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.)