Homo Mysterious: Evolutionary Puzzles of Human Nature (6 page)

There is yet another important hypothesis that remains to be considered. It is especially intriguing for several reasons. For one, it brings in a seemingly independent evolutionary mystery, namely, consciousness.
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For another, it confronts one of the enduring puzzles about concealed ovulation: It is one thing for ovulation to be hidden from others, but why in Darwin’s name should such important, biologically crucial information be kept from the woman herself? Why is this such a deep, dark secret, one that cannot even be shared with the person who presumably has—if not a legal or moral right to the information—at least a deep personal stake in obtaining it? And finally, this last hypothesis is somewhat counterintuitive and therefore great fun to examine.

It was first suggested by Nancy Burley, an evolutionary biologist currently at the University of California, Irvine.
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Let’s assume that far enough back in the human evolutionary line, there was a range of self-awareness when it came to one’s own ovulation, as there is for most things: Some women could tell when they were fertile, others—at the other extreme of the distribution—had little or no idea, and in between there was a range of ovulatory self-consciousness. Add to this the fact that among many traditional peoples today (hunter-gatherers and other members of nontechnological societies), women want fewer children than men do, mostly because of the downsides of pregnancy and childbirth, especially in an environment
lacking biomedical sophistication. The result is the following prospect: If, like today’s hunter-gatherers, our ancestral grandmothers disagreed with their mates in wanting fewer offspring, then those who detected their own “time of the month” might well have made special efforts at those times to fend off the advances of our would-be ancestral grandfathers. Call it the headache hypothesis.

By succeeding in limiting their reproduction, such women would unknowingly have sabotaged the self-awareness system in which they participated. Who would have gotten pregnant? Not those who could detect their own ovulation, but those who couldn’t, who were unaware of what was going on inside their own bodies. A case of matter over mind. Our maternal ancestors would thus have been those who didn’t reveal cues as to whether they were ovulating but who also couldn’t even tell, themselves.

Next, our attention turns to breasts. In the process, we have plenty of company. Whereas the mystery of ovulation is why it is so secret, the breast question is exactly the opposite: Why so obvious? Whereas ovulation is mysterious because something so important is so hidden, breasts are mysterious because something so unimportant (most of the time) is so prominent (most of the time) and gets so much undeserved attention (nearly all the time).

The most straightforward explanation for why women have prominent breasts even when not lactating is that they signal capacity to nourish offspring, so that bustier women would have been preferentially chosen by would-be fathers. But why hasn’t a similar process operated in other mammals? Except for human beings, there are no mammals in which nonlactating females sport prominent mammaries. Moreover, there is no correlation between the size of breasts, while not lactating, and the eventual ability to produce milk. This is because what appears as breast tissue is actually made up of fat; glandular structures only develop during pregnancy. But breast development among human beings occurs in conjunction with sexual maturation, and quite differently later when needed as milk producers. Why, then, do women possess such prominent additions to their anatomy?

As we have seen when talking about menstruation, even though its precise adaptive significance is still a mystery, most biologists agree that it occurs for some down-to-earth reason: to remove pathogens, maximize energy efficiency, eliminate subpar embryos, and so forth. By contrast, there does not appear to be any practical connection between the fat-filled breasts of nubile, nonpregnant, nonlactating women and their eventual role as purveyors of milk. The fat residing in human breasts is not readily mobilized into milk; under severe calorie deprivation, nursing women are far more likely to (figuratively) reach into their hips, thighs, and arms.

So, if breasts are not functionally mandated, why are they there? Or rather, why don’t they develop only during pregnancy, then recede when not needed, as in all other self-respecting mammals? The most likely answer is that prominent nonlactating breasts owe their evolutionary existence in our species not so much to the offspring they might help nourish but to men that they attract. I write this fully aware that indignant readers might complain, “There you go again, how like men, assuming that our breasts must be pointing at them.” But in fact, they probably are.

Note, for starters, that breasts are extraordinarily diverse, from huge and pendulous to small and taut, bilaterally symmetrical or not at all. Ditto for nipples, which vary in size, shape, color, and so forth. Given this remarkable structural variety, the likelihood is that breasts are not unidimensional in their biological role; otherwise, selection would almost certainly have narrowed their anatomical range.

Considering that human breasts have a very high ratio of fat to glandular tissue, a “reverse engineer” would have to conclude that milk production was not the goal. On the other hand, it is worth noting that in every human society (including avowedly bare-breasted ones), men find breasts erotically interesting, which suggests that somehow, male–female interactions—sexual selection, as Darwin termed it—must be involved.

But how? Some downright silly ideas have been advanced, one of the most notorious by British ethologist Desmond Morris in his best-selling book,
The Naked Ape
. According to Morris, conspicuous breasts evolved in part because natural selection favors emotional intimacy between men and women, as a result of the need for devoted biparental care of offspring. Most mammals mate
dorsoventrally (“doggy style”), which—although feasible for human beings, too—is less personal and thus less likely to generate emotional bonding than is face-to-face intercourse. To induce ventral–ventral, face-to-face lovemaking, then, evolution supposedly favored conspicuous bilaterally paired breasts, which essentially mimic the buttocks of “normal” quadrupeds and assist in the transition from dorsoventral to frontal copulation.

I wish I could embrace such a creative idea. Frontally. But the problems are too great. For one, there is no evidence of a correlation between women’s degree of breast development and their partner’s preferred sexual position. In addition, many nonhuman primates—including bonobos, gorillas, and orangutans—copulate in a variety of positions, including ventral–ventral, yet in all these cases, females are flat-chested. The supposed connection between sex while looking into each other’s eyes and devoted monogamy also appears to be sheer fantasy. Monogamy is rare among mammals, but when it occurs, in some species of foxes, beavers, otters, the California mouse, pygmy marmoset, and certain oddballs including the fat-tailed dwarf lemur and Malagasy giant jumping rat, the pairs mate dorsoventrally. So prominent breasts aren’t necessary for frontal mating, and frontal mating isn’t needed for pair bonding.

It is also difficult to imagine that males of any species, including our own, have ever needed the visual image of female buttocks to feel horny. Behavior and pheromones are more than sufficient. Not only that, but mammalian buttocks aren’t naked, rounded globes but rather flattened and hair-covered … in short, not very breastlike.

For another exercise in foolishness, author Elaine Morgan has long championed the bizarre idea that people evolved as “aquatic apes,” with breasts evolving as flotation devices.
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After all, during World War II, sailors called their life vests “Mae Wests.” Floating babies might have clung to their mother’s breasts as to water wings. And presumably men would have done the same; even now, they do so whenever they can! Seriously, however, if breasts evolved as life preservers, then men ought to have evolved them, too. Let’s return to reality.

Maybe human breasts evolved as calorie storage sites, a kind of pantry that preceded refrigeration. If so, then the ample breasts
of Earth Mother Goddesses would certainly have been preferred over those of today’s anorexic fashion models. There is a generally close correspondence between what people find sexually appealing and what ultimately leads to reproductive success, and so, it is not unlikely that our male ancestors preferred sexual partners whose well-upholstered bosoms suggested the ability to survive hard times, not to mention promising abundant nourishment for any eventual offspring.

But there are problems here as well. For one thing, breasts are unlikely to have evolved as storage sites; if calorie storage was evolution’s intent, it would have been far more efficient to use the hips, butt, or upper arms, where tissue could have been wrapped securely around bone instead of being left unsupported. (Any physically active woman will confirm that breasts are often a distinct liability.) Moreover, as we have already seen, fatty breast tissue doesn’t contribute to making milk, and nonlactating breasts—whose prominence we are trying to explain—are composed almost entirely of fat.

This leads to the fascinating, albeit dispiriting, proposition that breasts evolved as a kind of biological deception, with women taking advantage of male obtuseness by promising an amply-stocked, milk-soaked delicatessen that they may or may not be able to provide. Since breasts do in fact increase dramatically in size while lactating, it is not unreasonable that prehistoric men noticed and were readily persuaded to mate preferentially with women whose anatomy suggested more nourishment for the consequence of their mating. Even if guilty of false advertising, such a system could have worked, assuming that there was competition among women for access to the most desirable men and that these men, in turn, preferred big-busted women, even if such women likely promised more than they delivered.

Even under this scenario, men would not have been entirely helpless. To minimize being completely beguiled by the substitution of fat for gland, men have evidently evolved a fondness for the classic female “hourglass” figure, with narrow waist and relatively
wide hips. Since fat does accumulate in the waist, a comparatively high waist-to-hip ratio indicates overall body fat, and it turns out that cross-culturally, men prefer a waist-to-hip ratio of 70% or even lower.
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There are a few exceptions, including an Andean population in which men prefer more “tubular” women, but the overall worldwide preference is remarkably consistent. There has even been constancy over time. Measurements of ancient statues and paintings have confirmed that whether the societal norm favored a well-upholstered female form, á la Rubens, or absurdly slender, like current Western supermodels, the preferred. 70 ratio has remained quite steady.
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These findings, however interesting and suggestive, do not prove anything. In particular, the fact that men prefer “shapely” women does not necessarily tell us how they got to be shapely in the first place, which is our goal. If women’s body shape evolved for some entirely separate reason, it is plausible—indeed, likely—that men’s preference would have gone along, simply because men who invested preferentially in women with those traits would have been rewarded with greater reproductive success—of which one consequence would be yet more women constructed that way.

On the other hand, breasts might nonetheless provide accurate information, indicating a woman’s ability to accumulate and store calories. Pleistocene-era women who already had enough nutrition on board to readily expand their breasts—even if simply via fat deposition—would have been the most likely to stimulate males to provide yet more. This idea is especially compelling because it hints at a possible explanation for why prominent nonlactating breasts are so characteristic of human beings and not other mammals. The “explanation,” if valid, makes use of another trait that is especially characteristic of
Homo sapiens
: our intellect and imagination. Thus, it seems almost certain that prehistoric men would have noticed that lactating women develop enlarged breasts, and not unlikely that human cognition would therefore have made an association between large breastedness and effective milk production (even though such a correlation, as we have seen, turns out to be spurious).

And so, an early hominid female in a position to benefit from storing fat somewhere on her body might as well have done so via
her breasts, to stimulate additional male investment … all the more so insofar as men would have been predisposed to prefer women with relatively prominent breasts.

This brings to mind what has been called the “banker’s paradox,” so named because banks are least favorably disposed toward those prospective borrowers who especially need a loan, because the more needy the would-be borrower, the more liable he is to be a poor credit risk. Conversely, they eagerly bestow funds upon the wealthy, who need it least. In short, those who have, and don’t need much more, get. Something along these lines may have induced evolution to exaggerate the breasts of early female hominids, because those thereby endowed would have profited from the increased self-interested largesse of men, inclined to “lend” resources to a prospective mate deemed to be a good investment—whether or not they really are.

We turn now to breast evolution as an example of honest signaling. Happily, for anyone predisposed toward telling the truth, it may well be that honesty is the best policy, not just ethically, but also evolutionarily. There are, in fact, several ways in which human breasts may have evolved in the service of honest sexual selection.