Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (24 page)

The unreliability of big-game hunting therefore may make it a poor choice as the sole source of support for a family; saying you want to maintain your wife and children on it is the ancestral equivalent of claiming that you will be able to fulfill your familial responsibility on the proceeds of playing lead guitar in a band. But hunted meat is not simply food—it is a signal. As Hawkes and Bird put it, “More than its value as a source of nutrition, meat is a medium of communication through which the hunter transmits information to potential mates, allies, and competitors.”
²⁷
By contributing to the group, good hunters gain the respect of their peers. The anthropologists point out that “showing off” in this context does not have to involve actual bragging; in fact, among the Aché, meat is often brought into camp quietly and without much fanfare. But everyone knows who can, quite literally, deliver the goods.

This “status signaling” hypothesis has been critiqued by other anthropologists, who continue to discuss how best to calculate hunters’ contributions to their families, whether shared meat is repaid in kind, and which other factors play into the sexual division of labor. But the present-day perspective on hunting and gathering in modern—and presumably ancestral—societies still leads to three conclusions. First, sexual divisions of labor are widespread today, and very likely occurred in ancient humans, with men probably doing the more high-risk, potentially low-yield part of foraging. But second, and maybe more important, the tasks that men and women do are remarkably flexible across societies and over time. Anthropologist Jane Lancaster has noted that in various human cultures, women have been known to do virtually everything that men do, with a notable exception of metalwork, which she speculates requires too much single-minded concentration to be compatible with the presence of small children.
²⁸
Finally, nothing about these sex-specific tasks, whether gathering limpets, digging yams, or snaring monitor lizards, suggests a justification for all manner of modern gender stereotypes, from women liking to shop while men watch football to an ability (or lack thereof) to ask for directions. That paleofantasy of the cavewoman staying home with the kids while the caveman went out for meat would have ended up with no one getting enough to eat.

Our bodies, our genitals, ourselves

Our behavior is slippery stuff, with men and women acting differently in different societies and under different circumstances. And without preserved footage of Neandertal dating sites, we are left to speculate about the sexual proclivities of our ancestors. Or are we? Behavior may not fossilize, but bodies do, and we can infer a surprising amount about behavior from those remains. For instance, we are reasonably sure that some dinosaurs cared for their young, because of the discoveries of eggs or young dinosaurs associated with an adult.

We can also draw conclusions about how natural and sexual selection have acted on the sexes in the past, simply by looking at the kind and magnitude of differences between male and female bodies today, as well as by comparing those differences across species. Along with the rest of his focus on sex, Darwin was extremely interested in these differences, and he drew a distinction between what he called primary and secondary sexual characteristics. The primary sexual characteristics are what define us as male or female—the plumbing, so to speak, with human (and other mammal) males having testes and females having ovaries, for example. All animals have primary sexual characteristics, and they can be very obvious or quite subtle; in many rodents, for instance, males and females are difficult to tell apart even after inspection of the nether regions, at least by a nonexpert.

The secondary sexual characteristics are even more variable, and they were also of much interest to Darwin. These are all the other differences between the sexes—the ones that aren’t directly required for reproduction but are still sex-specific, such as the peacock tail I mentioned earlier or the songs of male frogs, crickets, or birds. Human secondary sexual characteristics include enlarged breasts in women, facial hair in men, and—most germane to our quest for the ancestral mating system—differences in body size, with men being on average 15 centimeters (about 6 inches) taller than women.

This body size difference is mirrored to greater or lesser degrees in many other species, though in some, including whales and many insects, it is the female that is the larger sex. (Larger females are believed to be favored when they can lay more eggs or otherwise provide more for the offspring.) Larger males are thought to be the result of sexual selection for better fighters, and the fighting is generally over access to mates. In elephant seals, for example, the 2-ton males are more than twice the size of the females, and the bulls spend hours battling for supremacy on the coastal breeding grounds. The champions are able to sequester and mate with a majority of the females that arrive on the beach, making the largest males big winners from an evolutionary perspective. So the general idea is that species with more male competition, and more polygyny, are likely to show a greater difference in body size between the sexes.

Among our primate relatives, the gorillas, which live in groups with a single male that mates with several females, have the most pronounced sexual size difference, with males about twice the size of females; similar sex differences are seen in orangutans. Males and females in the monogamous gibbons, by contrast, are roughly equal in size, as are the two sexes in the muriqui monkeys of Brazil, which have a polygamous mating system in which both males and females have several sexual partners during a season. Many primates also show sexual differences in their teeth, with male baboons, for example, having elongated, sharp canines that females lack. This difference, too, is thought to result from sexual selection, with the teeth used as weapons during fights between males.

Humans, along with chimps and bonobos, have a much more modest difference, which has led many researchers to conclude that we were only moderately to slightly polygynous in our evolutionary history. Fossil evidence also indicates that human sex differences in size have decreased over the last several hundred thousand years, though it can be difficult to draw conclusions, because skeletal remains are sometimes classified as male or female in the first place by comparing the size of the bones. Owen Lovejoy, an anthropologist at Kent State University in Ohio, extended this idea to suggest that human monogamy arose at least 4.5 million years ago, when the bipedal human ancestor
Ardipithecus ramidus
lived, and furthermore that it was facilitated by bipedalism, with men making use of the freeing up of their arms to hold tools, going off to hunt and bringing back meat for the women. Women, in turn, would reward their mates for the food with fidelity, à la Hrdy’s “sex contract,” tying things up in a nice tidy package.
²⁹

Of course, as I have already pointed out, this sharp division of labor is not upheld in modern hunter-gatherer societies, and furthermore, the similarity in relative sexual size between us and the chimps and bonobos suggests that we need more information before we can explain why we would end up monogamous and they would mate with many partners. Lovejoy also suggests that
Ardipithecus
was relatively peaceful, like the bonobos, but as de Waal notes, “unless the diggers come up with a male and female fossil holding hands and having wedding rings, the idea that these ancestors avoided conflict through pair-bonding remains pure speculation.”
³⁰
(One could nitpick that even this finding would not be conclusive, but de Waal’s point is well taken.)

Furthermore, although very highly sexually dimorphic species are thought to be polygynous (“dimorphic” is the technical term indicating size and shape differences), it’s not so clear what it means when the sexes are quite similar. Sometimes being bigger isn’t all that useful, as in hummingbirds or other species with aerial displays of aggression, where a more agile opponent wins the day. Other variables, such as the likelihood of mating within one’s group or outside it, can also influence the strength of selection on male competitiveness.

Even when we find evidence of sexual size differences, the degree of those differences is not necessarily the result of mating competition. Modern cultures vary in how dimorphic they are in height, with the Maya Indians of South America differing by nearly 10 percent, while the Taiwanese differ by only 5.5 percent. Claire Holden and Ruth Mace from University College London looked at seventy-six populations around the world for which they could find information on height, degree of polygyny, type of subsistence (hunting or agriculture), and the sexual division of labor during a preindustrial period.
³¹
If polygyny has made us dimorphic as a species, then it seems reasonable, if we look within human cultures, that the more polygynous cultures should also be more sexually dimorphic.

The latter two variables were of interest because the researchers hypothesized that polygyny might not be the only thing affecting sexual size differences; perhaps the way that men and women live is also a factor. And indeed, the more women contributed to food production in a culture, the smaller the difference in height was between the sexes, perhaps because such contributions meant that the women had more control over food distribution. Whether or not a society was polygynous made no difference, although the authors caution that their sample might not have allowed detection of a contribution by the mating system. They had to account for the historical relationships among the societies, as Fortunato did in her marriage analysis, described earlier; and once they did, they were left with a rather small sample.

The last piece of evidence about our mating history that we can glean from our bodies is a bit more personal than height. In many mammals, the size of the male testis is correlated with the number of females a male might potentially mate with over a short period of time. The testes, of course, produce sperm, and generally speaking, the larger the testis, the more sperm a male can manufacture. Usually, male animals produce enormous numbers of sperm cells (human ejaculates are 1.5–5.0 milliliters and contain anywhere from 20–150 million sperm per milliliter), but ejaculates need to be replenished, and might not be sufficient to fertilize the available females if a male is mating very frequently. In addition, if a female mates with more than one male in a short period of time, the sperm in her reproductive tract can compete with each other, in which case the male supplying the most competitors is at an advantage.

Several studies of animals, including primates, have found that species with more male competition for mates have larger testes than do species in which monogamy prevails. Interestingly, however, a 2010 study by Carl Soulsbury showed no relationship between the amount of mating outside a group—the “extra-pair paternity” that many paired-up animals show—and testes size among mammal species, suggesting that other factors besides sperm competition may be at play.
³²
Soulsbury also found that species with larger litters had larger relative testes, again suggesting that male competition is not the whole story behind the evolution of our genitalia.

Where do humans fit into this picture? Human testes are smaller relative to body size than those of chimpanzees and bonobos, but larger than those of either the monogamous gibbons or the gorillas. Although male gorillas are substantially larger than females, because they live in groups where only one male routinely mates with the females, the silverbacks have relatively little need for competition with other males. Most researchers have concluded that this finding supports our decreasingly polygynous history, although Ryan and Jethá try to make the case that it points to a life of polyamory, with simultaneous multiple partners for both sexes.
³³
They also note that men from different ethnic backgrounds differ in relative testis size, though since we do not know how each culture varied in the degree of multiple mating, it is hard to draw any conclusions from such information.

More recently, detailed examination of the stretches of DNA that are present in our ape relatives but absent in modern humans revealed a loss that women, at least, have cause to celebrate: the genes coding for “genital tubercles,” or more graphically, penis spines. In many other mammals, including chimpanzees, the penis has hardened growths that may serve to sweep away the sperm of previous mates. These structures are absent in humans because we lack the genes responsible for the hormone signals that would cause them to develop. The relative smoothness of the human penis is thought to be linked to a reduced frequency of sperm competition.

So, is monogamy swimming against an evolutionarily promiscuous tide? I doubt it. Lifetime fidelity to a single partner may be rare among animals, and even among humans (whether it should be likened to corn syrup, as the quote at the beginning of the chapter says, I leave up to the reader), but the sheer variation in mating systems among human societies in both space and time makes it unlikely that we have all been ignoring our true natures. If evolution favored a single marriage or sexual system, why would we not all have converged on that pattern?

The piece of the puzzle that still needs addressing is the real evolutionary payoff: the “F” word. No, not that “F” word; several of the musings on human sexuality are as explicit as anyone could want. I am referring instead to “fitness,” the biological term for success at passing on one’s genes. A mating system will persist if its followers are better than others at having offspring that survive and reproduce themselves. How humans do that is the topic of the next chapter.

8

The Paleofantasy Family

H
uman babies are extremely peculiar little organisms. Children and the way they are raised are among the most unusual things about us. Our sex lives, while obviously of highlighted importance to us, are nevertheless in keeping with many other species: we have definite preferences for those we find attractive, we show off to prospective mates, and we compete with each other for access to those we favor. If you held your head the right way and squinted, we could practically pass for marmosets. Well, the lack of fur and the use of indoor plumbing would give us away, but you get the point. Monogamy is rare among animals, as we have seen, but hardly unheard of.

In contrast, no other species—primate or not—has such disproportionately large, demanding, slow-growing offspring. A comment on Paleohacks.com declares, “Human babies are wimps! They take FOREVER to become independent too! There’s no other animal that takes anywhere CLOSE to the same length of time to start feeding themselves. What’s up with that?”
¹

What’s up with it indeed? Understanding how our slow development came about, and how our child-rearing practices do and don’t reflect our evolutionary history, goes far toward explaining what makes us human—sex lives and all. What’s more, sex means nothing in an evolutionary sense without, as I mentioned in Chapter 7, fitness, the bequeathing of our genes to succeeding generations. And fitness for humans means babies. If a practice enables our babies to be born and survive, evolution favors it. If a practice feels great, or even enables the practitioner to live longer, it doesn’t matter from the standpoint of evolution if the long life or enhanced well-being is not translated into offspring, the currency of the biological realm. (Appearances to the contrary, children are like money.)

Let’s examine the anomaly that is the human child. First, a comparison with our primate relatives. Anyone who has watched a mother chimp or gorilla at the zoo has seen the casual confidence with which infants are hauled around, their tiny faces bobbing behind or beneath the female as they cling to her fur. The mother does not have to work to keep the baby attached; as Sarah Hrdy notes, “Minutes after birth, possibly while the mother is still consuming the placenta, the tiny spidery newborn ape on the ground beside her will catch hold of her hairy belly and pull himself aboard.”
²
Baby monkeys and apes are born able to do things that human infants can only dream of doing—assuming, that is, that their nervous systems were advanced enough to allow them to imagine an alternative, which they are not. Numerous researchers have suggested that human infants are in effect always born prematurely, with the nine months
in utero
only the beginning and an additional nine to twelve months required for the infant to reach a stage comparable to that of most other primates at birth.

Such prematurity is thought to be a necessary consequence of our large brains. A human infant is about 6 percent of its mother’s body weight, while a baby chimpanzee is only 3 percent, although the adult females in both species are not so very different in size. Jeremy DeSilva of Boston University calculated the ratio of mother to infant body size in a number of fossil hominins, as well as apes, and suggested in a 2011 paper that our hefty babies evolved with
Australopithecus
, over 3 million years ago, considerably earlier than other anthropologists had believed.
³
DeSilva notes that lugging around such large infants would have been difficult for those early ancestors, lacking strollers and Snuglis as they did, which also means that
Australopithecus
would have had to give up life in the trees for a more terrestrial existence—a transition that again was thought to have occurred later in our evolution. Interestingly, anthropologist Tim Taylor suggests that the baby sling might have been invented by early humans, as a way to free the hands of bipedal animals, which in turn would have allowed the evolution of ever more large-brained and immobile infants.
⁴
This hypothesis is difficult to test, however, since the fibers that would have been used for such slings would not persist as archaeological artifacts.

That relative immaturity of our young persists into childhood. It is true that other apes nurse their offspring for longer than humans do, even in less westernized societies; chimpanzee infants, for example, wean at about four and a half years, while in many cultures children stop nursing after two to three years. But ceasing breast-feeding by no means signals independence in humans as it does in the other primates. Although they stop using their mother as an exclusive food source, young humans proceed to lollygag about before reaching sexual maturity. Other primates get on with their own reproduction relatively soon after weaning, with gorillas having their first baby just seven or so years later.

In effect, human beings invented childhood itself, an extended betwixt-and-between stage when a mother is less essential than in infancy but the youngster still cannot survive on its own. In contrast, a chimp orphaned or abandoned right after weaning at least has a fighting chance, as studies of wild primate populations show. And beyond childhood in humans is adolescence, another developmental stage virtually unknown among other mammals—occurring after sexual maturity but before having children of one’s own. Even in nonindustrialized societies, girls do not start having children the moment they reach menarche; the average age worldwide for a first baby is nineteen, according to a 2008 article in
Science
.
⁵

Play between adolescent chimp male Titan and little Gimli. Individuals besides the mother often interact with youngsters.
(Courtesy of Michael Wilson)

At the same time, ceasing nursing at a relatively early stage means that human mothers can have children more frequently than other primates can. Our “interbirth interval,” the formal term for the time between the births of offspring, averages just over three years, whereas gorillas take nearly four years and orangutans a whopping nine. This ability to reproduce quickly is part of why our species has been so successful; in the same amount of time, and with the same starting population size, you can make a lot more people than orangutans. And those children are more likely to survive to adulthood, again even discounting the influence of modern medicine and hygiene, probably because of that long period of dependence.

What, then, is that lengthy period between weaning and adulthood for? And what does it tell us about the evolution of the human family, and about our paleofantasies?

Successfully failing to launch

Anthropologists often invoke one unusual human attribute when trying to explain another. For example, our extraordinary capacity for complex communication is sometimes linked to our complicated social structure, or our tool use to our large brains. When it comes to childhood, therefore, it was reasonable to suggest that a prolonged period before independence was required once humans began to perform difficult tasks, like hunting or making pottery and baskets. Children could spend their time practicing these skills, which would better prepare them for success as adults in a hunter-gatherer society. In effect, this idea would mean that children are schooling themselves, and were doing so long before formal education was invented. Many childhood games seem to be dress rehearsals for adult activities, and for our ancestors, instead of playing doctor, teacher, or princess, they might have practiced tracking prey or finding edible plants. (This leaves aside the question of just what those princess mimics are hoping to achieve, since presumably most of the Cinderella wannabes lack even a trace of royal blood, but that is an issue for another day.)

The problem is that, reasonable though this theory sounds, little evidence exists to support it. In at least one modern-day hunter-gatherer society—the Hadza people of Tanzania—some children grow up in the bush while others attend boarding school, where they learn to read and write but have little opportunity to acquire skills such as bringing down animals with a bow and arrow. A 2002 publication by anthropologists Nicholas Blurton-Jones of UCLA and Frank Marlowe of Harvard compared the skills of the two groups, as well as those of women, who grow up digging tubers in anticipation of their adult role, with those of men, who do not.
⁶
They paid the Hadza to participate in a kind of hunter-gatherer Olympics, with contests in various skills.

It turned out that the men were just as efficient at digging as the women, despite their lack of experience. And climbing baobab trees, “an important and dangerous skill” according to the anthropologists, was performed no better by young men who had been practicing during childhood than by those who were novices. It isn’t that hunting and gathering aren’t difficult, but that the critical time period for learning these skills may not coincide with childhood at all; the most effective Hadza hunters were around forty years old, suggesting that men needed to practice not just while playing as children, but as adults.

Blurton-Jones and Marlowe do not completely dismiss the idea of childhood as rehearsal, noting that skills other than those they measured may require key practice during maturation. But, they caution, “it is not safe to assume that increases in skill with age are entirely due to learning or practice; they may instead be due to increases in size and strength.”
⁷
A similar comparison of adults and children of the Meriam people from the Torres Strait, Australia, also showed that the two age groups had comparable skills at fishing both with lines and with spears, despite the difficulty of these tasks.

Perhaps, then, the skills that must be learned during childhood are social, not physical. Given our complicated and subtle interactions, and the cutthroat nature of politics even in the smallest social groups, this is a reasonable supposition. But it, too, has had doubt cast on it; as anthropologist Meredith Small from Cornell University puts it, “Recent research suggests that children know more about social relations than we give them credit for, and they learn the ropes early.”
⁸
Joking about schoolyard politics aside, even toddlers can resolve conflicts on their own, and eight-year-olds have social skills that enable them to see things from another’s perspective and anticipate someone else’s intentions. Little support exists for the idea that children must spend a long time as social apprentices before they can be successful members of a group.

What if, then, childhood evolved not for the sake of the children themselves, but for their parents? Anthropologist Barry Bogin suggests a rather cold-blooded analysis along these lines, in which, as he puts it, “childhood may be better viewed as a feeding and reproductive adaptation for the parents of the child” than as a period of coddled maturation.
⁹
Children in many societies perform useful work, ranging from care of crops or animals to help with their younger brothers and sisters, and they usually receive no compensation beyond their keep; allowances are a modern invention. In an even more Dickensian spirit, Bogin points out that “children are relatively inexpensive to feed.”
¹⁰
Youngsters past weaning age thus can give back labor to help offset what they consume, enabling humans to reproduce at our uniquely rapid clip.

Meerkats, mothers, and sharing apes

In addition to having offspring with oddly prolonged childhoods, the human family is noteworthy for another unusual attribute: the common presence of multiple caregivers in addition to the mother. Human babies, like other infant primates, seek close and near-constant contact with their mothers. When anthropologists began studying maternal behavior in hunter-gatherer societies, they emphasized the way in which foraging women kept their babies close to them while they were digging for roots or picking berries. They were struck by how similar these women with infants slung on their backs seemed to be to any monkey or ape mother. This natural-seeming connection, apparently universal among our relatives, is part of what led to the attachment theory of parenting, in which the bonding between mother and infant is said to be an essential part of child development.

What the anthropologists did not notice, though, is potentially more significant than what they did. Like baby baboons and gorillas, human infants in contemporary hunter-gatherer societies are usually held by an adult, and babies spend little or no time entertaining themselves in the desert or forest equivalent of a playpen. The crucial difference is who’s doing the holding. Among nonhumans, that individual is the mother most of the time, except for brief periods in which other troop members examine a newborn addition to the group. Even this limited sharing of infants varies considerably among different monkey and ape species, with langurs (Old World monkeys native to India) passing their babies to a host of aunts, cousins, and siblings almost immediately after birth. Red colobus monkey mothers are far more reluctant and will not allow their babies to even approach another female until they are three or four months old. Marmosets and tamarins, diminutive New World monkeys, are exceptions, with the father taking on the majority of child care virtually from birth—an unusual circumstance that I will discuss further later in the chapter.