How We Do It

Human Exceptionalism

A meticulous account of the evolution of human sex, pregnancy and child-rearing.

By Marlene Zuk

How long does pregnancy last? Nine months, of course, or more precisely, 40 weeks, and we can use the date of last menstruation as a reliable indicator of when the pregnancy began. But as Robert Martin notes in "How We Do It," his meticulously researched account of human reproduction from conception to early childhood, "things are not always that simple." In many female primates, including women, monthly cycles persist into early pregnancy, for reasons still poorly understood. The date of conception is surprisingly hard to pin down, and due dates are as much guesswork as measurement.

Mr. Martin's humble but crucial acknowledgment that biology is unavoidably complicated—that we can't capture millennia of evolution or decades of research in glib sayings about the sexes' planetary origins or in single surveys of psychology undergraduates—is what makes "How We Do It" so compelling. It's not that Mr. Martin, a curator of biological anthropology at Chicago's Field Museum, claims that sexuality is such a morass of science, culture and mistaken beliefs that we should throw up our hands. Instead, he takes a calm, soothingly detached approach to the evolution of sex and child-rearing. No Mars and Venus, no extrapolations about why we evolved to love—or hate—strip clubs or whether bottle-feeding dooms a child to a life of puerile amusements and a career at the Kwik-E-Mart. Here instead are the facts of life as you may have never thought about them.

Consider, for example, the supposedly continuous sexual receptivity of women. In contrast to other mammals, such as dogs or cats, that have a discrete and easily detected period of estrus, or heat, women are supposedly equally ready to have sex whether or not it is likely to result in conception. Such a libertine separation between procreation and sex has been said to be uniquely human, something that elevates our sexuality into something more than the mere necessity of sperm meeting egg.

But Mr. Martin points out that many other primates besides humans show little external signs of ovulation, so humans are not unique in the supposed "concealment," as it is termed, of receptivity. Furthermore, in the 1930s, Georgios Papanicolaou, of Pap smear fame, documented consistent changes in the appearance of vaginal cells across the menstrual cycle and noted that women tended to report changes in sexual desire that paralleled this variation. In other words, women wanted sex more at certain times of the month. This might sound like a simple echo of our evolutionary ancestry showing up in unexpected places, except that the peak in desire did not correspond neatly with the time that conception is most likely to occur. Other studies report yet different results. Finally, despite the obsessive timing of intercourse by couples seeking to make a baby, it isn't all that clear exactly when the optimal time for fertilization is or to what degree it might vary from woman to woman. So human ovulation may be concealed, but no more so than in some other species, and even if it is, subtle distinctions persist in when women are more likely to want sex. And we still don't know exactly when intercourse is most likely to lead to pregnancy. Complicated enough for you?

Rather than develop stories about how human sexual behaviors might have been adaptive in the past, Mr. Martin persuasively argues that understanding the diversity of reproduction across mammals is essential to understanding it in humans. He repeatedly whisks away our smug sense of human exceptionalism and at the same time links many seemingly unrelated aspects of our biology to reproduction. The much-vaunted large human brain, he notes, evolved under constraints of pregnancy, not just cognitive capacity. All mammal mothers fuel growth of their infants' brains both before and after birth, so that longer pregnancy and/or suckling time usually means a larger brain size; Mr. Martin observes that mammals in general attain 90% of their final brain size by the time they're weaned. All primates are born with unusually large brains, and humans aren't even outliers among them: Taking body size into account, our 12-ounce brains are in keeping with those of newborn chimpanzees.

The brains of human infants, however, grow extremely quickly, increasing fourfold in the first year, a rate comparable to that of a fetus still in the womb. By rights, or at least by the same measure as other primates, a human pregnancy "should" be 21 months long to encompass all that brain growth—a conclusion likely viewed with horror by, well, anyone. It doesn't last that long, of course, probably because human pelvic girdles, having been adapted for walking upright on two legs, couldn't accommodate an infant with a head that big. Humans are thus born physiologically premature.

Brain growth has to happen within the limits of a woman's ability to supply energy to her baby. Although nursing continues to provide that energy after birth, the placenta is a much more efficient way to deliver nutrients to the baby. Thus evolution has pushed newborn head size to very nearly its upper limit. Babies spend as much time in the womb as possible, so that their heads just barely squeak through the pelvic canal. Not only does this make delivery unusually difficult and dangerous for human mothers, the "early" birth of humans leads to other problems, such as the widespread occurrence of earaches in newborns as a result of their relatively undeveloped middle ears.

In addition to debunking the conventional wisdom about questions we've all heard (little evidence exists for any of the many explanations for human females' enlarged breasts; other mammals' relatively flat chests work just fine for nursing), Mr. Martin poses questions we never thought to ask. Why, for example, do women have a single-chambered uterus, compared with the two chambers more commonly found in other mammals (even those that, like humans, typically produce only one offspring at a time)? And why does the fertilized egg implant deep in the uterine wall rather than simply adhering to the surface? Scientists still don't know the answers, perhaps because until someone like Mr. Martin comes along to contrast humans with our primate and other mammalian relatives, they might not have even seen a problem to be solved. It's in viewing ourselves along the continuum of primate evolution that our own uniqueness—and lack of it—becomes apparent.

Much of "How We Do It" puts what we consider "natural" under scrutiny. Being complicated, it turns out, is not only more accurate than being simple; it is far more interesting.

—Ms. Zuk is a professor of ecology, evolution and behavior at the University of Minnesota.

A version of this article appeared July 19, 2013, on page C8 in the U.S. edition of The Wall Street Journal, with the headline: Human Exceptionalism.

The book is:  How We Do It by Robert Martin