1.38-1.31 million years ago
As we noted in the last post, human females conceal ovulation (no chimp-style monthly sexual swellings) but advertise nubility (with conspicuous fat deposits). Presumably this has to do with sexual selection, via male mate choice. But sexual selection may have operated in the opposite direction, on male anatomy, as well.
Males of most primate species have a baculum, or penis bone. Human beings and spider monkeys are the exceptions. (A mnemonic: the mammals with penis bones are PRICCs – primates, rodents, insectivores, carnivores, chiropterans=bats.) The baculum helps to retract the penis when it’s not in use, so males in our species, lacking a penis bone, have more conspicuous dangling organs than most primate males.
This information comes from a recent book The Evolution of Beauty: How Darwin’s Forgotten Theory of Mate Choice Shapes the Animal World – and Us, by Robert Prum. Prum also cites a paper arguing that Adam’s “rib” (Hebrew tsela), the thing God used to make Eve (Genesis 2:21-23), was actually his baculum, providing a creationist explanation of “congenital human baculum deficiency.” The book contains lots of interesting tidbits like this, although its central argument – that sexual selection via mate choice is largely a result of non-adaptive aesthetic preferences – is shaky.
Men’s penises lack something else found in most primate species: most male primates have keratinized spines on their penises. But a gene involved in the development of penis spines got turned off in our evolutionary lineage, some time after our split with chimps, but before our split with Neanderthals. We’re not sure why. Penis spines might be favored in promiscuously mating species if they help one male dredge out sperm left by earlier matings with other males. So (relative) monogamy in our lineage might remove the evolutionary advantage of spines. But a non-spiny penis might also be less sensitive, and make for more prolonged intercourse.
If all this doesn’t answer the question “What do women want?”, it at least narrows down the possibilities a bit: not men with bony, spiny penises, apparently.