Wired for Culture: Origins of the Human Social Mind

by Mark Pagel

  But humans are naked. We have jettisoned this valuable fabric. An even greater irony is that having done so, we now hunt mammals and kill them so we can wear their fur. Even in our modern world, fur spun into the form of wool is still the preferred fabric as a base layer for skiing and other cold weather pursuits, and wolverine ruff is still the preferred insulator on the hats of polar explorers. So, why would humans have got rid of their fur, especially as being naked makes us more susceptible to cuts and bruises and to exposure to the sun? Surreptitiously look around you now if you are reading this book somewhere in public. Those bare patches of smooth skin you can see, and that appear self-evidently normal and even attractive in a human, seem ludicrous in any other mammal. Imagine your dog sheared of all its fur or a polar bear naked as a human. The ridiculous-looking image you have in your mind is what you look like to other animals.

  One suggestion is that we lost our fur when we moved out onto the savannah, early in our evolution, so that we could cool our bodies more effectively. Our Homo erectus ancestors were upright hunters and foragers, who would have spent much of the day baking in the hot sun, and so losing their fur and becoming naked might have been beneficial. For instance, no one knows how far back in time the persistence running style of modern-day San Bushmen existed, but if Homo erectus used it, then shedding their fur might have been an important way to protect themselves from overheating, although as it is males that do this hunting, this explanation fails to tell us why both males and females became naked. Another difficulty with this idea is that becoming naked should also then have benefited those other animals that lived in the noonday sun, and especially those that we chased, but they haven’t lost their fur. There are also some calculations that animals without fur would lose more heat at night—when they need to retain it—than they could give off during the day, returning a net loss for being naked. Finally, there is the problem that when we left the savannah and travelled around the world, our hair did not grow back as we inhabited colder climes. Natural selection would have had plenty of opportunities to pick people out who were slightly more hairy than others—they are easily spotted at the nearest swimming beach or public pool.

  A different idea to explain our nakedness gets our brains involved, but the link won’t be immediately apparent. It is a little-known fact that the single largest cause of death in animals, and probably all organisms, is parasites—viruses and bacteria, but also things like biting flies, or lice, ticks, worms, and other infectious organisms that transmit disease, suck our blood, and live in our guts where they eat our food. Because these parasites reproduce so quickly they can always stay one step ahead of our immune systems. Malaria, transmitted by a mosquito bite, still kills far more people around the world each year than wars. Recent estimates put the figure at around 1 million per year in Africa alone. The human immunodeficiency virus or HIV kills even more people than malaria, currently around 3 million per year. Few of us realize just how much energy we give over to our immune systems to fight these and other parasites, but farmers know that cows fed antibiotics get bigger. What? They get bigger because they do not have to use up precious energy fighting off infectious diseases, and freed of that burden they turn the excess energy into growth.

  Fur is a convenient and safe home for the parasites known as ectoparasites, creatures such as flies and ticks that plague us either by living on the outside of our bodies or by biting us and transmitting diseases such as Lyme or malaria. A measure of the burden of these flies is that many animals spend large portions of their days—over 25 percent in some cases—trying to remove ectoparasites. Monkeys huddle in groups to groom each other, not as a way to look better but to remove parasites. So bountiful is the harvest that one possible motivation for grooming someone is to be able to eat the parasites you get. But the more probable reason is to “curry favor”—the word “curry” here appropriately being an old expression for brushing a horse—by removing someone else’s ticks. Grooming is not always an option, and horses, cows, bears, and just about all large mammals rub themselves against trees at least partly to remove parasites. Some animals—horses among them—even have specialized muscles for twitching their skin to make flies jump off, or they can switch their tails to swat at them.

  Now, the connection between our brains and our nakedness is that our lack of hair might be a form of domestication by intelligence. Walter Bodmer and I have suggested that humans might have lost their fur as a way to reduce the burden of these annoying and disease-carrying ectoparasites. Our great intelligence—or our abilities at social learning—would have made us uniquely suited to replace the functions of hair or fur with our technologies. We can build fires, create shelters, and make clothes as ways of avoiding the loss of heat when we needed to retain it, or as a way of blocking the sun when its baking rays become too hot. Unlike fur, clothes can be changed and washed or even discarded if they get infested. Having these technological replacements for fur available at our fingertips might have set us on a trajectory of becoming less hairy as natural selection favored people who carried around fewer infections.

  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 sele
ction 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.