Homo Mysterious: Evolutionary Puzzles of Human Nature (13 page)

It is true, of course, that kangaroos, kingfishers, and cobras couldn’t apply mascara or wear high-heel shoes even if they wanted to, but the likelihood is that people—and especially women—do such things not simply because they can, but because doing so is consistent with natural selection. There is also the possibility that women’s penchant for public adornment corresponds, as well, to a private fondness for polyandry, that is, mating—perhaps on the sly—with more than one man. Recall our earlier consideration of the various potential evolutionary payoffs to concealed ovulation, a private hiddenness that might combine especially well with public attractiveness.

Carry this thought a step further: What if, rather than women having been selected to conceal their ovulation, chimpanzees and the like were selected to conspicuously signal theirs? Why, you ask, should evolution have favored female chimps announcing their sexual availability? Perhaps to attract the attention of the dominant male. This would seem an especially adaptive tactic among species in which male genetic quality and/or inclination to be a good parent varies substantially among individuals. If there were a big difference of this sort among males, females should be selected to do what they can to increase their chances of “getting” the better ones.

So what about this hypothesis: Women, biologically more camouflaged than chimps, make use of culturally created, man- and woman-made ornaments to achieve the same effect that chimpanzees get with their flagrant and presumably fragrant anatomy?
This would make particular sense if women’s penchants for polyandry were recently developed, so that natural selection hasn’t had time to evolve overt physical traits such as the chimpanzee’s gaudy genitals, and/or because of the various other payoffs that presumably come with being biologically more discreet.

Just as biologically generated traits have fitness costs—it is metabolically expensive and probably also dangerously predator attracting for a peacock to grow his fancy tail—it is often financially costly for people to ornament, adorn, and augment their bodies. But such costs are evidently seen as worthwhile: Clothing, cosmetics, and other personal adornment make up a large proportion of the budget of many people who might seem to have “better things to do” with their limited resources. For a species that is deeply, biologically concerned with sexual signaling, however, there can be evolutionary logic behind such expenditures.

For another mystery, not quite sexual but intimately linked nonetheless to maleness and femaleness, think about longevity. Women consistently outlive men, suggesting that even though men are stronger when it comes to measures of brute force, women are “biologically stronger.” Why?

Maybe it’s a result of hormones. Thus, there is some evidence that testosterone inhibits the human immune system, and that mentally retarded men who had been castrated in childhood live more than a decade longer than similar, intact men.
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Such an explanation, if confirmed, is nonetheless proximate only; it leaves unanswered the deeper “why” question. In addition, hormones alone don’t seem sufficient since boys are significantly more mortality prone than are girls even during their first decade, before “raging hormones” become relevant.

Or maybe genes are involved, not so much that women have better ones, but rather more of them. Aside from the sex chromosomes, there are no genetic differences between men and women. But don’t forget that women are XX and men, XY, and that the Y chromosome is a real slacker, almost lacking in useful genetic material. As a result, women have literally more DNA upon which
they can draw, a difference that might be consequential since a woman possessing a deleterious gene on one of her X chromosomes might find its harmful effect overridden by a more fitness-generating gene on her other X. (Contributing to this prospect is the fact that health-promoting genes tend to be dominant.) By contrast, men are stuck with an “unprotected” X, because their underachieving Y chromosomes cannot compensate for anything disadvantageous on the X. Men may therefore be more vulnerable to genetically related troubles, or simply they may have a shallower genetic “bench” when called upon to deal with life’s slings and arrows.

This would suggest that as a general rule, the “heterogametic sex” should have a shorter life span than the “homogametic” one, yet it turns out that among birds, in which females are heterogametic while males have a duplicated, and accordingly protected, sex chromosome, the latter still have shorter life spans. So much for the role of protected versus unprotected sex chromosomes.

Alternatively, maybe women are protected, not by their genome, but rather by cultural traditions and social expectations, which make it far more likely that men will engage in exhausting, physically dangerous, and likely life-threatening activities, whether job related or recreational. It is men, after all, who are prone to be coal miners, farmers, animal herders, construction workers, and soldiers. This undoubtedly accounts for some of the male–female differences in longevity, but not all. It doesn’t explain, for example, why the same consistent 7-year difference in male–female life span has been found when comparing cloistered monks and nuns, both of whom are isolated from the hurly-burly of modern life and who do much the same things.
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This leads us to another hypothesis for why women outlive men, based once more on evolutionary considerations. The point is that human beings show all the signs of being polygynous, that is, biologically inclined toward harem formation: Men are typically larger and more aggressive than women, and they become sexually mature significantly later (to be successful in mating with numerous partners requires defeating a comparable number of same-sex competitors, and selection therefore rewards would-be harem masters who enter into the reproductive fray when somewhat older and larger). In any event, it is characteristic of mildly
polygynous species such as
Homo sapiens
for males to be more likely than females to engage in vigorous same-sex competition, which typically manifests via huffing and puffing and sometimes literally trying to knock each other down, as well as engaging in show-offy behavior—all of which are, quite simply, risky.

To be sure, natural selection has dictated that for males generally and men in particular, such risks are worth running, if only because to a great extent for males—and not necessarily at all for females—evolutionary fitness is a zero-sum game. A woman is likely to become pregnant regardless of what transpires for other women; by contrast, reproductive success by a man is liable to occur at the expense of another man’s evolutionary fitness, since the reproductive potential of many women can be monopolized by a relatively small number of men.

Not only are men and boys more violent, but they are also more prone to risk taking, more inclined to take greater chances, because for them, the payoff to success is greater, as is the consequence of failure. As sperm makers and potential harem keepers, males simply play for higher stakes, and this, in turn, may help account for the higher morbidity and mortality that they experience.

This hypothesis is closely allied to one of the better-established theories in evolutionary biology, concerned with the question of senescence generally. One might ask (as many evolutionary biologists have), Why do organisms get old? More precisely, Why do they “senesce,” that is, experience higher morbidity and mortality over time? The most widely accepted explanation, first proposed by the great theorist George C. Williams, concerns the genetic phenomenon known as pleiotropy.
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Genes are pleiotropic when they influence more than one character. Accordingly, imagine a gene that contributes to an individual’s fitness early in life but which also has detrimental effects that show up only later, when the individual is postreproductive. Such a gene would on balance be selected for, since it would contribute to survival and reproduction, but would be only weakly selected against at the other end of life, when selection is greatly reduced or even absent altogether. Another way of looking at this is that selection is generally more potent early in life than later, resulting in accumulated costs that reveal themselves late in life.

This pleiotropic theory of senescence applies to all living things, not just human beings, and to both sexes. Thus, by itself, it doesn’t explain the male–female disparity in life span. However, combined with the other male–female differences just described, Williams’s concept could be insightful indeed. Since males have been selected to engage in male–male competition to a degree not found in females, selection for success in such competition—especially when young—would likely have been stronger among men than among women. If so, then men would be more vulnerable to comparatively early death not only as a direct result of their more risk-prone, competitive behavior, but also because genetic tendencies would be favored that generate reproductive success early in life, albeit at the expense of eventual longevity. Men, in a sense, have evolved to “live fast, love hard, and die young,” with younger dying being a consequence of the faster living and the harder loving.

Here is a final possible evolutionary explanation for the male–female life span discrepancy. Maybe it’s not a matter of evolution favoring men whose behavior shortens the average male life span, but of selection acting primarily on women, extending
theirs
. Think back to the grandmother hypothesis for the evolution of menopause, and view it from a different perspective: Instead of women having been selected to cease reproducing at a certain age, we might say that selection has acted upon women, more than upon men, to extend their postreproductive life, because of the genetic payoffs they can accrue, especially via their grandchildren. If so, then maybe women who are middle-aged and older can thank their grandchildren—and evolution, too—for their own longevity, no less than for their nonreproductive status.

Finally, let’s conclude with yet another mystery, one that surrounds sex itself, rather than being bounded by considerations of women and men as such. This one is among the major unsolved enigmas of evolutionary biology more generally: Why does sex exist at all?

At first glance, the question seems foolish, since sex serves reproduction, and without reproduction there would be no evolutionary
success and perhaps no life itself. But in fact, it is quite possible for living things to reproduce
asexually
, and quite a mystery why so few actually do so. Sex, after all, is a hassle. It requires that a would-be reproducing individual encounter another individual of the same species and the opposite sex and, moreover, that both be similarly motivated. It mandates that sexually reproducing individuals subject themselves to potentially dangerous intimacy with someone else, rendering themselves vulnerable to injury and illness, not to mention the risk of being deceived or exploited. By contrast, as we know from the numerous creatures that do so, asexual reproduction can be accomplished with safety, a high success rate, and without wasting all that time and energy on courtship and copulation.

The largest cost of sexual reproduction hasn’t even been mentioned: The fact that whereas a strawberry plant or amoeba who reproduces asexually produces offspring containing 100% of its genes, sexual breeders have to settle for a mere 50%, since each partner gets an equal, one-half share in the outcome.
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This is a whopping twofold cost of sex, and in a biological world in which “selection differentials” as small as one tenth of 1% have been shown to drive considerable evolutionary change (see the next chapter), it is exceedingly difficult to understand why natural selection favored sexual reproduction in the first place.

Such considerations drove the great evolutionary theorist George C. Williams to conclude glumly that sexual reproduction is probably not adaptive, at least not for large, complex, slow-breeding creatures such as human beings.
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It may, on the other hand, have evolved initially among small, simple, rapidly breeding organisms among whom sexual reproduction could have been advantageous. But now subsequent generations are simply stuck with it, essentially because having proceeded down that particular, peculiar anatomic and physiologic avenue, it simply wasn’t possible to turn back.

The issue is complex and fraught with arcane mathematical analysis as well as theoretical fine points, and of course, it isn’t
limited to human beings. But let’s gesture, at least, toward what seems a likely answer: the payoff of variety.

When a strawberry plant reproduces by sending out a runner, or an amoeba splits in two, the offspring are genetically identical to the parent. By contrast, sexual reproduction gives each parent, in a sense, a glass half full—as described above, this is perhaps the most formidable downside to sex. But it may also suggest a very large upside, since although each offspring is only “filled” with one-half the genotype of a given parent, it also contains one-half the genotype of the other parent, and, moreover, these genes will have been randomly reshuffled when eggs and sperm were produced and then combined. As a result, sexually reproducing individuals produce offspring who are different: from each other and from their parents. The benefit of this seems to be that when environments change, as they always do, parents who reproduced sexually end up with offspring that are genetically diverse rather than mere clones of the previous generation. Among such offspring, there is an enhanced likelihood that at least some individuals will find themselves well equipped to deal with the novelties ahead.

By analogy, if you were buying lottery tickets, it would be pretty foolish to purchase a dozen tickets all with the same number; this is essentially the strategy followed by asexual breeders.

Another, related argument has also been raised on behalf of reproducing sexually. Instead of focusing on external environmental changes (in climate, food availability, potential predators and prey, and so forth), let’s turn our attention to a living thing’s internal environment, including notably its pathogens and parasites. There is a constant arms race between free-living organisms (such as ourselves) and our various fellow travelers (viruses, bacteria, protozoa, nematode and trematode worms, etc.). These creatures, just like their “hosts,” are also evolving, “trying” to take advantage of our immune defenses while we seek to evolve ways of keeping the invaders at bay. If host bodies remain genetically unchanged from generation to generation—as happens with asexual reproduction—this makes it relatively easy for pathogens and parasites to home in on their characteristics, evolving ways of breaching their defenses. Sexually reproducing organisms, by contrast, are genotypically moving targets, whose offspring differ from each
other and from their parents, and are therefore more difficult to attack successfully.