The role of the female orgasm has been a head-scratcher for centuries. Case-in-point: Aristotle himself noted that the fact that human females don't need it to conceive clouded the quest for explanation. The statistics that show it's an "uncommon" occurrence during heterosexual intercourse and the lack of correlation between orgasm and number of offspring deepen the mystery—which scientists at Yale and the Cincinnati Children's Hospital may have "solved," reports the Guardian. Science reports there are about a dozen theories out there (read a few here); here's the upshot of this one, published in the Journal of Experimental Zoology: The human female orgasm is rooted in an ancestral kind of ovulation present in some mammals (cats, rabbits) today, in which sex triggers a surge of hormones which then triggers the egg's release.
Gunter Wagner and Mihaela Pavlicev explain that with male-induced ovulation, the surge of oxytocin and prolactin triggers eggs to be released. Human females obviously don't ovulate that way: Our spontaneous ovulation has no dependence on sexual activity. But the human female orgasm is still accompanied by that surge of oxytocin and prolactin, explains a press release. That suggests it's a remnant of that ancestral kind of ovulation, which the Guardian reports was replaced with spontaneous ovulation in the ancestor of primates 75 million years ago. As Science puts it, "because those hormonal surges no longer confer a biological advantage, orgasms during intercourse may be lost in some women." Wagner tells STAT the female orgasm isn't going anywhere though, because one body of tissue becomes either the penis or clitoris in the human fetus. "If the clitoris went away through evolution, so may the penis." (Sweden is trying to find out just how good sex is in Sweden.)