The Rise and Fall of the Third Chimpanzee Page 14
One of the prime supports of the climate theory is the pale skin of Scandinavians, living in the cold, dark, foggy North. Unfortunately, Scandinavians have been in Scandinavia for an even shorter time than American Indians have been in the Amazon. Until about 9,000 years ago, Scandinavia was covered by an ice-sheet and could hardly have supported any people, pale-skinned or dark-skinned. Modern Scandinavians reached Scandinavia only around 4,000 or 5,000 years ago, as a result of the expansion of farmers from the Near East (Chapter Ten) and of Indo-European speakers from southern Russia (Chapter Fifteen). Either Scandinavians acquired their pale skins long ago in some other area with a different climate, or else they acquired them in Scandinavia within half the time that Indians have spent in the Amazon without becoming dark-skinned.
The sole people in the world about whom we can be certain that they spent the last 10,000 years in the same location were the natives of Tasmania. Lying south of Australia, at the temperate latitude of Chicago or Vladivostok, Tasmania used to be connected to Australia until it was cut off by rising sea levels 10,000 years ago and became an island. Since modern Tasmanian natives did not have boats capable of going more than a few miles, we know that they were derived from colonists who walked out to Tasmania at the time of its connection to Australia, and who remained there continuously until they were exterminated by British colonists in the Nineteenth Century (Chapter Sixteen). If any people had enough time for natural selection to match their skin colour to their local temperate-zone climate, it was the Tasmanians. Yet they had blackish skins, supposedly adapted to the Equator.
If the case for natural selection of skin colour seems weak, that for hair colour and eye colour is virtually non-existent. There are no consistent correlations with climate, and not even any half-plausible theories for the supposed advantage lent by each colour type. Blonde hair is common in cold, wet, dimly lit Scandinavia and also among Aborigines of the hot, dry, sunny desert of central Australia. What do those two areas have in common, and how does being blonde help both Swedes and Aborigines to survive? Do freckles and red hair help Irishmen catch leprechauns? Blue eyes are common in Scandinavia and supposedly help their owners see farther in dim, misty light, but that speculation is unproven, and all my friends in the even dimmer, mistier mountains of New Guinea see just fine with their dark eyes.
The racial traits for which it seems most absurd to seek an explanation based on natural selection are our variable genitalia and secondary sex characteristics. Are hemispherical breasts an adaptation to summer rainfall and conical breasts an adaptation to winter fog, or vice versa? Do the protruding labia minora of Bushmen women protect them against pursuing lions, or reduce their water losses in the Kalahari Desert? You surely don’t think that men with hairy chests can thereby keep warm while going shirtless in the Arctic, do you? If you do think so, then please explain why women do not share hairy chests with men, since women also have to keep warm.
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Facts such as these were what made Darwin despair of imputing human racial variation to his own concept of natural selection. He finally dismissed the attempt with a succinct statement: ‘Not one of the external differences between the races of man are of any direct or special service to him.’ When Darwin came up with a theory that he preferred, he termed it ‘sexual selection’ to contrast with natural selection, and he devoted an entire book to explaining it.
The basic notion behind this theory is easily grasped. Darwin noted many animal features that had no obvious survival value but that did play an obvious role in securing mates, either by attracting an individual of the opposite sex or by intimidating a rival of the same sex. Familiar examples are the tails of male peacocks, the manes of male lions, and the bright red buttocks of female baboons in oestrus. If an individual male is especially successful at attracting females or intimidating rival males, that male will leave more descendants and will tend to pass on his genes and traits – as a result of sexual selection, not natural selection. The same argument applies to female traits as well.
For sexual selection to work, evolution must produce two changes simultaneously: one sex must evolve some trait, and the other sex must evolve in tandem a liking for that trait. Female baboons could hardly afford to flash red buttocks if the sight revolted male baboons to the point of their becoming impotent. As long as the female has it and the male likes it, sexual selection could lead to any arbitrary trait, just as long as it does not impair survival too much. In fact, many traits produced by sexual selection do seem quite arbitrary. A visitor from outer space who had yet to see humans could have no way of predicting that men rather than women would have beards, that the beards would be on the face rather than above the navel, and that women would not have red and blue buttocks.
That sexual selection really can work, at least in birds, was proved by an elegant experiment carried out by the Swedish biologist Malte Andersson on the long-tailed widowbird of Africa. In this species the male’s tail in the breeding season grows to 20 inches long, while the female’s tail is only 3 inches. Some males are polygamous and acquire up to six mates, at the expense of other males who get none. Biologists had guessed that a long tail served as an arbitrary signal by which males attracted females to join their harem. Andersson’s test was to cut off part of the tail from nine males until their tails were only 6 inches long. He then glued those cut segments to the tails of nine other males to give them 30-inch tails, and he waited to see where the females built their nests. It turned out that the males with the artificially lengthened tails attracted on the average over four times as many mates as the males with artificially shortened tails.
Perhaps our first reaction to Andersson’s experiment is: those dumb birds! Imagine a female selecting a particular male to father her offspring merely because his tail is longer than other males’ tails! But before we get too smug, let’s consider again what we learned in the last chapter about how we humans select our own mates. Are our criteria such good indicators of genetic worth? Do not some men and women set disproportionate value on the size or form of certain body parts, which are really nothing more than arbitrary signals for sexual selection? Why did we evolve to pay any attention at all to a beautiful face, which is useless to its owner in the struggle for survival?
In animals some of the traits that vary racially are ones produced by sexual selection. For instance, lions’ manes vary in length and in colour. Males of the astrapia birds of paradise in New Guinea have fancy tails to display to females, but different populations evolved tails of different shapes and colours. From west to east, the tails are broad and purple, short and white-based, very long and white, long and purple, and broad and purple again. Similarly, snow geese occur in two colour phases, a blue phase commoner in the western Arctic and a white phase commoner in the eastern Arctic. Birds of each phase prefer a mate of the same phase. Could human breast shape and skin colour similarly be the outcome of sexual preferences that vary arbitrarily from area to area?
After 898 pages Darwin convinced himself that the answer to this question was a resounding ‘yes’. He noted that we pay inordinate attention to breasts, hair, eyes, and skin colour in selecting our mates and sex partners. He noted also that people in different parts of the world define beautiful breasts, hair, eyes, and skin by what is familiar to them. Thus, Fijians, Hottentots, and Swedes each grow up with their own learned, arbitrary beauty standards, which tend to maintain each population in conformity with those standards, since individuals deviating too far from the standards would find it harder to obtain a mate.
Darwin died before his theory could be tested against rigorous studies of how people actually do select their mates. Such studies have proliferated in recent decades, and I summarized the results in Chapter Five. There I showed that people tend to marry individuals who resemble themselves in every conceivable character, including hair and eye and skin colour. To explain that seeming narcissism of ours, I reasoned that we develop our beauty standards by imprinting on the people we see aroun
d us in childhood – especially on our parents and siblings, the people of which we see the most. But our parents and siblings are also the people to whom we bear the strongest physical resemblance, since we share their genes. Thus, if you are a fair-skinned, blue-eyed blonde who grew up in a family of fair-skinned, blue-eyed blondes, that is the sort of person whom you will consider most beautiful and will seek as a mate. In the meantime, my dark-skinned, dark-haired New Guinea friends were growing up with other New Guineans and learning to regard fair-skinned, blue-eyed blondes as grotesquely revolting.
To test that imprinting theory of human mate choice rigorously, one would have to do experiments like shipping some Swedish babies to adoptive parents in New Guinea, or painting some Swedish parents permanently black. Then, after waiting twenty years for the babies to grow up, one could study whether they preferred Swedes or New Guineans as sex partners. Alas, once again, the Search for Truth about humans founders on practical problems, but such tests can be performed with full experimental rigour on animals.
Take snow geese, for example, with their blue or white colour phases. Do white geese learn or inherit their preference in the wild for white geese over blue ones? Canadian biologists hatched gosling eggs in an incubator, then put the goslings into a nest of goose ‘foster-parents’. When those goslings grew up, they chose a mate with the colour of the foster-parents. When goslings were reared in a large mixed flock of both blue and white birds, they showed no preference between blue and white prospective mates on reaching adulthood. Finally, when the biologists dyed some white parents pink, their offspring came to prefer pink-dyed geese. Thus, geese do not inherit but learn a colour preference, by imprinting on their parents (and on their siblings and playmates).
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How, then, do I think that people in different parts of the world evolved their differences? Our insides remained invisible to us and were moulded only by natural selection, with results such as that of tropical Africans but not Swedes evolving the anti-malarial defence of a sickle-cell haemoglobin gene. Many visible features of our outsides also got moulded by natural selection. But, just as in animals, sexual selection had a big effect in moulding the external traits by which we pick our mates.
For us humans those traits are especially the skin, eyes, hair, breasts, and genitals. In each part of the world those traits evolved in tandem with our imprinted aesthetic preferences to reach different, somewhat arbitrary results. Which particular human population ended up with any given eye or hair colour may have been partly an accident of what biologists term the ‘founder effect’. That is to say, if a few individuals colonize an empty land and their descendants then multiply to fill the land, the genes of those few founding individuals may still dominate the resulting population many generations later. Just as some birds of paradise ended up with yellow plumes and others with black plumes, so some human populations ended up with yellow hair and others with black hair, some with blue eyes and others with green eyes, some with orange nipples and others with brown nipples.
I do not mean thereby to claim that climate has nothing whatsoever to do with skin colour. I acknowledge that tropical peoples tend on the average to have darker skins than temperate-zone peoples, though there are many exceptions, and that this is probably due to natural selection, though we are unsure of the exact mechanism. Instead, I am saying that sexual selection has been strong enough to render the correlation between skin colour and sun exposure quite imperfect.
If you are still sceptical about how traits and aesthetic preferences can evolve together to different and arbitrary end points, just think about our changing fashion preferences. When I was a schoolboy in the early 1950s, women rated men with crew-cuts and clean-shaven faces as handsome. Since then, we have seen a parade of men’s fashions, including beards, long hair, earrings, purple-dyed hair, and the Mohawk hair style. A man daring to flaunt any of those fashions in the 1950s would have revolted the girls and enjoyed zero mating success. That is not because crew-cuts were better adapted to atmospheric conditions of Stalin’s last years, while a purple Mohawk has higher survival value in our post-Chernobyl era. Instead, men’s appearances and women’s tastes changed in tandem, and the changes occurred far more rapidly than evolutionary changes in skin colour, since no gene mutations were required. Either women came to like crew-cuts because good men had them, or men adopted crew-cuts because good women liked them, or something of both happened. The same goes for women’s appearances and men’s tastes.
To a zoologist, the visible geographic variability that sexual selection produced in humans is impressive. I have argued in this chapter that much of our variability is a by-product of a distinctive feature of the human life-cycle, our choosiness with respect to our spouses and sex partners. I do not know of any other wild animal species in which eye colour of different populations can be green, blue, grey, brown, or black, while skin colour varies geographically from pale to black and hair is either red, yellow, brown, black, grey, or white. There may be no limits, except those imposed by evolutionary time, on the colours with which sexual selection can adorn us. If humanity survives another 20,000 years, I predict that there will be women with naturally green hair and red eyes – plus men who think such women are the sexiest.
SEVEN
WHY DO WE GROW OLD AND DIE?
We constantly invest resources in the repair of our bodies, just as we do with our cars. Unfortunately for us and for all other animals, there is a limit to the resources that natural selection found it worthwhile to programme into our self-repair. As a result, we eventually grow old and die, but at least we age more slowly than our ape relatives.
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‘MOTHER, WHY DID Grandpa have to die? Will you die some day? Will I die too? Why?’
Death and aging constitute a mystery that we often ask about as children, deny in youth, and reluctantly come to accept as adults. I scarcely reflected on aging when I was a college student. Now that I am fifty-three years old, I find it decidedly more interesting. Life expectancy among US white adults is presently about seventy-eight years for men, eighty-three for women. But few of us will survive to 100. Why is it so easy to live to eighty, so hard to live to 100, and almost impossible to live to 120? Why do humans with access to the best medical care, and animals kept in a cage with plenty of food and no predators, inevitably grow infirm and die? It is the most obvious fact of life, but there is nothing obvious about what causes it.
In the bare fact of our aging and dying, we resemble all other animals. In the details, however, we have improved considerably over the course of our evolutionary history. Not a single individual of any ape species has been recorded as achieving the current life expectancy of US whites, and only exceptional apes reach their fifties. Hence we age more slowly than do our closest relatives. Some of that slowdown may have developed recently, around the time of the Great Leap Forward, since quite a few Cro-Magnons lived into their sixties while few Neanderthals passed forty.
Slow aging is crucial to the human lifestyle because the latter depends on transmitted information. As language evolved, far more information became available to us to pass on than previously. Until the invention of writing, old people acted as the repositories of that transmitted information and experience, just as they continue to do in tribal societies today. Under hunter-gatherer conditions, the knowledge possessed by even one person over the age of seventy could spell the difference between survival and starvation or defeat for a whole clan. Thus, our long lifespan was important for our rise from animal to human status.
Obviously, our ability to survive to a ripe old age depended ultimately upon advances in culture and technology. It is easier to defend yourself against a lion if you are carrying a spear than just a hand-held stone, and easier yet with a high-powered rifle. However, advances in culture and technology alone would not have been enough, unless our bodies had also become redesigned to last longer. No caged ape in a zoo, enjoying all the benefits of modern human technology and veterinary care, reaches
eighty. We shall see in this chapter that our biology became remoulded to the increased life expectancy that our cultural advances made possible. In particular, I would guess that Cro-Magnon tools were not the sole reason why Cro-Magnons lived on the average longer than Neanderthals. Instead, around the time of the Great Leap Forward our biology must have changed so that we aged more slowly. That may even have been the time when menopause, the concomitant of aging that paradoxically functions to let women live longer, evolved.
In short, cultural and biological change had to develop hand-in-hand to permit our long lives. Along with the changes in our sexual anatomy, physiology, behaviour, and preferences discussed in Chapters Three to Six, retarded aging is the last of the life-cycle changes that made possible the third chimpanzee’s rise.
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The way in which scientists think about aging depends on whether they are interested in so-called proximate explanations or ultimate explanations. To appreciate this difference, consider the question, ‘Why do skunks smell bad?’ A chemist or molecular biologist would answer,
‘It’s because skunks secrete chemical compounds with certain particular molecular structures. Due to the principles of quantum mechanics, those structures result in bad smells. Those particular chemicals would smell bad no matter what the biological function of their bad smell was.’
But an evolutionary biologist would instead reason,
‘It’s because skunks would be easy victims for predators if they didn’t defend themselves with bad smells. Natural selection made skunks evolve to secrete bad-smelling chemicals; those skunks with the worst smells survived to produce the most baby skunks. The molecular structure of those chemicals is a mere incidental detail; any other bad-smelling chemicals would suit skunks equally well.’