Being yet another excerpt from the ongoing project that is The Beowulf Effect: Fossils, Evolution and the Human Condition. This part concerns the evolution of bipedality. Why, precisely, did humans get up on their hind legs and walk? I think it’s all about sex, as things so often are.
It happened a long time ago but the experience was so traumatic that I remember it as if it were yesterday – the moment when the outraged, elderly professor pinned me against a wall and harangued me for having rejected his paper on why human beings got up on their hind legs and walked. Human beings became bipeds, yelled the Prof, to free the hands so that mothers could cuddle infants close to their chests. How could I have had the temerity, screamed the empurpled sage, to have rejected a paper that made so much sense?
One of the problems with human evolution, as opposed to – say – rocket science – is that everybody feels that their opinion has value irrespective of their prior knowledge (the outraged academic in the encounter above was a scientist, but not a biologist, still less an evolutionary biologist.) It’s obvious to see why – we are all human beings, and we are all bipeds, so we think we know all about it, intuitively. What we think about bipedality ‘stands to reason’. Now, I’d be the last to disparage anyone who wanted to express an opinion, however cockeyed, but it is sometimes the case that the most perplexing problems are those that seem the simplest at first sight.
It is always a wonder to me that there is still much to be discovered about something so screamingly obvious as the way we humans walk. However, much about human walking is squarely in the realms of the known unknown. Why, for example, do we walk the way we do? Why, when moving faster than a certain speed, do we start to run? Why do we walk at all, when other animals get by perfectly well on all fours? These and other such questions are still being debated by scientists. I remember publishing a research paper showing that there was a perfectly feasible gait, somewhere between walking and running, that people never used. I enjoyed demonstrating the gait to my colleagues, as if it were something out of the famous Monty Python sketch about the Ministry of Silly Walks. We might not know the arcane secrets of the universe, but we are all perfectly familiar with walking and running, so how could there be a third, distinct gait, available all the time for our use, and we somehow missed it?
As anyone who has watched a cruising toddler will attest, simply the act of standing up on two feet requires a great degree of control, and scientists still have a great deal to learn about how this is achieved – and this is with modern human subjects who can be watched and their activities measured. And even after all this, robots that can walk with anything like the natural grace of a human have yet to be built. How much harder it is to learn much about how bipedality evolved, still less why. The very fact of bipedality remains a taxing problem for those versed in fields as diverse as evolutionary biology, mechanical engineering and robotics. It’s not the easy problem that people so often imagine.
The common-or-garden explanations put forward by armchair theorists tend to avoid the problems that engage serious scientists – problems of energetics, and posture, and balance, and anatomy, and neuromuscular control – in other words anything that might require some actual scientific training and a facility with at least the basics of mechanics – and cut to the chase of why the ancestors of humans became bipedal. These explanations are invariably teleological. That is, they are driven by some inherent purpose or striving, in the manner of Lamarck – or, indeed, of the popular model of evolution as ‘progressive’.
For example, humans got up so that they could free their hands in order to make tools; or in order to cuddle babies close to their chest; or in order to see longer distances; or in order to live better in open country rather than in forests, as our ape cousins still do. All such arguments are easily demolished – many animals make tools, irrespective of whether they have hands; non-human animals of all sorts have no problem cosseting their young close to their chests; many animals are tall, or can make themselves so, without being bipeds; many large primates such as baboons live in open country and do so on all fours without extravagant distress. So why should bipedality be in any way remarkable, a qualitative advance over what other animals can achieve? Why not stay on four legs and evolve longer legs? Or longer necks? Why not evolve jumping or hopping?
Another idea is that bipedality evolved in order to make it easier for people to keep cool in hot climates. A biped presents a much smaller cross-section to the Sun – just the top of the head rather than the whole body. The rest of the body, not pointing sunwards, is thus free to radiate away any excess heat. This idea makes sense, in part, because it seeks to explain a suite of other features of humans that don’t immediately seem connected with bipedality. Features such as our relative hairlessness, and the presence of large numbers of sweat glands in our skin. Taken together, you can see how a creature with exposed skin and plenty of sweat glands could have stood up in a breeze to cool off – an advantage in the hot, dry climates of Africa in which the human lineage is thought to have evolved.
This all seems fine, except that there are lots of other animals that live in hot climates that are both quadrupedal and very furry. The problem is that you can come up with any number of other ideas that ‘explains’ any suite of features you choose; all of which have much to recommend them, and none of which can be shown to have any more scientific validity than any other. Just come up with a scenario, and then cherry-pick the features of modern humans you need to make the theory work, and ignore any others.
An example of this kind of approach is the ‘aquatic ape’ theory, promoted for many years by Elaine Morgan. Morgan selects a range of features of modern human physiology and behaviour to suggest that there was once a period in human evolution during which humans were aquatic – that is, lived in and around water, and became adapted to an aquatic environment in a way that our close ape cousins did not. This idea is perhaps the most developed of all the various ideas I have described as teleological, and the subject of several books that have gained a degree of respectable support.
The anatomy of humans is peculiar in many ways relative to that of apes. 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. 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 out of town. 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 fish can ride bicycles. However, it so happens that goats are surprisingly good at climbing trees. Professor Neil H. Shubin of the University of Chicago told me a story about a drive in Morocco, during which he and the other passengers noticed a tree in the distance that had been colonised by what looked like large birds. Vultures, maybe. As they got closer, it became clearer that the birds were in fact a herd of goats that filled the entire tree from the trunk to he outermost twigs.
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. Morgan uses this difference to her advantage in a book called The Descent of Woman, exposing her views as in part politically motivated as well as allowing them to fall into the trap of the myth of progression. Women are fattier and less hairy than men because they are more ‘evolved’. However, sex differences in fat content and hairlessness are intriguing and beg 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. This is the tendency for the different sexes to exhibit their own traits which 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. 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 looker drabber and more droopy than those in the peak of health. 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. 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 bower-birds? 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.
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, nor that it’s original.
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 oestrus – ‘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.
Oestrus 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 oestrus is concealed.
It remains a mystery why oestrus is concealed in humans – 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 flaws. 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.
In these respects human sexual habits have more in common with nesting birds than those of apes. Much work on nesting birds shows complex multigenerational family structures reminiscent of human ones, including the tendency to overt monogamy and covert extra-pair copulation – which by definition happens in private. Like birds, 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 appearances and behaviour not just to attract males, but to compete with other females – another notable human trait which I shall discuss again later. All this aside, it seems possible that bipedality is related to hairlessness, sexual display – particularly the display of breasts – and the still unsolved problem of the concealment of oestrus 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. As any middle-aged male reader will know, males are in generally rather lean, and if fat starts to accumulate, it is round 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 – some 25% of a woman’s body weight is fat, compared with 15% in a man. Body fat in women is spread all over, just under the skin (‘subcutaneous’), and one effect of this is that the skins of females are on average paler in tone than the skins of males of similar ethnic background. 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 emboinpoint is proverbial, from the Venus figurines of the Palaeolithic to the luscious nudes of Titian, Rubens or Renoir. It might be suggested that standards of beauty are in part conditioned by culture. For example, 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 big breasts and behinds. Those men who had moved to urban centres, and who had been exposed to western advertising, festooned as it is in slender models, tended to find slimmer women more attractive.
It’s easy 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 afterwards. 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, 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 womens’ 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. 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 oestrus 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 (note that I didn’t write ‘nobody’ – we humans can get turned on by the darndest things, and it remains a known unknown, at least to me, why so many women of my acquaintance are so enraptured by, of all things, shoes) 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 débutante. I suspect that the evolution and development of clothing is connected with sexual selection, the strange fact of the hidden human oestrus, 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 probably the single biggest cause of worker absenteeism in the world. 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. To suppose that bipedality evolved ‘for’ some utilitarian 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. Only sexual selection has the power to generate something so maladaptive, so seemingly 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 more true 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 effectively.