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What do these experiments on quail, mice, and rats show? The message is clear. Animals of those species learn to recognize their parents and siblings as they grow up, then are programmed to seek out an individual fairly similar to the parent or sibling of the opposite sex – but not Mother or Sister herself. They may inherit some search image of what constitutes a rat, but they evidently learn their search image of who in particular is a beautiful, eligible rat.
We can immediately appreciate what experiments are needed to get unequivocal proof of this theory for humans. We should take an average happy family, spray Father every day with Parma Violet, spray Mother’s nipples daily with lemon oil while she is nursing, and then wait twenty years to see whom the sons and daughters marry. Alas, we would be frustrated by the many obstacles to establishing Scientific Truth for humans. But some observations and accidental experiments still let us tip-toe towards the truth.
Take the incest taboo. Scientists debate whether the taboo itself in humans is instinctive or learned. However, this chapter is concerned with a separate question: given that we somehow acquire an incest taboo, do we learn to whom to apply it, or do we inherit that information in our genes? Normally we grow up with our closest relatives (parents and siblings), so our subsequent avoidance of them as sex partners could equally well be genetic or learned, but adoptive brothers and sisters also tend to avoid incest, suggesting learned avoidance.
This conclusion is strengthened by an interesting set of observations made in Israeli kibbutzim – the collective settlements whose members house, school, and care for all their children together as a large group. Thus, kibbutz children live from birth until young adulthood in intimate association with each other, like a gigantic family of brothers and sisters. If propinquity were the main factor influencing whom we marry, most kibbutz children should marry within the kibbutz. In fact, a study of 2,769 marriages contracted by kibbutz-reared children turned up only thirteen between children from the same kibbutz. All the other children married outside the kibbutz on reaching maturity.
Even those thirteen cases turned out to be the exceptions that proved the rule: all involved couples of whom one had moved into that kibbutz only after the age of six! Among children reared in the same peer group since birth, there were not only no marriages, but also no adolescent or adult heterosexual activity at all. This is astonishing restraint on the part of nearly 3,000 young men and women who enjoyed daily opportunities for sexual involvement with each other, and who had far fewer opportunities for involvement with outsiders. It illustrates dramatically that the period between birth and the age of six is a critical time for formation of our sexual preferences. We learn, however unconsciously, that our intimate associates from that period are ineligible as sex partners when we become mature.
We also appear to learn the part of our search image that tells us whom to seek, not just the part that tells us whom to avoid. For instance, a friend of mine who is 100% Chinese herself happened to grow up in a community in which every other family was white. Eventually she moved as an adult to an area with many Chinese men, and for some time she dated both Chinese and white men, but came to realize that it was the whites who attracted her. She has been married twice, both times to white men. Her own experiences led her to ask her Chinese women friends about their backgrounds. It turned out that most of her friends reared in white enclaves also ended up marrying white men, while those reared in Chinese neighbourhoods married Chinese men – although all had plenty of men of both types from whom to choose during their young adult years. Hence those who surround us as we grow up, though ineligible themselves as eventual mates, nevertheless shape our standards of beauty and search image.
Think to yourself: what sort of men or women do you find physically attractive, and where did you develop that taste? I would guess that most people, like myself, can trace their preference to the appearance of parents or siblings or childhood friends. So do not be discouraged by all those old generalizations about sex appeal – ‘Gentlemen prefer blondes,’ ‘Men seldom make passes at girls who wear glasses,’ etc. Each such ‘rule’ applies only to some of us, and there are plenty of men out there whose mothers were myopic brunettes. Fortunately for my wife and me – both of us brunettes requiring glasses, born of brunette glass-wearing parents – beauty is in the eye of the beholder.
SIX
SEXUAL SELECTION, AND THE ORIGIN OF HUMAN RACES
People from different parts of the world can be distinguished at a glance by so-called racial characteristics. But those same traits – ones such as the colour of our skin and hair and eyes, or the shapes of our breasts and genitals – play a big role in how we select our mates and sex partners. Thus, our outward appearances and our beauty standards have evolved in tandem to different local end points.
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‘WHITE MAN! LOOKIM this-feller line three-feller man. This-feller number-one he belong Buka Island, na ‘nother-feller number-two he belong Makira Island, na this-feller number-three he belong Sikaiana Island. Yu no savvy? Yu no enough lookim straight? I think, eye-belong-yu he bugger-up finish?’
No, damn it, my eyes-belong-me were not ruined beyond repair. It was my first visit to the Solomon Islands in the Southwest Pacific, and I told my scornful guide through the medium of pidgin English that I saw perfectly well the differences between those three men in a row over there. The first one had jet-black skin and frizzy hair, the second had much lighter skin and frizzy hair, and the third had straighter hair and more slanty eyes. The only thing the matter with me was that I had no experience of what people from each particular Solomon island looked like. By the end of my first trip through the Solomons, I too could match people to their islands by their skin and hair and eyes.
In those variable features, the Solomons are a microcosm of humanity. Simply by looking at a person, even laymen can often tell what part of the world that person comes from, and trained anthropologists may be able to ‘place’ him or her in the right part of the right country. For example, given one person each from Sweden, Nigeria, and Japan, none of us would have any trouble deciding at a glance which person was from which country. The most visibly variable features in clothed people are of course skin colour, the colour and form of the eyes and hair, body shape, and (in men) the amount of facial hair. If the people to be identified were undressed, we might also notice differences in amount of body hair, the size and shape and colour of a woman’s breasts and nipples, the form of her labia and buttocks, and the size and angle of a man’s penis. All those variable features contribute to what we know as human racial variation.
Those geographic differences among humans have long fascinated travellers, anthropologists, bigots, and politicians, as well as the rest of us. Since scientists have solved so many arcane questions about obscure unimportant species, surely you might expect them to have answered one of the most obvious questions about ourselves: ‘Why do people from different areas look different?’ Our understanding of how humans came to differ from other animals would remain incomplete if we did not also consider how, in the process, human populations acquired their most visible differences from each other. Nevertheless, the subject of human races is so explosive that Darwin excised all discussion of it from his famous 1859 book On the Origin of Species. Even today, few scientists dare to study racial origins, lest they be branded racists simply for being interested in the problem.
There is another reason why we do not understand the significance of human racial variation: it proves to be an unexpectedly difficult problem. Twelve years after Darwin wrote his book attributing the origin of species to natural selection, he wrote another book 898 pages long, attributing the origin of human races to our sexual preferences which I described in the last chapter, and entirely rejecting a role of natural selection. Despite that verbal overkill, many readers were unconvinced. To this day, Darwin’s theory of sexual selection (as he called it) remains controversial. Instead, modern biologists generally invoke natural selection
to explain the visible differences among human races – especially the differences in skin colour, whose relation to sun exposure seems obvious. However, biologists cannot even agree on why natural selection led to dark skin in the tropics. I shall explain why I believe natural selection to have played only a secondary role in our racial origins, and why Darwin’s preference for sexual selection seems to me correct. I therefore consider visible human racial variation to be largely a by-product of the remodelled human life-cycle that forms the subject of Part Two of this book.
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Firstly, to place matters in perspective, let’s realize that racial variation is not at all confined to humans. Most animal and plant species with sufficiently wide distributions, including all higher ape species except the geographically localized pgymy chimp, also vary geographically. So marked is variation in some bird species, such as North America’s white-crowned sparrow and Eurasia’s yellow wagtail, that experienced bird-watchers can identify an individual bird’s approximate birthplace by its plumage pattern.
Variation in apes encompasses many of the same characteristics that vary geographically in humans. For example, among the three recognized races of gorillas, western lowland gorillas have the smallest bodies and rather grey or brown hair, while mountain gorillas have the longest hair, and eastern lowland gorillas share black hair with mountain gorillas. Races of white-handed gibbons similarly vary in hair colour (variously black, brown, reddish, or grey), hair length, tooth size, protrusion of the jaws, and protrusion of the bony ridges over the eyes. All these traits that I have just mentioned as varying among gorilla or gibbon populations also differ among human populations.
How does one decide whether recognizably distinct animal populations from different localities constitute different species, or belong instead to the same species and just constitute different races (also known as subspecies)? As explained in Chapter Two, the distinction is based on interbreeding under normal circumstances: members of the same species may interbreed normally if given the opportunity, while members of different species do not. (But closely related species that would not normally interbreed in the wild, like lions and tigers, may do so if a male of one is caged with a female of the other and given no other choice.) By this criterion, all living human populations belong to the same species, since some interbreeding has occurred whenever humans from different regions have come into contact – even people as dissimilar in appearance as African Bantus and Pygmies. With humans as with other species, populations may intergrade into each other, and it becomes arbitrary to decide which populations to group as races. By the same criterion of interbreeding, the large gibbons known as siamangs are a distinct species from the smaller gibbons, since both occur together in the wild without hybridizing. This is also the criterion for considering Neanderthals possibly as a species distinct from Homo sapiens, since hybrid skeletons have not been identified despite apparent Cro-Magnon/Neanderthal contact (see Chapter Two).
Racial variation has characterized humans for at least the past several thousand years, and possibly much longer. Already around 450 BC, the Greek historian Herodotus described the Pygmies of West Africa, the black-skinned Ethiopians, and a blue-eyed red-haired tribe in Russia. Ancient paintings, mummies from Egypt and Peru, and bodies of people preserved in European peat bogs confirm that people several thousand years ago differed in their hair and facial features much as they do today. Origins of modern races can be pushed back still further, to at least ten thousand years ago, since fossil skulls of that age from various parts of the world differ in many of the same respects that modern skulls from the same regions differ. More controversial are the studies of some anthropologists, contested by others, reporting continuity of racial skull characteristics for hundreds of thousands of years. If those studies are correct, then some of the human racial variation that we see today may predate the Great Leap Forward, and may have gone back to the times of Homo erectus.
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Now let’s turn to the question of whether natural selection or sexual selection has made the larger contribution to those visible geographic differences of ours. Take first the arguments about natural selection, the selection of traits that enhance survival. No scientist denies today that natural selection does account for many of the differences between species, such as why lions have paws with claws while we have grasping fingers. No one denies either that natural selection explains some geographic variation (‘racial variation’) within some animal species. For instance, Arctic weasels that live in areas covered by winter snow change colour from brown in summer to white in winter, while more southerly weasels stay brown all year. That racial difference enhances survival, because white weasels against a brown background would be glaringly conspicuous to their prey if they were not camouflaged against snow.
By the same token, natural selection surely explains some geographic variation in humans. Many black Africans but no Swedes have the sickle-cell haemoglobin gene, because the gene protects against malaria, a tropical disease that would otherwise kill many Africans. Other localized human traits that surely evolved through natural selection include the big chests of Andean Indians (good for extracting oxygen from thin air at high altitudes), the compact shapes of Eskimos (good for conserving heat), the slender shapes of southern Sudanese (good for losing heat), and the slit-like eyes of northern Asians (good for protecting eyes against cold and against sun glare off the snow). All these examples are easy to understand.
Can natural selection similarly explain the racial differences that we think of first, those in skin colour and eye colour and hair? If so, one might expect that the same trait (for instance, blue eyes) would reappear in different parts of the world with similar climates, and that scientists would agree on what the trait is good for.
Seemingly the simplest trait to understand is skin colour. Our skins run the spectrum from various shades of black, brown, copper, and yellowish to pink with or without freckles. The usual story to explain this variation by natural selection goes as follows. People from sunny Africa have blackish skins. So too (supposedly) do people from other sunny places, like southern India and New Guinea. Skins are said to get paler as one moves north or south from the equator, until one reaches northern Europe, with the palest skins of all. Obviously, dark skins evolved in those people who were exposed to much sunlight. That is just like the skins of whites tanning under the summer sun (or in tanning salons!), except that tanning is a reversible response to sun rather than a permanent genetic one. It is equally obvious what good a dark skin does in sunny areas: it protects against sunburn and skin cancer. Whites who spend lots of time outdoors in the sun tend to get skin cancer, and they get it on exposed parts of their body like their head and hands. Does that not all make sense?
Yes, but … it is really not so simple at all. To begin with, skin cancer and sunburn cause little debilitation and few deaths. As agents of natural selection, they have an utterly trivial impact compared to infectious diseases of childhood. Hence many other theories have been proposed to explain the supposed pole-to-equator gradient in skin colour.
One favourite competing theory notes that the sun’s ultraviolet rays promote vitamin D formation in a layer of our skin beneath the main pigmented layer. Thus, people in sunny tropical areas might have evolved dark skin to protect them against the risk of kidney disease caused by too much vitamin D, while people in Scandinavia with its long dark winters evolved pale skins to protect them against the risk of rickets caused by too little vitamin D. Two other popular theories are that dark skins are to protect our internal organs against overheating by the tropical sun’s infrared rays, or – just the opposite – dark skins help keep tropical people warm when the temperature drops. And if those four theories are not enough for you, consider four more: that dark skins provide camouflage in the jungle, or that pale skins are less sensitive to frostbite, or that dark skins protect against beryllium poisoning in the tropics, or that pale skins cause deficiency of another vitamin (folic acid) in the
tropics.
With at least eight theories in the running, we can hardly claim to understand why people from sunny climates have dark skins. That in itself does not refute the idea that, somehow, natural selection caused the evolution of dark skins in sunny climates. After all, dark skins could have multiple advantages, which scientists may sort out some day. Instead, the heaviest objection to any theory based on natural selection is that the association between dark skins and sunny climates is a very imperfect one. Native peoples had very dark skins in some areas receiving relatively little sunlight, like Tasmania, while skin colour is only medium in sunny areas of tropical Southeast Asia. No American Indians have black skins, not even in the sunniest parts of the New World. When one takes cloud cover into account, the world’s most dimly lit areas, receiving a daily average of under three-and-a-half hours of sunlight, include parts of equatorial West Africa, southern China, and Scandinavia, inhabited respectively by some of the world’s blackest, yellowest, and palest peoples! Among the Solomon Islands, all of which share a similar climate, jet-black people and lighter people replace each other over short distances. Evidently, sunlight has not been the sole selective factor that moulded skin colour.
The first response of anthropologists to these objections is to raise a counter-objection, the time factor. This argument tries to explain away the cases of pale-skinned people in the tropics by claiming that those particular peoples migrated to the tropics too recently to have evolved black skins. For example, the ancestors of American Indians may have reached the New World only 11,000 years ago (Chapter Eighteen): perhaps that has not been long enough to evolve black skins in the tropical Americas. But if you are going to evoke the time factor to explain away objections to the climate theory of skin colour, then you also have to consider the time factor for peoples who supposedly support that theory.
The Rise and Fall of the Third Chimpanzee Page 13