Brains

[Joel Olson]

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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.
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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.
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