Thumbs, Toes, and Tears

by Chip Walter

  The point is that in the never-ending struggle to win over the animal they want to mate with, species develop some interesting behaviors and physical traits. Our early ancestors were no exception.

  Our sex organs are unusual, for example. Take the human penis, which, to be blunt, is enormous compared with its peers in the rest of the primate world. Unlike humans, if male gorillas, orangutans, chimpanzees, and bonoos aren’t sexually excited, their members are so small and hidden from view you would have no way of knowing they have any at all.

  The same applies to human female breasts, which are large and defined compared with other primates. There is no obviously practical reason for this. Their increased size isn’t, as you might expect, necessary for lactation and feeding. All other female primates suckle their young but never develop full, round breasts.

  Our buttocks are a third, strangely human adaptation. We are the only primates that have a round, muscled bum. Arguably it developed when we stood up because we needed those muscles to help support and balance all the weight we were now carrying above our pelvises. But they may also have evolved as a sign of health that made us more attractive to the opposite sex. In men, solid, rounded buttocks might have signaled the strength needed for running and hunting, traits that make a good provider. But in females the dynamics may have been different. Scientists have found that males across all cultures rate women as consistently more attractive if the circumference of their waist is 70 percent of their hip circumference. The theory is that the unconscious, primal message an hourglass figure sends is that the owner of such a body is healthy and fertile and therefore desirable.2 This may explain why women as different as Marilyn Monroe, Twiggy, Sophia Loren, Kate Moss, and the Venus de Milo are all considered attractive. Every one of them has a waist-to-hip ratio of about 0.7. Apparently if size doesn’t matter, at least sometimes ratio does.

  However we calculate the mathematics of human sexuality, the point is that these peculiarly human features evolved not simply because they helped us to survive but because they helped us to multiply. They emerged because they enabled our ancestors to better compete among themselves to find an appropriate mate so they could pass along their particular set of genes. Or put another way, they made us sexy and attractive.

  In his book The Prehistory of Sex, Timothy Taylor theorizes that human males evolved large penises to show the males that they were competing with and, possibly, other females, what they had to offer. It became a signal of manliness and fertility. The issue wasn’t so much what the penis could do—ejaculate and impregnate; all penises do that—it was what it symbolized. A visible penis was a reminder of its purpose, a sexual cue. But if that is the only reason for evolving large penises, why did our ancestors begin to develop them when apes didn’t?

  Taylor holds that upright walking made the penis more visible and that, in turn, made its symbolic meaning easier to communicate. There it was for everyone to see, even when it was “at ease.” Not only that, but if locker room behavior is any indication, it also became a source of bonding among males because it was an obvious and unique sexual feature common to all of them. Despite the old saw that “size doesn’t matter,” Taylor argues that at least some females from generation to generation must have preferred bigger over smaller; otherwise the “bigger” genes would never have been passed along that eventually led to increased size being a common human trait.3

  The same may be true of female breasts. Perhaps they came to serve a symbolic meaning similar to penises, although understanding how that happened requires following a particularly circuitous, evolutionary path.

  Many primate females, but not humans, have what is known as estrus skin between their legs that engorges and becomes much more visible when they are fertile. In evolutionary terms it’s a straightforward way of saying, “I’m ready when you are to continue the species.” During these times sex can, like voting in Chicago, happen early and often.

  But once savanna apes stood up, estrus skin wouldn’t have been nearly as noticeable, hidden as it became between the two legs of an upright body. If our ancestors were simultaneously developing larger, more muscular buttocks, then the estrus skin would have been even less obvious, and that would have opened the door to other adaptations.

  There is a theory, for example, that sometime in our past, as we were in the process of standing upright, we began, slowly, to grow less hairy. Darwin believed this happened early in human evolution, but there is no absolute way we can be sure. For the sake of argument, though, let’s assume it did. (As we will see, there is reason to believe it happened at least two million years ago.) If estrus skin was hidden, and if buttocks were just beginning to evolve, scientists speculate that two changes may have occurred. First hominid bottoms may have grown more muscular to power our new form of locomotion, and more fleshy to store body fat, something crucial to survival in an environment where it was never clear where the next meal would be coming from. In females, this fleshiness may also have substituted for the engorged estrus skin that had become less visible.

  Zoologist Desmond Morris speculated in his book The Naked Ape that our ancestors’ bottoms would have become naked as a way to help this new version of sexual skin to stand out from other, more hairy parts of the body. A hairless, rounded rump would have indicated it belonged to a female with a certain level of fat, and a certain level of fat would have been attractive because it was an indicator of health and fertility. We know today that women need to have minimum amounts of fat in their systems to ovulate. Studies reveal that some long-distance female runners who have very low fat levels sometimes stop menstruating and ovulating. Other studies show the same happening to women who are on extremely low-fat diets.4 So a rounder rump would have been a solid indicator that this female not only had a worthy set of genes, but also was fertile enough to pass them along.

  All of this circles back to the evolution of female breasts, but again the route is indirect. Species sometimes develop sexual skin on one part of the body that reflects or recapitulates sexual skin on another part. Female gelada baboons, for example, show off brightly colored patches of skin around their nipples that look very similar to the estrus skin that flashes on their bottoms. If they are sitting down, which is something they do a lot, this gives them a second way to attract sexual attention. Mandrills also have ribbed, brilliant blue, white, and scarlet muzzles that recall the colors of their genital area, a kind of natural neon sign advertising their sexuality.5

  Full, round female breasts might have evolved as a recapitulation of their newly developed bottoms. Once we stood upright, chests, which are mostly hidden on animals that walk on all fours, were now front and center. Because our ancestors had gotten up on their hind legs, they had more upright, face-to-face contact. For females, their chests would have made perfect billboards for recalling the round, fleshy buttocks that had recently evolved to symbolize womanhood, health, and fertility without compromising the original purpose of their breasts.d

  We can’t be sure that these particular features evolved this way. Fossilized bone isn’t very helpful here, and other factors were certainly part of the evolutionary mix. The heat of the savanna undoubtedly played a role in our nakedness, and the need to capture and share certain scents and odors for sexual purposes may help explain why we haven’t become entirely hairless. Eventually some hominids, between three million and two million years ago, developed, and wrote into their genetic code, sexual signals and features that found their way into the long chain of DNA that shapes us today. We have bottoms, breasts, and penises, and their effect on our psyche, culture, and behavior remains large. They send powerful sexual signals, even in modern society, and they are crucial erogenous zones and pleasure centers that continue to shape the way we act and what we value.

  At this point in their evolution, the hominids from which we descended must have felt like adolescents caught in the grip of raging hormones, sexual desire, and social turmoil. Here they were, struggling to survive from one day to the
next on the grasslands of Africa, battling predators, disease, and the elements on the one hand, and one another on the other. Although standing upright had undoubtedly saved the species, it also had introduced new physical, social, and sexual forces that made life more complicated. And still there was more. All of the anatomical realignments our big toes had set in motion were creating yet another evolutionary bottleneck that threatened to strangle the species.

  The Taung Child skull. Because of his youth, Taung looked more human than his apelike parents, the result of a strange phenomenon in nature called neoteny. We may be neotenous chimps. (Used by permission of the Smithsonian Institution.)

  …

  In 1924, workers from a South African limestone quarry handed Raymond Dart, a young anatomy professor at the University of Witwatersrand in Johannesburg, a peculiar skull. The laborers had sifted it out of the debris of dynamited lime deposits in their home, a village called Taung, on the faraway edges of the Kalahari Desert. Lime deposits in this part of Africa were already well known for their fossils—mostly baboon skulls and ancient animal limbs—but when Dart saw this particular two-and-a-half-million-year-old skull, he knew he was looking at something no one had ever seen before. And it was not a baboon.

  What amazed Dart was how human the skull looked. It was small, but its cranium was rounded and unusually large compared with the rest of the face. There was no thick brow ridge, as one might expect in a primate that was so ancient, and the forehead was vertical from the nose up. He also noticed that the foramen magnum, that spot where the spine enters the skull, was far closer to the center than he had seen in any other primate except humans. Such a creature would have walked far more upright than chimps and gorillas.

  Dart named his find Australopithecus africanus (the southern ape from Africa) and concluded that he had found “an extinct race of apes intermediate between living anthropoids and man.” Still, that didn’t explain its perplexingly human form. Almost no other fossils of human predecessors had been found up to this time, so scientists could only conclude that this was simply the way protohumans must have looked. But in this, they were wrong.

  What Dart had done, without knowing it, was step back in time twenty-five-hundred millennia and then glimpse the future. His discovery would not only provide new insight into our ancestry, it also would provide important clues about why we look, and are, so different from other apes.

  The reason the primate skull that Dart had been handed included so many thoroughly humanlike features was because it was a baby, a toddler, perhaps two or three years old. Dart called the fossil the Taung Child. Two puncture marks in his head indicated Taung had met his fate when a leopard, or possibly an eagle, carried him off and made a meal of him. But had this child survived and grown up, he would not have looked nearly as human as he did the day he died. As he matured his jaw would have grown larger and more snoutlike, his eyes would have risen up in his face to be hooded by a bony brow ridge, and his forehead would have sloped back apelike from his nose.

  But why should a hominid toddler that lived more than two million years ago look more human than a fully grown adult? Because adult humans resemble infant, even fetal, apes. The reason we do is because, in some ways, that is precisely what we are. The Taung Child’s resemblance to a modern human is the result of a curious evolutionary phenomenon called neoteny, which has over time nudged us into being born earlier in our development than other primates. The result is that we tend to retain many of the youthful physical and behavioral traits of our species (and our ancestors) well into adulthood. In short, we are born younger, and stay that way longer. And the reason we are born younger is because it has evolutionary advantages, and because of our big toes.

  …

  A human being’s entry into the world is the most dangerous and difficult in nature. Because standing up rearranged the pelvic bones of our ancestors, and because of the rather large dimensions of our heads, human babies have to rotate in the birth canal from facing forward to facing sideways as they begin to emerge. Then they have to turn still another ninety degrees until they face their mother’s back as they are born. If the baby rotates the opposite way, the sharp bends in the birth canal could twist the baby’s spine backward and do enormous damage.

  For gorillas and chimpanzees the process isn’t nearly as difficult. Monkeys deliver their babies through a considerably broader birth canal while squatting or on all fours. Because their heads are smaller, infant chimps, gorillas, and orangutans can exit facing the stomach of their mothers, and even help pull themselves out of the birth canal. As these babies do a controlled fall, mothers often reach down and guide the infant out.

  The twisting, turning route we humans take was already in the works as far back as four million years ago. Fossils as old as Lucy’s reveal that standing upright had narrowed the birth canal enough that even for relatively small-brained australopithecines, babies would have had to rotate either forward or backward to fit their shoulders through the narrowed canal. Karen Rosenberg at the University of Delaware and Wenda Trevathan at New Mexico State University, two experts in the evolution of birth, speculate that this also would have drawn the troop more closely together because pregnant australopithecines would have needed help bringing their young into the world.6

  If birth was growing more difficult for Lucy and her kind, it would only have grown considerably more difficult for the next major species of savanna primate to emerge—Homo habilis, the first of our genus and our first known direct ancestor. Homo habilis (the handy man) walked out the mists of time about two million years ago with a brain that had nearly doubled, to an average size of 750 cc.7 That made an already tight and dangerous journey into the world considerably more difficult. The birth canals of Homo habilis could not grow larger to accommodate the larger heads of their children. If they did, the upright walking that the big toe had made possible would have become impossible. Hips would have grown too wide and bipedalism would have become mechanically un-workable. On the other hand, going back to smaller brains was not in the cards. Our ancestors faced a dilemma. The same evolutionary forces that were making them more mobile and more intelligent were also making birth more difficult. Without a solution, increasingly intelligent, upright walking apes would become extinct. Something would have to give. And, fortunately, something did.

  While developing in the womb, the skulls of primates don’t form in one piece. They develop instead as separated plates that drift over the brain. Humans have eight plates, each of which over time knits together after birth to form the hard, brain-protecting shield of our cranium. Before birth these disconnected plates make our skulls pliable enough to slip through the tight circle of the birth canal.

  Chimps and gorillas have some unknitted skull plates, so it is likely that our ancestors did as well. They were an elegant solution to a nasty evolutionary problem, and a perfect example of neoteny at work. It meant that Homo habilis didn’t have to develop a larger pelvis or smaller brains, or die off. Instead their young simply entered the world premature. And the larger the brain, the more premature their births had to be.

  We humans are the most extreme case of this in the primate world. If we were born as fully formed and physically mature as the babies of contemporary great apes, human gestation would last not nine months, but twenty-one! This means we are born a full year premature. We may define “full term” as nine months in the womb, but by ape standards we are fetuses that have arrived twelve months earlier than we should have.

  A young chimp (left) and a mature chimp (right). Humans have retained many of the physical traits of young chimpanzees. (Reprinted from Ontogeny and Phylogeny by Stephen Jay Gould.)

  Because of our premature birth, we come into the world almost totally helpless. Our brains are small and underdeveloped; our limbs, fingers, and toes are cartilage rather than mature bone. We are born nearly blind, our nervous systems are not even close to fully formed, and we continue to grow for approximately a third of our lives, years after oth
er primates have reached their majority. And while most of the plates in our skulls knit within the first several years of life, some don’t close until age thirty, and in a few cases can remain open when we are past ninety years of age.8

  All of the evidence suggests that unlike other species, our brains never stop adjusting to the world around us. Recent studies confirm the unusual plasticity of the human brain, and new evidence reveals that, contrary to popular belief, it does in fact replenish certain types of its own cells. Other research has shown that the prefrontal cortex, the most recently evolved part of our brain, continually rewires itself in response to new experiences until the day we die.

  The way Stephen Gould saw it, this lifelong youthfulness represented a powerful form of evolutionary selection that enabled humans to enjoy the benefits of a highly adaptive brain long after birth. Jacob Bronowski called these neotenous qualities “the long childhood.” Rather than binding our behavior irretrievably to our genes, they make us adaptable and mentally nimble. They enable us to change our personal behavior in reaction to our personal experience, and as a species, they keep us curious, playful, creative, and restless; in a word, youthful. And that youthful exuberance is, to a large degree, the bedrock of human culture and the creativity it represents.

  …

  Louis Bolk, a professor of anatomy from Amsterdam, first saw how these changes seemed to apply to humans, at least physically. Bolk was a great believer in neoteny, and in 1926 he assembled a list of features he saw in fetal and baby apes that also seemed to show up in the anatomy of human beings: a less snoutlike lower face, for example, our higher foreheads, and the greater ratio of brains to body mass. He pointed out that we are largely hairless like baby apes and that ape fetuses had external ears more like ours, a thinner facial bone structure, a foramen magnum more centrally located, and, strangely enough, a straight big toe.9,e “Man,” Bolk observed, “is a primate fetus that has become sexually mature.”