What makes us different from all the other animals? Is it our swollen brains, our idle hands, or perhaps our limber thumbs? In 2011, a research team reviewed the quirks of human DNA and came across another oddly shaped appendage that makes us who we are: I mean, of course, man's smooth and spineless member. The penises of lots of mammals are endowed with "horny papillae," hardened bumps or spikes that sometimes look like rows of studs on a fancy condom. These papillae enhance sensation, or so it has been claimed, and shorten a mating male's delay to climax. Since humans lost their phallic bumps several million years ago, it could be that we evolved to take it slow. And it could also be the case that longer-lasting sex produced more intimate relationships.
So (one might argue that) the shedding of our penis spines gave rise to love and marriage, and (one could also say that) our tendency to mate in pairs pushed aside the need for macho competition, which in turn gave us the chance to live together in large and peaceful groups. Life in groups has surely had its perks, not least of which is that it led to bigger brains and a faculty for language, and perhaps a bunch of traits that served to civilize and tame us. And so we've gone from horny papillae to faithful partners—from polygamy to monogamous humanity.
I like this story well enough, but it may or may not be true. In fact, not all penis spines in nature serve to quicken sex—orangutans have fancy ones but waste a quarter of an hour in the act—so we don't know what to make of our papillae or the lack thereof. That won't stop anyone from wondering.
Since we like to think that how we mate defines us, the sex lives of ancient hominids have for many years been examined in computer simulations, by measuring the circumferences of ancient bones, and by applying the rules of evolution and economics. But to understand the contentious field of paleo-sexology, one must first address the question of how we mate today, and how we’ve mated in the recent past.
According to anthropologists, only 1 in 6 societies enforces monogamy as a rule. There's evidence of one-man-one-woman institutions as far back as Hammurabi's Code; it seems the practice was further codified in ancient Greece and Rome. But even then, the human commitment to fidelity had its limits: Formal concubines were frowned upon, but slaves of either sex were fair game for extramarital affairs. The historian Walter Scheidel describes this Greco-Roman practice as polygynous monogamy—a kind of halfsy moral stance on promiscuity. Today's Judeo-Christian culture has not shed this propensity to cheat. (If there weren't any hanky-panky, we wouldn't need the seventh commandment.)
In The Myth of Monogamy, evolutionary psychologists David P. Barash and Judith Eve Lipton say we're not the only pair-bonding species that likes to sleep around. Even among the animals that have long been known as faithful types—nesting birds, etc.—not too many stay exclusive. Most dally. "There are a few species that are monogamous," says Barash. "The fat-tailed dwarf lemur. The Malagasy giant jumping rat. You've got to look in the nooks and crannies to find them, though." Like so many other animals, human beings aren't really that monogamous. Better to say, we're monogamish.
That –ish has caused no end of trouble, for lovers and for scientists. Efforts to define our sexual behavior often run afoul of humans’ in-between-ness. Take one common proxy measure of how a primate species copulates: testis size. A male that's forced to share its partners might do well to make each ejaculation count by firing off as many sperm as possible. Chimpanzees mate rather freely and show a high degree of male-male competition. They also have giant balls, for blowing away their rivals'. Gorillas, on the other hand, have their sexual dynamics more worked out: The alpha male has all the sex; the other males are screwed. Since there's less chance of going head-to-head on ejaculations, tesis size isn't so important. Gorilla balls are pretty small. And what about a man's testes? They're not so big and not so little. They're just eh.
Male gorillas may not one-up each other with their testes, but they do rely on other traits to get and keep their harems. That's why male gorillas are so huge and fearsome: so they can fight off other males for social dominance. Within a species, the difference between the male and female body type yields another proxy for mating habits: The bigger the gap in body size, the more competitive the males, and the greater the inclination toward polygynous arrangements. So how does the split between human men and women compare to that of other primates? We're sort of in the middle.
Seeing as we're neither one thing nor the other, scientists have been left to speculate on how our ancestors might have done their thing. Were they like gorillas, where most males suffered while one dude enjoyed the chance to spread his seed? Or more like chimpanzees—sleeping around, with males competing for multiple partners? Or is there another possibility, like the one championed by Christopher Ryan and Cacilda Jethá in their best-selling and soundly criticized paean to free love, Sex at Dawn? According to that book's authors, our ancestors did as bonobos do: They had rampant sex without much bickering.
Such discussions tend to dead-end quickly, though, since we just don't know for sure. Our most recent relatives in common with these other primates lived about 6 million years ago. (I suppose if bonobos could be anthropologists, one of them might write a book on whether bonobo sexuality evolved from something humanlike.) "What this really is," says Barash, "is a Rorschach test for the people asking the question."