dr Oppenheimer shows (see chart 2), most of this brain expansion happened early in human evolutionary history, in Homo habilis and Homo erectus. The brains of modern people are only about 6% larger than those of their immediate African predecessors. Perhaps more surprisingly, they are smaller than those of Neanderthals. There is no doubt that this early brain growth set the scene for what subsequently happened to Homo sapiens, but it does not explain the whole story, otherwise Homo erectus would have built cities and flown to the moon.
Flying to the moon may, in fact, be an apt analogy. Just as a space rocket needs several stages to lift it into orbit, so the growth of human intelligence was probably a multi-stage process, with each booster having its own cause or causes. What those causes were, and when they operated, remains a matter of vehement academic dispute. But there are several plausible hypotheses.
The most obvious idea—that being clever helps people to survive by learning about their surroundings and being able to solve practical problems—is actually the least favoured explanation, at least as the cause of the Great Leap Forward. But it was probably how intelligence got going in the pre-human primate past, and thus represented the first stage of the rocket.
Many primates, monkeys in particular, are fruit-eaters. Eating fruit is mentally taxing in two ways. The first is that fruiting trees are patchily distributed in both space and time (though in the tropics, where almost all monkeys live, there are always trees in fruit somewhere). An individual tree will provide a bonanza, but you have to find it at the right moment. Animals with a good memory for which trees are where, and when they last came into fruit, are likely to do better than those who rely on chance. Also, fruit (which are a rare example of something that actually wants to be eaten, so that the seeds inside will be scattered) signal to their consumers when they are ready to munch by changing colour. It is probably no coincidence, therefore, that primates have better colour vision than most other mammals. But that, too, is heavy on the brain. The size of the visual cortex in a monkey brain helps to explain why monkeys have larger brains than their weight seems to warrant.
The intelligence rocket's second stage was almost certainly a way of dealing with the groups that fruit-eating brought into existence. Because trees in the tropics come into fruit at random, an animal needs a lot of fruit trees in its range if it is to avoid starving. Such a large range is difficult for a lone animal to defend. On the other hand, a tree in fruit can feed a whole troop. For both these reasons, fruit-eating primates tend to live in groups.
But if you have to live in a group, you might as well make the most of it. That means avoiding conflict with your rivals and collaborating with your friends—which, in turn, means keeping track of your fellow critters to know who is your enemy and who your ally. That, in turn, demands a lot of brain power.
One of the leading proponents of this sort of explanation for intelligent minds is Robin Dunbar, of Liverpool University in England. A few years ago, he showed that the size of a primate's brain, adjusted for the size of its body, is directly related to the size of group it lives in. (Subsequent work has shown that the same relationship holds true for other social mammals, such as wolves and their kin.) Humans, with the biggest brain/body ratio of all, tend to live in groups of about 150. That is the size of a clan of hunter-gathers. Although the members of such a clan meet only from time to time, since individual families forage separately, they all agree on who they are. Indeed, as Dr Dunbar and several other researchers have noticed, many organisations in the modern world, such as villages and infantry companies, are about this size.
Living in collaborative groups certainly brings advantages, and those may well offset the expense of growing and maintaining a large brain. But even more advantage can be gained if an animal can manipulate the behaviour of others, a phenomenon dubbed Machiavellian intelligence by Andrew Whiten and Richard Byrne, of the University of St Andrews in Scotland.
Size isn't everything
Monkeys and apes manage this to a certain extent. They seem to have a limited “theory of mind”—the ability to work out what others are thinking—which is an obvious prerequisite for the would-be simian politician. They also engage in behaviour which, to the cynical human zoologist, looks suspiciously like lying. But it is those two words, “cynical” and “suspiciously”, that give the game away. For it is humans themselves, with their ability to ponder not only what others are thinking, but also what those others are thinking about them, who are the past masters of such manipulation.
And it is here that the question of language enters the equation. Truly Machiavellian manipulation is impossible without it. And despite claims for talking chimpanzees, parrots and dolphins, real language—the sort with complex grammar and syntax—is unique to Homo sapiens.
Dr Dunbar's hypothesis is that language arose as a substitute for the physical grooming that other group-living primates use to maintain bonds of friendship. Conversation—or gossip, as he refers to it—certainly does seem to have the same bond-forming role as grooming. And, crucially for the theory, groups rather than just pairs can “groom” each other this way. Dr Dunbar sees the 150-strong group size of Homo sapiens as both a consequence and a cause of verbal grooming, with large groups stimulating the emergence of language, and language then permitting the emergence of larger groups still. Language, therefore, is the result of a process of positive feedback.
Once established, it can be deployed for secondary purposes. Furthering the Machiavellian ends outlined by Dr Whiten and Dr Byrne would be one such purpose, and this would drive other feedback loops as people evolve more and more elaborate theories of mind in order to manipulate and avoid manipulation. But language would also promote collaborative activities such as trade and the construction of sophisticated artefacts by allowing specialisation and division of labour.
Not everyone agrees with the details of this thesis, but the idea that the evolution of mental powers such as language has been driven by two-way feedback loops rather than one-way responses to the environment is a powerful one. Terrence Deacon, a researcher at the University of California at Berkeley, for instance, thinks that language evolved in a feedback loop with the complex culture that it allowed humans to create. Changes in culture alter and complicate the environment. Natural selection causes evolutionary changes that give people the means to exploit their new, more complex circumstances. That makes the cultural environment still more complicated. And so on. Dr Deacon believes this process has driven the capacity for abstract thought that accounts for much of what is referred to as intelligence. He sees it building up gradually in early hominids, and then taking off spectacularly in Homo sapiens.
The peacock mind
Perhaps the most intriguing hypothesis about the last stage of the mental-evolution rocket, though, is an idea dreamed up by Geoffrey Miller, of the University of New Mexico. He thinks that the human mind is like a peacock's tail, a luxuriant demonstration of its owner's genetic fitness.
At first sight this idea seems extraordinary, but closer examination suggests it is disturbingly plausible. Lots of features displayed by animals are there to show off to the opposite sex. Again, this involves a feedback loop. As the feature becomes more pronounced, the judge becomes more demanding until the cost to the displayer balances the average reproductive benefit.
Frequently, only one sex (usually the male) does the showing off. That makes the sexually selected feature obvious, because it is absent in the other sex. Dr Miller, though, argues that biologists have underplayed the extent to which females show off to males, particularly in species such as songbirds where the male plays a big part in raising the young, and so needs to be choosy about whom he sets up home with. Like male birds, male humans are heavily involved in childrearing, so if the mind is an organ for showing off, both sexes would be expected to possess it—and be attracted by it—in more or less equal measure.
Dr Miller suggests that many human mental attributes evolved this way—rather too many, according to some of his critics, who think that he has taken an interesting idea to implausible extremes. But sexual selection does provide a satisfying explanation for such otherwise perplexing activities as painting, carving, singing and dancing. On the surface, all of these things look like useless dissipations of energy. All, however, serve to demonstrate physical and mental prowess in ways that are easy to see and hard to fake—precisely the properties, in fact, that are characteristic of sexually selected features. Indeed, a little introspection may suggest to the reader that he or she has, from time to time, done some of these things to show off to a desirable sexual partner.
Crucially, language, too, may have been driven by sexual selection. No doubt Machiavelli played his part: rhetoric is a powerful political skill. But seduction relies on language as well, and encourages some of the most florid speech of all. Nor, in Dr Miller's view of the world, is the ability to make useful things exempt from sexual selection. Well-made artefacts as much as artful decorations indicate good hand-eye co-ordination and imagination.
Whether Dr Miller's mental peacock tails have an underlying unity is unclear. It could be the ability to process symbols; or it could be that several different abilities have evolved independently under a single evolutionary pressure—the scrutiny of the opposite sex. Or it could be that sexual selection is not the reason after all, or at least not the main part of it. But it provides a plausible explanation for modern humanity's failure to interbreed with its Neanderthal contemporaries, whether or not such unions would have been fertile: they just didn't fancy them.