SURVEY: HUMAN EVOLUTION
The concrete savannah
Dec 20th 2005
From The Economist print edition
Evolution and the modern world
THE eruption of Toba marked the beginning rather than the end of hostilities between Homo sapiens and the climate. Views differ about whether the eruption was the trigger, but it is clear that an ice age started shortly afterwards. Though the species spread throughout Asia, Australia and Europe (the populating of the Americas is believed by most researchers to have happened after the ice began to retreat, although not everybody agrees), it was constrained by ecological circumstances in much the same way as any other animal. The world's population 10,000 years ago was probably about 5m—a long way from the imperial 6-billion-strong species that bestrides the globe today.
The killer application that led to humanity's rise is easy to identify. It is agriculture. When the glaciers began to melt and the climate to improve, several groups learned how to grow crops and domesticate animals. Once they had done that, there was no going back. Agriculture enabled man to shape his environment in a way no species had done before.
In truth, agriculture turned out to be a Faustian bargain. Both modern and fossil evidence suggests that hunter-gatherers led longer, healthier and more leisured lives than did farmers until less than a century ago. But farmers have numbers on their side. And numbers beget numbers, which in turn beget cities. The path from Catalhoyuk in Anatolia, the oldest known town, to the streets of Manhattan is but a short one, and the lives of people today, no matter how urbane and civilised, are shaped in large measure by the necessities of their evolutionary past.
That fact has, however, only recently begun to be widely recognised. For many years, psychology, like anthropology, operated in a strange intellectual vacuum. Psychologists did not deny man's evolutionary past, but they did not truly acknowledge it, either. Many in the field seemed to feel that humanity had somehow transcended evolution. Indeed, those of a Marxist inclination more or less required that to be true. How else could people be perfectible? Dissenters were usually treated with disdain. But, at about the time that Dr Cann was publishing the work that would expose the fallacy of multiregionalism, a group who dubbed themselves “evolutionary psychologists” began to stick their heads above the academic parapets.
Studying the behaviour of humans is more difficult than studying that of other animals, for two reasons. One is that the students come from the same species as the studied, which both reduces their objectivity and causes them to take certain things for granted, or even fail to notice them altogether. The other is that human culture is, indeed, far more complex than the cultures of other species. There is nothing wrong with studying that culture, of course. It is endlessly fascinating. But it is wrong to assume that it is the cause of human nature, rather than a consequence; that is akin to mistaking the decorative finishes of a building for the underlying civil engineering. The aim of evolutionary psychology is to try to detect the Darwinian fabric through the cultural decoration, by asking basic questions.
Many of those questions, naturally, address sensitive issues of sex and violence—another reason evolutionary psychologists are not universally popular. David Buss, of the University of Texas, demonstrated experimentally what most people know intuitively—that women value high status in a mate in a way that men do not. Helen Fisher, of Rutgers University, has dissected the evolutionary factors that cause marriages to succeed or fail. She thinks, for example, that the tendency of females to prefer high-status mates is at odds with the increasing economic independence of women in the modern world. Laura Betzig, of the University of Michigan, put an explicitly Darwinian spin on the tendency of powerful men to accumulate harems.
Randy Thornhill, of the University of New Mexico, has shown that physical beauty is far from being in the eye of the beholder. In fact, those features rated beautiful, most notably bodily symmetry, are good predictors of healthy, desirable attributes such as strong immune systems—in other words, aesthetic sensibilities have evolutionary roots.
Karl Grammer, of the Ludwig Boltzmann Institute of Urban Ethology, in Vienna, has shown that body odour, too, is correlated with symmetry and linked to immunological strength. Dr Thornhill, meanwhile, has raised quite a few hackles by arguing that a propensity to rape is an evolved characteristic of men rather than a pathology. Even murder has not escaped the attention of the evolutionary psychologists. Martin Daly and Margo Wilson, of McMaster University in Hamilton, Ontario, showed that adults are far more likely to kill their stepchildren than their biological children—a fact that had escaped both police forces and sociologists around the world. They then dared to propose a Darwinian explanation for this, namely that step-parents have no direct interest (in the evolutionary sense) in the welfare of stepchildren.
However, something similar to this list of human behaviours that have yielded to evolutionary psychology could be found in many species. Indeed, it was often comparisons with other species that sparked the investigations in the first place. The males of many other species gather harems, but females rarely do so; female swallows prefer their mates to have symmetrical tails and they are also more faithful to high-status males; both male lions and male baboons kill the infants of females in groups they have just taken over; and so on. Where evolutionary explanations of behaviour become really interesting is when they home in on what is unique to humanity.
Playing games with the truth
One uniquely human characteristic is the playing of games with formal rules. Evolutionary psychology has not yet sought to explain this, but it has exploited it extensively to develop and test its ideas. In their different ways, the games devised by Leda Cosmides and John Tooby, of the University of California at Santa Barbara, and Robert Axelrod, of the University of Michigan, have underpinned that part of evolutionary psychology devoted to uniquely human behaviour. For not all games are about competition. Many also require trust, a sense of justice and sometimes self-denial.
Cases of animals apparently making sacrifices, occasionally of their own lives, to help others are not rare in nature, but at first sight they are surprising. What is in it for the sacrificer? The usual answer, worked out in the 1960s by William Hamilton, is that the beneficiary is a relative whose reproductive output serves to carry genes found in the sacrificer into the next generation, albeit at one remove. Translated into human terms, this is good old-fashioned nepotism. In a few species, though—mankind being the most obvious—people will make sacrifices for non-relatives, or “friends”. The assumption is that the favour will be paid back at some time in the future. The question is, how can the sacrificer be sure that will happen?
Dr Axelrod used a branch of maths called game theory to come up with at least part of the answer. He showed mathematically that as long as you can recognise and remember your fellow creatures, it makes sense to follow the proverb “fool me once, shame on you; fool me twice, shame on me” and trust them provided they don't cheat you. (Sometimes in science it is necessary to prove the obvious before you go on to the less obvious.) Dr Cosmides and Dr Tooby used a different sort of game to show that humans are thus, as Dr Axelrod's model suggests they should be, acutely sensitive to unfair treatment. They did this by presenting some problems of formal logic to their experimental subjects as a card game. When the problems were presented using cards with letters and numbers on opposite faces, and the subjects had to work out which cards needed to be turned over to yield the required answers, people found them hard to do and more often than not got them wrong. However, when the problems were presented in a form that required the subjects to decide whether people were being treated fairly or not, they found them really easy. The researchers' conclusion is that humans are hard-wired not for logic but for detecting injustice.
Trust, and the detection and punishment of injustice, lie at the heart of human society. They are so important that people will actually harm their own short-term interests to punish those they regard as behaving unfairly. Another game, for example, involves two people dividing a sum of money ($100, say). One makes the division and the other accepts or rejects it. If it is rejected, neither player gets any money. On the face of it, even a 99:1 division should be accepted, since the second player will be one dollar better off. In practice, though, few people will accept less than a 70:30 split. They will prefer to punish the divider's greed rather than take a small benefit themselves.
This makes no sense in a one-off transaction, but makes every sense if the two participants are likely to deal with each other repeatedly. And that, before the agricultural population boom (and also, for the most part, after it) was the normal state of affairs. The people an individual dealt with routinely would have been the members of his circle of 150. Strangers would have been admitted to this circle only after prolonged vetting. Such bonds of trust, described by Matt Ridley, a science writer, as “the origins of virtue” in his book of that name, underlie the exchanges of goods and services that are the basis of economics. They may also, though, underlie another sensitive subject that social scientists do not like biologists treading on: race.
Robert Kurzban, a colleague of Dr Cosmides and Dr Tooby, took the racial bull by the horns by reversing the old saw about beauty. Dr Thornhill's work overturned the folk wisdom that beauty is in the beholder's eye by showing that universal standards of beauty have evolved, and there are good reasons for them. Dr Kurzban, by contrast, thinks he has shown that race really does exist only in the eye—or, rather, the mind—of the beholder, not the biology of the person being beheld, and does so for good Darwinian reasons.
First impressions count
Dr Kurzban observes that the three criteria on which people routinely, and often prejudicially, assess each other are sex, age and race. Judgments based on sex and age make Darwinian sense, because people have evolved in a context where these things matter. But until long-distance transport was invented, few people would have come across members of other races. Dr Kurzban believes that perceptions of racial difference are caused by the overstimulation of what might be called an “otherness detector” in the human mind. This is there to sort genuine strangers, who will need to work hard to prove they are trustworthy, from those who are merely unfamiliar members of the clan. It will latch on to anything unusual and obvious—and there is little that is more obvious than skin colour. But other things, such as an odd accent, will do equally well. Indeed, Dr Dunbar thinks that the speed with which accents evolve demonstrates that they are used in precisely this sort of way.
If Dr Kurzban is right (and experiments he has done suggest that assessments of allegiance are easily “rebadged” away from skin colour by recognisable tokens such as coloured T-shirts, as any sports fan could probably have told him), it explains why race-perception is such a powerful social force, even though geneticists have failed to find anything in humans that would pass muster as geographical races in any other species. In fact, one of the striking things about Homo sapiens compared with, say, the chimpanzee is the genetic uniformity of the species. The only “racial” difference that has a well-established function is skin colour. This balances the need to protect the skin from damage by ultraviolet light (which requires melanin, the pigment that makes skin dark) and the need to make vitamin D (which results from the action of sunlight on a chemical in the skin). This explains dark, opaque skins in the tropics and light, transparent ones nearer the poles. The test is that dark-skinned arctic dwellers, such as the Inuit of North America, have diets rich in vitamin D, and so do not need to make it internally. As to other physical differences, they may be the result of founder effects, as described by Dr Ambrose, or possibly of sexual selection, which can sometimes pick up and amplify arbitrary features.
Darwinian thinking can lead in other unexpected directions, too. Pursue Dr Buss's observation about women preferring high-status males to its logical conclusion, and you have a plausible explanation for the open-endedness of economic growth. Psychologists of a non-evolutionary bent sometimes profess themselves puzzled by the fact that once societies leave penury behind (the cited income level varies, but $10,000 per person per year seems about the mark), they do not seem to get happier as they get richer.
That may be because incomes above a certain level are as much about status as about material well-being. Particularly if you are a man, status buys the best mates, and frequently more of them. But status is always relative. It does not matter how much you earn if the rest of your clan earn more. People (and men, in particular) are always looking for ways to enhance their status—and a good income is an excellent way of doing so. Aristotle Onassis, a man who knew a thing or two about both wealth and women, once said: “If women didn't exist, all the money in the world would have no meaning.” Perhaps the founding father of economics is not really Adam Smith, who merely explained how to get rich, but Charles Darwin, who helped to explain why.