The female orgasm has been shouldering its 'mysterious' reputation for centuries. The male orgasm is simple to understand - it quite literally facilitates human reproduction.
Why the female orgasm exists, and how it survived millions of years of evolution is a whole lot less clear, but a pair of evolutionary biologists think they might finally have the answer.
It might not feel like it right now, as you slump over your computer screen and feel annoyed about your various aches and pains and why you're always tired, but your body represents the most effective version of the human species that has ever existed.
Every part of you is there for a reason - your ears hear sounds, your kidneys process waste, and your body hair helps regulate your internal temperature.
Even the parts that don't play any discernible role other than taking up space - your wisdom teeth, appendix, and tailbone - are there for a reason. Not because we need them, but because they're not 'costly' enough to have been phased out by natural selection through the millennia of human evolution.
Unlike wisdom teeth, the female orgasm still serves plenty of important roles, particularly in terms of strengthening intimate relationships, and at the most basic level, it's a source of free, healthy, good old-fashioned pleasure.
But with all the parts that have to come together to achieve an awesome combination of muscle contractions, hormone release, and intense pleasure - which only occurs 69 percent of the time in the average heterosexual encounter - it's a lot of work, evolutionarily speaking, for a bit of fun.
Now, evolutionary biologists Mihaela Pavličev from the Cincinnati Children's Hospital and Günter Wagner from Yale University have come up with a new hypothesis that could explain why the female orgasm came to be - it might once have been as psychologically vital to human reproduction as the male orgasm is now.
"It is important to stress that it didn't look like the human female orgasm looks like now," Pavličev told Nicola Davis at The Guardian. "We think that [the hormonal surge] is the core that was maybe modified further in humans."
Pavličev and Wagner decided to look at the female orgasm in the context of other placental mammals. In rabbits and cats, hormonal surges also occur during sex, but instead of imparting pleasure, their role is to signal to the ovaries to release eggs - something known as male-induced ovulation.
While rabbits and cats only release an egg during sex, in humans, our eggs are released spontaneously each month - and not because of a male-induced hormone signal.
But in tracing the history of ovulation through the mammalian evolutionary tree, Pavličev and Wagner found that male-induced ovulation actually existed earlier than spontaneous ovulation, and pinpointed its origin in a common ancestor of primates and rodents that lived some 75 million years ago.
"That, they say, suggests that human female orgasm could have its roots in a mechanism for the release of eggs during sex - a mechanism that became redundant with the evolution of spontaneous ovulation, with female orgasm potentially going on to acquire other roles," says Davis.
Interestingly, the position of the clitoris in women today supports this hypothesis, the researchers say, because when male-induced ovulation was the norm, the clitoris was located inside the vagina, and has since moved outside, as Carl Zimmer explains for The New York Times:
"When early mammals mated, the clitoris could send signals to the brain, triggering hormones that released an egg. Once the egg became fertilised, the hormones may have helped ensure it became implanted in the uterus."
Once spontaneous ovulation kicked in for humans, the clitoris moved away from this position, so as not to confuse the body with conflicting signals. "You don't want to have the old signal sending noise at the wrong time," Wagner told Zimmer.
Of course, this is just an hypothesis for now, and it will need to be debated by other scientists and supported by evidence before it can rise above the dozens of other hypotheses about the female orgasm that have failed to achieve consensus.
And, as Sarah Emerson points out at Motherboard, this whole hypothesis is based on us trying to equate the function of the female orgasm with the function of the male orgasm, and that could be sending us down the wrong road entirely.
Maybe it doesn't need to have an evolutionary purpose at all, as Elisabeth A. Lloyd, a philosopher at Indiana University argues. Maybe like nipples, the female orgasm evolved alongside the male orgasm, one version ending up with a very strong evolutionary purpose, the other, not so much.
"It all seems to be rather purposeless - except for the enjoyment, obviously," she told The Guardian. "It doesn't mean it is not important, it just means it doesn't have an evolutionary purpose."
The study has been published in The Journal of Experimental Zoology.