Some of the most intriguing (and NSFW) back ends are all-purpose anus analogues called cloacae, which merge the terminal parts of the digestive, urinary, and reproductive tracts into a single opening—essentially an evacuation foyer for outbound feces, urine, eggs, and sperm. Cloacae are fixtures among birds, reptiles, and amphibians, and although they tend to get a bad rap, their internal architecture is actually quite sophisticated, Patricia Brennan, a cloaca expert at Mount Holyoke College, in Massachusetts, told me. They can also be quite convenient: When female birds mate with unsatisfactory males, they can simply eject the subpar sperm and begin the process anew. Cloacae have been around for so long, Hejnol added, that they could even represent the evolutionary bridge between the reproductive and digestive tracts that helped lead to some of the first anuses.
Still, cloacae come with risks: “You have all your digestive waste pretty much in direct contact with genitalia,” basically a gnarly infection just waiting to happen, Brennan said. Any live young who pass through the reproductive tract could also be imperiled by the proximity to poop-borne pathogens. Perhaps that’s why human anuses ventured off on their own.
Whatever the reason behind it, the partitioning that did away with the cloaca made human anuses, as Manafzadeh said, “completely boring.” As far as exit holes go, ours are standard-issue, capable of little more than extruding waste from the gut, with no frills to speak of.
The only redeeming quality of humans’ humdrum posterior hole is the feature we evolved to cushion it: our infamous buttocks, the most voluminous one documented to date, thanks to our bizarre tendency to strut around on our two primate legs. “Our bipedalism is obligate; it’s special; it’s the only way we get around,” Darcy Shapiro, an anthropologist, told me. That pattern of locomotion reshaped the pelvis, which in turn reoriented our muscles. The gluteus maximus—the hefty muscle that powers our ability to run and climb—swelled in lockstep, and blanketed itself in a cozy layer of fat that some scientists think serves as an energy reserve. Anuses aside, “our buttocks are the real innovation here,” Manafzadeh said.
Evolution blew the human butt out of proportion; our cultural norms quickly followed suit. We regard one another’s bums with lust, disgust, and guilty fascination. We shrink them, we sculpt them; we sexualize them. We rap about them with abandon. They, in return, make it much easier to sprint, but much harder to keep our rear ends clean. Our anus is a sheep dressed in a very fabulous wolf’s clothing, and we simply cannot deal.
Maybe that’s part of why humans are so often embarrassed of their posteriors, and, by extension, so many others. We even opt for butt as a euphemism for anus in casual conversation. Buttocks aren’t anuses, but they do cloak them, physically and perhaps figuratively. They obscure the idea that, from its very start, our digestive end has been a wonder. It cracked open our ancestors’ evolutionary path, and made our own existence possible. Maybe it’s time we made like a pearlfish, and got comfortable with what’s between those cheeks.