by Henry Gee
The anatomy of humans is certainly peculiar in many ways relative to that of apes such as chimpanzees and gorillas. Humans are much fattier than apes and are much less hairy. In contrast to almost all other primates, humans are capable swimmers, and newborn babies appear to have an inborn capacity for swimming. In these respects humans are less like apes than, say, seals and other aquatic mammals, which are relatively fatty and hairless, compared with their purely terrestrial cousins. Humans, in contrast with apes, have historically eaten a lot of seafood, a diet that offers minerals and fatty acids essential for development, especially for the brain and nervous system, but otherwise hard to come by unless humans once spent a great deal of time in and around water.
There is a lot more to the “aquatic ape” idea than that, of course, but from this brief description you can, I expect, already identify some flaws. The first is that it’s always a problem identifying features that humans have now and inferring that they must have had some adaptive value in the past. It’s entirely true that humans seem to have an unusual fondness for seafood—but we still do, and it remains an important part of our diet. But we humans also consume an extraordinary range of foodstuffs compared with other animals, including substances that are noxious or even bad for us, such as capsaicin (the substance that makes chili peppers hot), alcohol, tobacco, and dangerous drugs. What of body fat and hairlessness? If they were once selectively advantageous for water-loving humans, why are we still relatively fatty and less hairy than other apes? Presumably other factors have come into play that might have nothing in particular to do with an aquatic stage in our history.
Second, it’s notoriously hard to infer habits from anatomical structure. If a busload of Martian anatomists came across the skeleton of a goat, the one who said that goats would be good at climbing trees would be laughed off the planet. With their long, spindly legs and complete lack of grasping hands or feet or tail, you’d think that goats could climb trees as handily as giraffes can ride unicycles. However, it so happens that goats are surprisingly good at climbing trees.6 And you’d never guess from her fur that my golden retriever, Heidi, is a capable and strong swimmer, regularly braving pounding North Sea surf to retrieve a stick or ball. Heidi is very far from hairless—indeed, she can brave the cold and wet of the sea because she has more fur, rather than less, an extra layer of underfur that keeps the cold and wet away from her skin.
The relative hairlessness of humans is complicated, however, by sex. It’s easy to say that being relatively fatty and less hairy might be a sign of aquatic ancestry, but this doesn’t explain why human females are very much fattier and less hairy than human males. In addition, men become more hirsute as they mature (excepting male-pattern baldness, as I mentioned above). Sex differences in fat content and hairlessness are intriguing and demand explanation. Their presence might even shed light on why we humans are bipeds.
Jared Diamond suggests that relative hairlessness, combined with differences in fat distribution, might be connected with what Darwin called sexual selection.7 This is the tendency for the different sexes to exhibit their own traits that they use to attract the attentions of the opposite sex. The most famous example is the peacock with his extravagant tail, which he displays to attract the attentions of the much less extravagantly endowed peahens. Sexual selection arises because males and females contribute different amounts to the next generation. Typically, a male will contribute sperm, which are easy and cheap to make in large quantities, and will seek to inseminate as many females as possible. Females, on the other hand, contribute eggs, which are expensive to make and rare, and so will have much more at stake. This is why males are showy, and females are choosy—females make a much greater investment in the next generation, so have more to lose if this investment isn’t recouped in terms of numbers of strong, healthy offspring. It is incumbent on males, therefore, to demonstrate to females that they would be appropriate mates, usually by some proxy such as a display (showy plumage, mating calls, and so on) that illustrate their suitability.
Sexual selection is a vibrant subject of study in modern evolutionary biology.8 Evolutionary biologists are still arguing about what it is, precisely, that choosy females are selecting in prospective mates. We know that birds (say) suffering from parasitism or disease look drabber and more droopy than those in the peak of health.9 Is showy plumage therefore a reliable mark of a healthy genetic constitution? Does a sports car, an indicator of high status or a fat bank balance, mark a man as a better potential mate than had he been seen riding a rusty bicycle? This is the “good genes” idea—females choose males based on signs of good general health.
Or is the association of a showy male trait somehow linked—perhaps by chance, to begin with—with female choosiness for that particular trait, reinforcing one another down the generations? This is the “runaway process” first elaborated by the brilliant geneticist and statistician Ronald A. Fisher.10 To put it another way—need there be any particular reason why red sports cars are sexier than (say) blue ones? Peacock tails confer no obvious advantage on a peacock apart from attracting peahens, but why the elaborate tail rather than (say) a mating call, as in nightingales; or the construction of a bower, as in bowerbirds? The answer could be that the male display trait, perhaps entirely random to begin with, has become linked, genetically, with the female preference for that trait. Successful partners have male offspring that display the trait strongly, and daughters that are strongly attracted to that trait, so that the two traits have become reinforced down the generations. Initially, there is no reason why the selected trait has any selective advantage at all, and its choice might be completely fortuitous. If sports cars, why red sports cars?
There is another aspect to this, too, related to self-advertisement. Extreme traits such as the long tails of peacocks are actually a disadvantage. They are expensive to make and maintain in terms of resources, and interfere with important aspects of daily life—such as the ability of the peacock to fly away from predators. Such displays seem to say that the male is not only fit to be a mate, but so fit that he can survive perfectly well despite having to support such seemingly profligate habits, as if he has fitness to burn.11 Sports cars are expensive not just to buy but to maintain, as anyone attempting to buy an insurance policy for one will attest.12
Irrespective of its internal mechanics, nobody doubts that sexual selection exists. Here, by way of making my own contribution to the shuddering pile of teleological arguments that purport to explain why humans are bipedal, I’d like to suggest how sexual selection might have played its part. I’m not suggesting it’s correct, or even that it’s original.13
Still less would I pin anyone against a wall and shout at them about it.
If standing upright does one thing, it exposes one’s breasts or genitalia to full view—especially if one has relatively little fur, and no clothes. No other ape is as habitually bipedal or as hairless as humans, and these features might be connected with another human peculiarity, that human females do not show any physical sign of when they are in estrus14—“in heat”—that is, sexually receptive such that sex has a high probability of producing offspring. Other apes are not only hairy and quadrupedal, but their females make it perfectly obvious when they are in heat, by displaying large swellings in the genital area. The breasts of ape females are also tiny, covered with fur, and swell only when they are pregnant or lactating. When apes mate, they do so in full view of other apes.
Estrus in human females is concealed, even from the female herself. No external sign betrays when a human female is more or less likely to conceive. In addition, the breasts of human females are more or less prominent at all times, and the fact of hairlessness makes them more prominent still. Being bipedal makes breasts more obvious even as estrus is concealed.
It remains a mystery why estrus is concealed in humans15—just as it is not obvious why humans tend to have sex in private. The usual explanations concern the tendency in humans to be monogamous and form stable pair
bonds, but this argument has its own problems. Human societies are particularly variable as regards their mating systems—polygamy is widespread—and even when societies are nominally monogamous, both males and females cheat on their partners (what scientists call “extra-pair copulations”) more frequently than polite society admits.16
In these respects—cheating, and having sex in private—human sexual habits have more in common with nesting birds than with apes. Much work on nesting birds reveals multigenerational family structures that are much more complex than anything seen in apes, but highly reminiscent of human proclivities, including the tendency to overt monogamy and covert extra-pair copulation17—which by definition happens in private. Like birds (but unlike apes), humans are prone to elaborate sexual display by males, with consequent choosiness by females, and evidence has also come to light that in birds, females use their own appearance and behavior not just to attract males, but to compete with other females18—another notable human trait that I shall discuss again later. All this aside, it seems possible that bipedality is related to hairlessness, sexual display, and the still unsolved problem of the concealment of estrus in females.
How is this related to sexual selection? Let’s look more closely at the secondary sexual characteristics of humans inasmuch as they relate to body fat and hairlessness.19 As any middle-aged male reader will know, males are in general rather lean, and if fat starts to accumulate, it is around the gut and internal organs, and then generally after a male has done all his reproducing. Females, though, even when young, are much fattier than males. The percentage of body mass that is fat is 10.4 points greater in females than in males of the same body mass index.20 Body fat in women is spread all over, just under the skin, and the skins of females are on average smoother than the skins of males of similar ethnic background, a difference that might have been maintained by sexual selection.21 Females also have skin that is significantly paler than that of males of similar ethnicity. Apart from that, fat deposits in females are concentrated in the upper arms, breasts, thighs, and buttocks. This difference in fat deposition leads to very obvious differences in appearance, and it seems likely that they have some connection with sexual selection. The historical male preference for plump, rounded women with ample embonpoint is proverbial, from the “Venus” figurines of the Paleolithic to the well-upholstered nudes of Titian, Rubens, and Renoir.
However, might it not be the case that standards of beauty are in part conditioned by culture, rather than purely by sexual selection? Contemporary “Western” standards of female attractiveness tend to emphasize a leaner physique, so does the conventional picture of amply bosomed women have more to do with changing cultural norms than a more general, more ingrained tendency? A recent study of Peruvian men unexposed to Western media showed that their idea of feminine attractiveness was strongly associated with fat. They preferred women with a pronounced “hourglass” figure, big busts and behinds.22 Those men who had moved to urban centers, and who had been exposed to Western advertising, festooned as it is in slender models, tended to find slimmer women more attractive.
It’s simple to find pat answers to such preferences. Historically, fatness in women has been associated with reproductive success. Women with more fat would have the nutritional reserves necessary to nurture a fetus to term, and nurse it afterward. In the past, and in traditional societies, to be thin was to be ill—suffering from some threatening disease such as tuberculosis, or laboring under a large parasitic load. It’s easy to see why men have traditionally found fatter women attractive. Only today, when nutritional resources are more abundant and less episodic, is fatness seen as a disadvantage.
It might also be the case that men are looking for different things in women than women are looking for in one another. Competition between females over appearance has been documented in birds,23 and in this context it is noteworthy that pictures of slim, attractive women are aimed not just at men (in pornography, for example) but at women: in women’s magazines, advertisements for beauty products, fashion plates, and so on.
If it seems all too easy to find reasons why fat is attractive, it’s harder to understand hairlessness, or, at least, patterns of hair distribution. If humans are generally hairless, they retain hair on their heads, and, when adult, under their armpits and around the genitals. Why? Armpit and genital hair makes sense in terms of devices to trap secretions meant to attract members of the opposite sex, or deter rivals—except that the role of pheromones in human beings is very much an open question.24 Head hair is another problem entirely. In many cultures, luxuriant head hair is seen as attractive in women, whereas it is common for men to lose much of their head hair in adulthood. What is head hair for? There seems no good, adaptive reason for the presence of hair on the head (as opposed to anywhere else) than sexual selection, and this illustrates how secondary sexual characters need have no adaptive reason except that they are attractive to the opposite sex, very much in tune with Fisher’s runaway process. This applies as much to the distribution of fat as hair. Consider—why do men find women with pale skin, luxuriant locks, and curvaceous figures attractive? One can come up with examples based on nutritional status, but only after the fact. There is no reason, a priori, why gentlemen don’t prefer bald women with hairy ears and enormous feet.
If females standing upright expose their breasts to view, men standing upright expose their penises. The connection between bipedality and penis display seems less fraught than that between bipedality and the hidden estrus of females. Males are always sexually receptive—their penises do not lengthen and shorten with the seasons. The connection between penis display and sexual selection should be too obvious to underline. And it is a curious fact that the penis of the human male is much larger as a proportion of body mass than that of any other ape. This combination of unusually large size, open display, and relative lack of body hair seems to speak loudly of sexual selection as well as habitual bipedality. It is perhaps noteworthy that there are tribesmen in New Guinea in which the men are naked except for elaborate sheaths worn on the penis that emphasize their presence and exaggerate their size.
This topic touches on another distinctive feature of humans, which is clothing. Conventional explanations for clothing include protection against harsh environments, as well as compensation for lack of body hair (and the two might be related). Such explanations are, like conventional explanations for bipedality, prone to teleology. To be sure, few will find Inuit parkas, space suits, or protective goggles sexy,25 but I suspect that clothing in general is as much about sexual display as anything more utilitarian. The penis sheaths of New Guinea tribesmen conceal as well as emphasize sexual features, just as much as the swimwear displayed by a glamour model, the basque of a burlesque diva, or the bustle and corsetry of a Victorian debutante. I suspect that the evolution and development of clothing is connected with sexual selection, the strange fact of the hidden human estrus, and, beyond that, bipedality.
If none of that convinces, try this. Sexual selection is distinct from natural selection because, in sexual selection, features can be emphasized, which in normal circumstances would be highly maladaptive. The tail of the peacock is just such a feature. Bipedality is another. Standing upright introduces a potential for all kinds of injurious stresses to the head, spine, and limbs that simply don’t apply to quadrupeds. Back pain, related directly to bipedality, is a significant burden on the economy.26 Bipedality becomes even more problematic for women during pregnancy, and the evolution of the particular kind of spinal curvature typical of humans can be related to the need for extra lumbar support during pregnancy.27 To suppose that bipedality evolved for some reason or another is to belittle the immense changes in bodily form that the human frame underwent simply to stand upright as of habit, and the considerable disadvantages accrued in so doing. All parts of the body have been profoundly influenced by the acquisition of bipedality, even the head.28 Only sexual selection has the power to generate something so maladaptive, so seeming
ly pointless, as a peacock’s tail—or human bipedality.
Much of the foregoing is written at least in part in jest. I do not claim that bipedality evolved for the purpose of sexual display. The point I am trying to make is one that armchair theorists of bipedality fail to understand: that there can be no simple relationship between a proposed cause and a proposed effect. The consequence of one change has an impact on many other traits or adaptations, until the whole body is affected. In no trait does this seem truer than in bipedality. Bipedality means more than just standing on two legs. It requires the wholesale modification of the body, not all of it very effective.
But bipedality has evolved considerably since the first appearance of bipeds: it did not appear all at once. The awkward gait of the very primitive fossil hominin Ardipithecus ramidus (at 4.4 million years old, the earliest for which good skeletal evidence is known) shows that the first bipeds were not as refined as modern humans.29 They could stand upright, they could walk, though not as upright as modern humans, but they probably could not run very well. However, footprints attributed to the fossil human Australopithecus afarensis (Lucy) from a million years later show that by this time, creatures close to the human lineage could walk just about as well as modern humans. Even so, the skeleton of this creature was still very different from modern humans: Lucy could walk, but her skeleton suggests that she might have been a better tree climber than modern humans are.30
The act of standing upright was followed, in sequence, by walking and then running—two gaits that demand very special, and rather different, adaptations. Daniel Lieberman and Dennis Bramble have recently proposed that many features of modern humans appear to be adaptations not to walking, as such, but to long-distance running.31 These include a range of features throughout the body not directly connected with the legs and feet.
Here are just two examples. Homo erectus and modern humans have barrel-shaped rib cages, in contrast to the cone-shaped, wide-bellied rib cages of earlier hominins. This means that later hominins had “waists,” which would have allowed the counterrotation of the arms relative to the legs while running. This is an extremely important aid to balance. Such counterrotation, however, would move the head from side to side with each stride, if it weren’t for a corresponding reduction in the neck musculature to allow the head to be suspended independently. In human beings there is a ligament—the nuchal ligament—that connects the back of the skull with the back and shoulders. This allows the posture of the head to be maintained without effort. This ligament is not found in apes. It is found, however, in predators such as dogs, which track and hunt over long distances without tiring—just as traditional hunters do.
