Wired for Culture: Origins of the Human Social Mind (36 page)

It might not be a coincidence that where humans do still get ectoparasite infections, it is on those parts of our bodies covered with hair. We get head lice, and some unfortunate or adventuresome ones among us even get pubic infestations, but no one gets arm lice, or colonies of them on their legs, face, or backs. Another hairless mammal is the apparent exception to the mammalian norm of having fur, but it might just prove the ectoparasite rule. Naked mole rats, as their name implies, have lost their fur. They live in large and densely populated underground burrows and rarely venture out. The burrows are warm and humid places where parasites can easily multiply and move from host to host. Quite possibly, then, naked mole rats have been able to lose their fur as a way to control parasites, but without the threat of freezing to death.

Lice might even allow us to put a date to when we became naked. Body lice plague all animals with fur, but humans, uniquely it seems, are blessed with two distinct varieties. One is specifically adapted to living in thick hair—this is the so-called head lice species—but the other has evolved adaptations for living in clothing. The molecular biologist Mark Stoneking ingeniously recognized that the two species might have separated when humans adopted the habit of wearing clothes. If this habit arose because, owing to a lack of fur, we were getting cold at night or perhaps shivering in strong winds, then clothes put a date on our nakedness. Because clothes do not normally remain preserved at archaeological sites for more than a few thousand years, there is no good evidence as to when this happened. But by careful comparison of slight differences that had accumulated in the genes of these two otherwise closely related species, Stoneking was able to infer that they separated about 107,000 years ago. It is remarkable to think that our burgeoning nakedness around that time created not just modesty but an industry that accounts for billions in sales worldwide today. And it is all down to our ingenuity at ridding ourselves of parasites.

(Just to show how difficult research on these topics can be, Alan Rogers with S. Wooding undertook a different approach to trying to estimate when we became naked. Rogers studied the melanocortin 1 receptor, which is a gene that influences skin color. People of African descent all share a particular variant of this gene that produces their darker skin and confers a strong resistance to sunburn. Non-Africans have many different varieties of this gene, but none of them produces dark skin. The version of this gene in chimpanzees, which are covered with fur, differs from any human form but also fails to provide resistance to the sun. This led Rogers to wonder if the sun-resistant form of the gene might have arisen when we became hairless and thus exposed to the sun. His analysis suggested the sun-resistant form might have appeared around 500,000 years ago. Rogers has gently teased me that if we accept his analysis and the clothes-lice story is correct, we might have stood around naked and cold for about 400,000 years! The comment might not be as far-fetched as it sounds. When Darwin visited Tierra del Fuego at the southern tip of South America, he found the native Fuegians essentially naked in this cold and harsh climate. They seemed not to have a tradition of making clothes; to stay warm, they smeared their bodies with seal fat and slept curled up together in groups. They also made fires. Indeed, in 1520 Magellan called this region the “land of smoke” after the hundreds of beach fires he observed from offshore in his ship. Only later was the name changed to Tierra del Fuego or “land of fire.”)

If having less hair did grant the naturally selected advantage of reducing our burden of parasites, it would probably quickly have become part of our tastes in a prospective mate. That big hairy guy might just be carrying ticks. Sexual selection is the process by which natural selection favors traits that make it more likely you will attract a mate. Sexual selection normally acts on one sex more than the other, and often it is males. The reason is that males typically have greater reproductive potential because a male can easily produce lots of children with many different females, while females are limited by how many babies they can gestate in their reproductive lifetime. This difference in reproductive potential means that males will normally be forced to compete with each other to attract females because there are more males than are needed. Females, in their turn, can then afford to become choosy about who they mate with. And this is why the large and gaudy ornaments, songs, trills, odors, and sexual displays of most animals are found in males—they are all ways of persuading females to mate with them rather than some other fellow. Indeed, we might expect that females will have evolved expensive tastes, and all because of this difference in reproductive potential.

But when it comes to hairlessness, both women and men are expected to prefer less hair in a partner for the simple reason that both want a healthy mate, and neither wants to catch the other’s parasites. These considerations could explain why many people find hirsuteness unattractive, and why products for removing hair from our bodies are such a big industry, sending huge advertising revenues to television stations all too happy to broadcast commercials about the latest five-bladed razor or hair removal cream for women. Even some of our preferences in fashion might reveal ancient tendencies to avoid people who could be carrying parasites. One of the most enduring features of women’s fashion is the backless dress. We do not normally think of backs as secondary sexual characteristics like breasts or hips, so why all the interest in them? It might not be an accident that our backs expose the single largest patch of bare skin on our bodies. A backless dress, without our even being aware of it, acts as a billboard broadcasting one’s healthy—and hairless—skin.

Our nakedness exposes another genetic trait that we have lost, or nearly so, and again domestication by our brains might be the reason. Among the most striking artistic or symbolic objects that our ancestors produced were the Venus statues that have been found from as far back as 24,000–30,000 years ago. They depict women with exceptionally large thighs and bottoms, and some also have large breasts and other hypertrophied sexual characteristics. The statues are often interpreted as exaggerated or symbolic forms, representing ideals to be upheld or sought out. Throughout most of our history as hunter-gatherers, starvation or near starvation was a daily fact of life, at least until the invention of agriculture. A woman who could store enough fat to attain a shape like those depicted in the Venus statues would have been a walking advertisement for her ability to acquire food and to provide for her children. This might seem trivial to us today when food stores for many of us are often no more than a few minutes away, but not to hunter-gatherers.

There is good reason to believe that the Venus statues might not have been exaggerations, or not just exaggerations. A now rare morphological trait known as
steatopygia
produces nearly exactly the hip shape depicted in Venus statues. Women throughout our history might have been more at risk of starvation than men because they would normally be providing food for themselves but also for any child they might have been gestating, nursing, or rearing. Indeed, a hunter-gatherer female would have been in one or more of these circumstances for nearly all of her adult life. Steatopygia is an example of natural selection not just providing these women with insurance against starvation but an exquisitely fine-tuned one. Fat stored on the hips requires less energy to carry around because this is where our center of gravity lies. Steatopygia might very well have been the normal shape of some African and Andaman Island women until as recently as 10,000 years ago. But ever since our brains came up with the idea of agriculture, it has not been as advantageous to store fat and the trait has nearly disappeared. Even so, we still see ancient remnants of it in our tendency to store excess fat on our hips when we gain weight.

Our domestication continued when the plants we domesticated, also beginning around 10,000 years ago, turned around and changed us. The
alcohol dehydrogenase gene
or
Adh
helps animals, including humans, to metabolize alcohol. This protects our livers but also our brains. Common fruit flies carry this gene because they are regularly exposed to naturally fermenting fruit. Genetic studies of Han Chinese and Tibetan populations show that around 10,000 years ago natural selection began to act strongly on these people, favoring a variant of the
Adh
gene that improved their ability to degrade alcohol. This corresponds to a time when rice crops were being domesticated and rice production was spreading across what is now southern China. It is not known whether these people acquired their
Adh
genes from regular consumption of rice wines they produced or simply to protect them against routine exposure to alcohol from naturally fermenting rice. But among contemporary Han Chinese, those carrying the selected variant are less likely to suffer from alcoholism.

The trend for culture to select our genes by domesticating us means that modern humans are far more closely related on many of their genes than the passage of time might suggest. The reason is that “natural selection” is really just a euphemism for selective death. Strong selection means those who lack certain combinations of genes are more likely to die before reproducing, while those lucky few who have them become the progenitors of the rest of us. Modern humans entered Europe sometime around 40,000 years ago, but owing to selection, that does not mean that if you are of European origin your genes are separated by that amount of time from those of other Europeans. On many of your genes you will share common ancestry with
all
other Europeans as recently as a few thousand years ago, or even more recently than that. If you are reading this book on a train or airplane, the stranger next to you might be far more closely related than you think, at least on some of your genes.

One of the best-known examples of this is the ability to digest milk as an adult. The 5,000 or so animal species that make up the mammals are the only animals that produce milk. All mammal infants can digest it because they have a gene that makes an enzyme called lactase, and this enzyme breaks down the lactose sugars found in milk. After weaning, mammals no longer have access to milk, and so the gene that makes the enzyme gets switched off. This would have been true of humans throughout our evolutionary past, but an ability to digest lactose milk sugars as adults is now common in people of European and African descent. What they share is a cultural history of having ancestors who, sometime around 10,000 years ago, began domesticating animals. In what would prove to be a double act of domestication, these animals went on to domesticate their owners. Cows, sheep, and even camels could provide a ready supply of meat, but also of milk. The meat was edible, but the first groups of people to domesticate these animals would have found the milk largely indigestible, at least to the adults.

But then someone in Europe and someone else in Africa each got lucky. It was in fact a 1 in 3 billion chance for each of them. Our genome is made up of about 3.3 billion of the chemicals called bases or nucleotides. It turns out that a mutation or change to a single one of them confers the ability to digest milk as an adult. The solution was simple: ensure that the genetic switch that turned off the infant’s ability to digest milk sugar got disabled, meaning that the ability to digest milk persisted throughout life. Natural selection found just the right switch in both of these people, and they are in slightly different places on the same gene. The two variants of this gene are among some of the most rapidly evolving that have ever been studied. They might have arisen only around 6,000 years ago, but so great is the advantage of being able to digest milk as an adult that all of us who can do so are recent descendants of these two lucky people who were around at that time.

Chances are that if you see someone near you drinking a latte and you are both either of European or African ancestry, the two of you will share a very recent common ancestor—someone who probably lived in the last few thousand years and was lucky enough to have had this gene. There could not be a clearer demonstration of the power of human culture to shape and select our genes. Nothing in our evolutionary history or in the entire history of the mammals would have seen this coming. Indeed, new evidence confirms that Neanderthals were not tolerant to lactose as adults, so the ability arose only in our lineage. But like domestication in general, this one has also made us more juvenile or even infantile, as we now somewhat lazily rely on animals for energy that in the past we would have had to use our brains to hunt and forage for.

STILL EVOLVING

IT IS
sometimes said that the question of whether we are still evolving can be rephrased as, Are we all having the same number of offspring? Looking around the world, some groups are having more children than others. But that alone is not sufficient to say human populations are evolving. Natural selection maximizes the number of
grandchildren
you leave, not the number of children. I might produce ten children and you might produce three. But if I cannot provide for my ten, they might not produce as many of their own children as your three. Time will tell who among us is leaving the most grandchildren, but for now the question is not easy to answer.

Still, there is reason to believe that humans are still evolving, and the reasons come back to our brains: the real message of our evolution is written into our responses to the cultural changes our brains have unleashed.
HAR1
will almost certainly prove to be just one of many genes affecting the structure and organization of our brains. Two that have already been identified are
ASPM
and microcephalin. A variant form of microcephalin that arose 37,000 years ago is currently sweeping through human populations. The timing of its probable origin corresponds to the full flowering of culture in fully modern humans. A variant of the
ASPM
gene, also sweeping through human populations, arose just 5,700 years ago, coinciding with the spread of agriculture and animal domestication, the development of cities, and early writing. Its remarkably young age implies that the human brain is still evolving and evolving rapidly. If you are of European, Middle Eastern, or Far Eastern descent, including Iberians, Basques, Russians, North Africans, and South Asians, chances are you have this variant in your brain. But if so you should not conclude that you have higher intelligence—the variant form might simply confer some sort of metabolic or energetic difference. No one yet knows.