Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (21 page)

What about the runners, Olympic-caliber or not, of tomorrow? As Downey argues, and I agree, it is hard “to talk about . . . what the human body is ‘designed’ to do.”
⁶⁵
French Nobel Prize–winning geneticist François Jacob famously said, “Nature is a tinkerer, not an engineer.”
⁶⁶
He meant that evolution can work only with the parts at hand, rather than inventing the best possible solution to a problem. Hence, people in the Pleistocene may well have had knee trouble, because our knees are jury-rigged joints that have been modified from our quadruped ancestors. That doesn’t make bending down unnatural. And we use our limbs, and our bodies, for a wide variety of activities, depending on our place in history and on the planet. It is therefore futile to look for the single best type of exercise, given our evolutionary heritage, though it’s a safe bet that we would be healthier if we got up off the couch.

7

Paleofantasy Love

W
hat is our true sexual nature? Humans have highly demanding, precociously born babies; although a mother chimp or gorilla can raise an infant on her own, in all human societies women usually have help with their offspring, often in the form of the father. One could take this to mean that monogamy is both natural and necessary. But a great many popular and scientific sources disagree. Some suggest that men and women are in conflict, with naturally faithful women fighting a losing battle to keep men from straying. As one commenter on an article titled “Is Infidelity Natural? Ask the Apes”
¹
says:

Men are wired to spread seed far and wide. All men know this, we get hormones that run through our bodies that do this. There is no denying that at all, and it would be stupid to deny it. Men are also logical and can hold those feeling [
sic
] back. It doesn’t always happen.
²

Alternatively, the best-selling book
Sex at Dawn
would have it that both men and women are sexual gourmands, with multiple partners the real norm and monogamy a miserably failed experiment. According to authors Christopher Ryan and Cacilda Jethá, “The campaign to obscure the true nature of our species’ sexuality leaves half our marriages collapsing under an unstoppable tide of swirling sexual frustration, libido-killing boredom, impulsive betrayal, dysfunction, confusion, and shame.”
³

(© Kim Warp/The New Yorker Collection/www.cartoonbank.com)

On Cavemanforum.com, readers chime in:

It’s actually also true than [
sic
] girls are more likely to settle for the less alpha-type guys, but if they are going to cheat they do it with the more macho guys. We’re biologically wired that way. Blame evolution.
⁴

The paleo norm was for less than half the men to reproduce. Having 80–90% of men reproduce is a very recent phenomenon (even closer in time than agriculture). Lifetime Monogamy is as artificial as bread or corn syrup.
⁵

What all of these views have in common is an evolutionary perspective. They seek to explain our modern behavior in terms of our past, recognizing that how we mate and have babies is at the heart of who we are. Sex is the way in which we can seem most like our animal ancestors and relatives; all creatures reproduce in some fashion, and we obviously are descended from primates that chose mates, gave birth, and raised their children. What is not so clear is how they did it, and by extension what behaviors and roles we have inherited from our ancestors.

Perhaps not surprisingly, many of the paleo-lifestyle advocates point wistfully to a time when “sex was a lot more egalitarian and promiscuous than you’d think it was.”
⁶
One woman confessed that, since eating a paleo diet, “I’m VERY attracted to more aggressive (but still respectful), more capable men, strong of intellect and body, physically larger, hair on their chests, who would make good protectors and providers—not so much financially, but in the home.”
⁷
Exactly how a change in diet leads to a preference for more hirsute partners is not clear.

We may not really believe that men are from Mars and women from Venus, but we have a lot of opinions about what each sex can do or is likely to enjoy, from reading maps to watching tearjerker movies. Men and women are often supposed to differ even more in their sexuality, whether that means the kind of partner they want, their sexual appetites, or what they think a good marriage looks like. Evolution has been invoked to explain all of these characteristics—maybe not the movies per se, but supposed sex differences in traits, ranging from the capacity to appreciate nuances in relationships or have empathy for the travails of others, to the ability to perform spatial reasoning tasks, to a love of shopping for shoes, have been attributed to our history as hunter-gatherers.

As with the question regarding other aspects of human behavior, however, how much do we know about the sex lives of our ancestors? And how much of that behavior is still manifested today, in a world with speed dating and sperm banks? Is it instructive to look at our ancestry, or will we simply see what we want to see?

Darwin, peacocks, and pipefish

To understand the evolutionary basis for sex differences, we need to pay a brief visit to Charles Darwin. Although he is best known for his theory of natural selection as it applies to the origin and diversity of species on Earth, Darwin was also extremely interested in sex. One might almost say he was troubled by it, not necessarily in his own life (though some biographers have speculated about problems in his relationship with his wife, Emma, as well as his Victorian-era prudery), but because many things about sex in animals seem perplexing.

Take, for example, the peacock, with his splendid shimmering train of green, blue, and gold. Anyone who has seen a peacock displaying at a zoo will have no difficulty imagining that the huge fan of feathers impedes the male’s movements, and would make it difficult for him to escape, say, a tiger creeping up in the forests of the peacock’s native India. The females lack this encumbrance, as is the case for many kinds of animals: though exceptions occur, males are the ones with the elaborate plumage, colorful patterns, and loud songs—all characteristics that require a fair amount of energy to produce and that render their bearer more conspicuous to predators. In short, these traits seem disadvantageous, and hence would not be expected to evolve under natural selection.

Darwin’s solution was that another process besides natural selection was at work, called sexual selection. Just like natural selection, sexual selection causes some organisms to leave more genetic representations of themselves than others do, but instead of operating on characteristics such as a greater ability to blend in with the bark of a tree and hence avoid detection by a hungry bird, sexual selection is all about who gets more and/or better mates. If a female peafowl (the correct term for the species; “peacock,” strictly speaking, refers only to the male, while “peahen” is the word for the female) is more likely to mate with a male bearing an ornamented train than one with more modest plumes, the genes conferring the lovely feathers are more likely to appear in succeeding generations. The process works even if the traits hamper survival, though if the advantage in mating is outweighed by a disadvantage in survival, sexual selection won’t continue to act to exaggerate the trait.

Sexual selection is thought to account for many of the differences that we see between males and females: male deer with enormous racks of antlers, brightly colored birds of paradise, and frogs that croak the night away. It also explains why it is mainly the males that sport fancy advertisements, and not the females. Darwin’s original idea relied on the aforementioned Victorian notions of coy females needing to be persuaded by macho males, and it has been modified by his successors.

In my opinion, the best framework for understanding sexual selection comes from the more modern interpretation by Robert Trivers, an evolutionary biologist at Rutgers University. In a book chapter published in 1972, Trivers pointed out that although both sexes devote a great deal of effort, time, and energy to perpetuating their genes, the way in which they do so often differs.
⁸
Evolution means changing the genes that occur in the population, which means that individuals who have higher reproductive output are more likely to be represented. What limits that output? For females, it’s the number of offspring they can produce and, in some cases, rear, because by definition the female is the egg-producing sex and the eggs, once fertilized, are usually (though not always) cared for by the mother. Because eggs, and embryos and fetuses, are expensive and time-consuming to manufacture, even if females operate at maximum capacity their production is fairly small.

Imagine, for example, the highest number of puppies or kittens a female dog or cat could have in her lifetime, assuming that she mated every time she came into heat. That number, while obviously higher than the number of children a human female could bear over her lifetime, is still vastly smaller than the number of young that, at least in theory, could be sired by a given male of the species, since his investment, as Trivers would refer to it, is only the relatively smaller amount of time and energy required to mate with the females. Given the opportunity, a single male could inseminate many if not all the females in a population. Therefore, a male can win much bigger, evolutionarily speaking, than a female, because her upper limit is smaller. But of course, one male’s win is another’s loss, and in many species, most males do not mate with even a single female. Sexual selection therefore favors males that are able to compete with members of their own sex to get the opportunity to mate at all, because the stakes are high.

Females, on the other hand, will often do better from an evolutionary perspective if they mate with only the highest-quality partner, because a male that either has genes conferring a greater ability to survive or provides for the offspring directly—say, by feeding them—will make it more likely that her own genes persist in future generations. With this dichotomy in mind, many evolutionary biologists talk about competitive males and choosy females. Exceptions abound, of course, and Trivers himself emphasized that it’s the level of investment that is important, not the sex of the individual doing the investing. If, as is the case in some insects, the male must present the female with a nutritious package that he produces with his own body fluids before she will accept him as a mate, his investment becomes quite high, and males of species that offer these “nuptial gifts,” as they are called, are quite choosy about their mates. Seahorses and their relatives the pipefish are also good examples of this reversal of the usual state of affairs, with males receiving the eggs of females in their pouches so that they become pregnant and give birth to tiny replicas of themselves after the young develop.

Some researchers and popular writers took the idea of selection favoring different behaviors in the sexes to mean that women are expected to be rather unenthusiastic about sex itself, or at least to want it only as a means to gain a male’s investment in children, while men are expected to enjoy sex for its own sake. Presumably, the thinking goes that if men invest only sperm in mating (the more the merrier, as it were) while women have to weigh the consequences of their pleasure in a potential pregnancy, selection should favor men who seek out one encounter after another. Ryan and Jethá, the authors of
Sex at Dawn
, are among the most explicit about this claim, and the most eager to debunk it. They refer to Darwin’s “anti-erotic bias” and, more directly, claim that “Darwin says your mother’s a whore.”
⁹

Evolutionary psychologists, who extend some of the principles of evolutionary biology to the mental and behavioral adaptations of our own species, also come in for criticism by Ryan and Jethá, who note that many works coming from that field also assume that the female libido is sluggish, at least in comparison to that of the male. My own opinion is that while it’s certainly true that biologists, like everyone else, have brought their own biases to the study of human sexuality, whether in early evolution or not, the differential investments of the sexes into offspring do not automatically lead to a reduction in female lust. Just because the consequences of sex are different for men and women does not mean that either one enjoys it less.

Who, really, is your daddy?

Another reason males are said to be more likely to roam is central to the difference in how males and females reproduce. With rare exceptions, any offspring produced by a female is guaranteed to carry half of her genes, which makes any investment into that offspring a sound one in evolutionary terms. The same is not necessarily true for a male; he cannot be sure, anthropomorphically speaking, that a given offspring is his, because the mating happened days, weeks, or months before the young are hatched or born, and the female could have mated with another male in the meantime.

This disparity between the sexes means that what is called confidence of paternity can be rather low in some species, especially those in which females mate more than once before their eggs are fertilized. If a male is not the genetic father of the offspring a female produces, he is better off, evolutionarily speaking, finding another female to mate with than sticking around to help rear the young of his first mate. Females, on the other hand, would benefit from having a male’s help in taking care of their babies, regardless of whether he is their genetic father. Hence, the story goes, females are more inclined to monogamy and males to playing the field. And indeed, in many species the males perform a variety of activities that seem to have evolved to increase the likelihood that they are indeed the only one to have mated with a female; in bluebirds, for example, males stick close to the female during the critical few days surrounding ovulation, chasing any other suitors away.

We humans seem to be no less concerned than the bluebirds. So-called paternity fraud, with women fooling men about their relationship to the women’s children, has become a staple of talk shows and TV crime series. Billboards provocatively ask, “Paternity questions?” and advise that the answers are for sale at your local pharmacy in the form of at-home DNA paternity tests. Some fathers’ rights groups in Australia have called for mandatory paternity testing of all children at birth, with or without the mother’s consent or even her knowledge.