Homo Mysterious: Evolutionary Puzzles of Human Nature (2 page)

Detail from frontispiece of the
Encyclopédie
, published in 1772. Truth is surrounded by a bright light (a frequently employed symbol of the Enlightenment). Reason and Philosophy, on the right, are undressing Truth, by pulling off her veil.

Just as Diderot’s
Encyclopédie
was a paean to Enlightenment values—notably the upside of human knowledge—
Homo Mysterious
is similarly enamored of reason, science, and progress, even though it is superficially dissimilar, and not merely because its goals and reach are much more limited. Nonetheless, in
Homo Mysterious
, we shall continue the tradition of the
Encyclopédie
’s frontispiece, attempting to get a closer look at Truth, but instead of undressing her, we shall proceed nonvoyeuristically, pointing out where, despite the best efforts of Reason and Science, she continues to be at least somewhat clothed. “All women, O, are beautiful,” wrote
Theodore Roethke, “when they are half-undressed.” The same applies to Truth itself.

Unlike the
Encyclopédie
, which took all knowledge as its subject, in
Homo Mysterious
we shall limit ourselves to human beings.
ⁱ
Not only that, we’ll be concerned only with certain characteristics of
Homo sapiens
as seen through the lens of evolutionary biology.

The traits in question are fundamental to being human, stubborn stigmata of our species’ unique evolutionary heritage, yet their basis is neither understood by scientists nor for the most part even acknowledged by the public as the puzzles that they are. Most people are unaware that female orgasm, for example, and even religion are biological mysteries, simply because nearly everyone takes the most intimate aspects of his or her life for granted, so deeply woven into our substantive human being that they are rarely identified as legitimate perplexities.

We members of
Homo sapiens
are almost literally immersed in mystery; the evolutionary enigmas of humankind are the seas in which we swim. In
Homo Mysterious
, you will be introduced to this ocean of unknowns, as well as to the major hypotheses that currently occupy scientists who are attempting to unravel each puzzle (including some proposed here for the first time). Like science courses, nearly all science books describe what we know, thereby giving the impression that we know nearly everything, whereas the reality is exactly the opposite: We know very little compared to how much we don’t.
Homo Mysterious
is designed for readers likely to be challenged by the blank spots on the human evolutionary map, the
terra incognita
of our own species.

At the risk of channeling former Defense Secretary Donald Rumsfeld, there are “unknown unknowns,” things we don’t understand and that we don’t even know that we don’t know! Since we can’t identify them, “unknown unknowns” are difficult—perhaps impossible—to write about. On the other hand, there are also “known unknowns,” things that we don’t yet understand but that we at least have the wisdom to acknowledge as such. Prior to Albert Einstein, Niels Bohr, Enrico Fermi, and others, nuclear energy
was a known unknown, just as evolution was before Darwin. Today, the underlying basis for homosexuality is similarly a mystery—a phenomenon whose scientific basis is unknown—as is consciousness, the cross-cultural universality of religious belief, the evolutionary basis of artistic creativity, and so forth.

Homo Mysterious
will examine these and other perplexities from a perspective that many readers will find unusual. Thus, we shall be less concerned with
how
these things came about than with
why
, not “why” in the metaphysical sense, but that of evolutionary biologists. Were we to ask “how” people became bipedal, the answer would involve changes in muscle attachments, bone structure, and nerve growth; to be concerned instead with the evolutionary “why” is to ask about the likely adaptive pressures that must have conveyed a reproductive advantage to those of our ancestors who walked on two legs, regardless of exactly
how
that posture was achieved.

Similarly, we’ll be less concerned with
how
religious belief or homosexuality comes about—the nerves, muscles, brain regions, hormones, or specific genes responsible—than with
why
these behaviors, regardless of their underlying mechanisms, appeared and have persisted in the human population, despite their seeming evolutionary disadvantages.

This approach is familiar to evolutionary biologists and, less so, to most anatomists, physiologists, and the intelligent lay public as well. Not that scientists aren’t likely to be “evolutionary” in their thinking; rather, they are more prone to ask questions—and to answer them—in terms of immediate causal mechanisms. Instead, we’ll raise questions—and answer them—in evolutionary terms. The underlying conceptual theme is, therefore: “In what way has _______ (female orgasm, concealed ovulation, homosexuality, consciousness, religious belief, etc.) contributed to the ultimate reproductive success of human beings, thus in all likelihood explaining why this particular trait evolved?”

One way of conceptualizing this process is to think about what is sometimes called reverse engineering. In normal engineering, a problem is identified, after which some sort of device or structure is engineered as a solution. Reverse engineering operates, not surprisingly, in reverse: A device or structure (or, in the case of
Homo Mysterious
, a behavior) is identified, after which the evolutionary
biologist tries to figure out how this particular phenomenon came to be “engineered”—which is to say, why it evolved.

In many cases, the process is straightforward and obvious, so that it is rarely even attempted. Why do we love our children? Almost certainly, ancestors who lacked parental love didn’t do a very good job of caring for their offspring and therefore didn’t become ancestors. In that sense, love may actually be less mysterious than is generally thought! For all its storied and supposedly enigmatic nature, there are other human characteristics far more mysterious than love; once we identify them, we are faced with the question of “reverse engineering” them.

In nearly every such case, we will identify multiple potential evolutionary explanations—although we’ll refrain from coming up with
the
explanation, simply because at this point, despite the world’s impressive store of accumulated scientific knowledge, final answers just aren’t yet available. Hence the mystery, the pleasure, and, eventually, the illumination. “There is a crack in everything,” writes poet/songwriter Leonard Cohen. “That’s how the light gets in.”

Just one modification: There are
many
cracks and, when it comes to some of the most interesting mysteries of human evolution, no reason why a single one should necessarily provide the only illumination.
Homo Mysterious
therefore cannot claim to be exhaustive, just scientifically accurate as far as it goes and, I hope, fun.

Consistent with our slightly lascivious reflections on the frontispiece to the
Encyclopédie,
we’ll begin our exploration with some human sexual mysteries. Then, we move above the waist. I hope you enjoy the trip and that along the way, you might even come up with a suggestion or two that could help solve some of the evolutionary mysteries we are about to explore together.

C
HAPTER
T
WO
Sexual Mysteries I: Menstruation,
Concealed Ovulation, and Breasts

M
OST PEOPLE WOULD AGREE
that sex is a mystery. But they would probably be thinking of romantic perplexities rather than scientific enigmas. The fact is, however, that when Winston Churchill famously described Russia in 1939 as “a riddle wrapped in a mystery inside an enigma,” he could as well have been speaking of some hard-wired aspects of human sexuality. Particularly mysterious, at least from the perspective of evolutionary biology, are some aspects of the sexual biology of women.

The first notable mystery begins when a girl becomes a woman: menstruation. Although a few other species bleed slightly at mid-cycle, no other organism does so as prominently as
Homo sapiens
. Breast development is another perplexity: Although we take it for granted that women have conspicuous breast tissue even when not nursing, no other mammal is comparably bosomed. Only human beings are blessed (or, in the opinion of some, cursed) with prominent nonlactating breasts.

The mysteries continue. Go to a zoo and take a look at the chimpanzees, gorillas, or baboons (or, indeed, nearly any nonhuman primate). There is no question when an adult female is ovulating.
It is as obvious as the bright pink cauliflower on her behind. Not so for our own species. Given the great importance—socially, biologically, evolutionarily—of reproduction, and thus of ovulation, it is extraordinary and as yet unexplained why even now, in our medically sophisticated 21st century, it is exceedingly difficult to tell something so basic as when a woman is fertile. For reasons unknown, human beings conceal their ovulation and are unique among mammals in doing so. Not only that, but in the great majority of cases, the exact time of a woman’s ovulation is even hidden
from herself
. As with menstruation and nonlactating breasts, hypotheses abound, but no one knows the answer.

Ditto for female orgasm. Its male counterpart is a no-brainer (almost literally!), since without ejaculation there would be no fatherhood and thus no evolutionary success. But the data are quite clear: There is no correlation between female orgasm and female fitness in the evolutionary sense. In other words, orgasmic women are no more successful reproductively than their less fortunate, nonorgasmic “sisters.” So, why does female orgasm occur at all?

Proceeding along the trajectory of a woman’s life, we come to yet another mystery: menopause. By around age 50, a woman can anticipate that she will cease ovulating. Why does reproduction inevitably end, even for the healthiest women, at a time in middle age when many can anticipate several decades of continued and vigorous life? This is especially perplexing since reproductive success is the name of the Darwinian game and simple calculations show that producing just one additional child, compounded over time, would convey a huge evolutionary advantage. Yet menopause is not only a cross-cultural human universal but also one not shared by any other living thing, except possibly for the short-finned pilot whale (but of course, you already knew that).

Let’s consider these mysteries, and some possible solutions, one at a time.

It has been suggested that all mammals may in fact shed some of their uterine lining between ovulations, but no other species comes close to
Homo sapiens
in the volume of blood flow and amount of
tissue disruption, which requires women to literally rebuild their uterine lining with each reproductive cycle. The amount of blood loss—about 40 ml—is not enormous but is enough to force many to take iron supplements. It is as though every sexually competent woman is obliged to suffer an automatic monthly deduction from her metabolic checking account. Not only that, but it seems likely that the shedding of fresh blood, sometimes in copious quantities, would make menstruating women more susceptible to predators, many of which are exquisitely sensitive to cues that indicate potential prey.
ⁱ
Given that evolution is a stern task-mistress, constantly sifting and sorting through alternatives to find the most efficient and fitness-enhancing way to accomplish the various tasks of living, why have we signed on to such expensive and possibly even risky monthly taxation? And this doesn’t even count the cramps, which in some cases are so severe as to be temporarily disabling.

One might even say that menstruation almost literally deserves its old-fashioned descriptor, “the curse.” More scientifically, we would expect that women who did
not
menstruate would have been more successful over evolutionary time than those who did. Yet the opposite has clearly been true, since menstruation is something that all healthy premenopausal women do. It may be an annoyance but it is not an illness or a biomedical problem; rather, prolonged amenorrhea—
failure
to menstruate—indicates that something is wrong.

In addition, about 10% of otherwise healthy women suffer from endometriosis, a painful and potentially life-threatening condition caused when cells of the uterine lining are discharged in the wrong direction, into the pelvic cavity instead of outside the body. And of course, menstruation necessitates that for a particular duration (typically 3 to 5 days), pregnancy cannot occur. This is yet another cost of menstruation, which further italicizes the mystery that it exists at all.

For starters, let’s ignore the suggestion, repeated through generations of folk “wisdom,” that menstruation indicates the “weeping of a disappointed uterus” that has failed its reproductive role.
(This notion is especially foolish since the likelihood is that without menstruation, more cycles would be “successful.”) Ditto for the theological assertion that it constitutes part of the punishment inflicted by a vengeful deity upon a disobedient Eve. There are, in fact, several possible scientific hypotheses for menstruation.