The Body’s Most Embarrassing Organ Is an Evolutionary Marvel

A cat in rear view
Arne Svenson, Stray #55

To peer into the soul of a sea cucumber, don’t look to its face; it doesn’t have one. Gently turn that blobby body around, and gaze deep into its marvelous, multifunctional anus.

The sea cucumber's posterior is so much more than an exit hole for digestive waste. It is also a makeshift mouth that gobbles up bits of algae; a faux lung, latticed with tubes that exchange gas with the surrounding water; and a weapon that, in the presence of danger, can launch a sticky, stringy web of internal organs to entangle predators. It can even, on occasion, be a home for shimmering pearlfish, which wriggle inside the bum when it billows open to breathe. It would not be inaccurate to describe a sea cucumber as an extraordinary anus that just so happens to have a body around it. As Rebecca Helm, a jellyfish biologist at the University of North Carolina at Asheville, told me, “It is just a really great butt.”

But the sea cucumber’s anus does not receive the recognition it deserves. “The moment you say ‘anus,’ you can hear a pin drop in the room,” Helm said. Bodily taboos have turned anuses across the tree of life into cultural underdogs, and scientific ones too: Not many researchers vocally count themselves among the world’s anus enthusiasts, which, according to the proud few, creates a bit of a blind spot—one that keeps us from understanding a fundamental aspect of our own biology.

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The appearance of the anus was momentous in animal evolution, turning a one-hole digestive sac into an open-ended tunnel. Creatures with an anus could physically segregate the acts of eating and defecating, reducing the risk of sullying a snack with scat; they no longer had to finish processing one meal before ingesting another, allowing their tubelike body to harvest more energy and balloon in size. Nowadays, anuses take many forms. Several animals, such as the sea cucumber, have morphed their out-hole into a Swiss Army knife of versatility; others thought that gastrointestinal back doors were so nice, they sprouted them at least twice. “There’s been a lot of evolutionary freedom to play around with that part of the body plan,” Armita Manafzadeh, a vertebrate morphology expert at Brown University, told me.

But anuses are also shrouded in scientific intrigue, and a fair bit of squabbling. Researchers still hotly debate how and when exactly the anus first arose, and the number of times the orifice was acquired or lost across different species. To tap into our origins, we’ll need to take a squarer look at our ends.

In the beginning, there was nothing. The back ends of our animal ancestors that swam the seas hundreds of millions of years ago were blank, relegating the entry and exit of all foodstuffs to a single, multipurpose hole. Evolutionary echoes of these life-forms still exist in corals, sea anemone, jellyfish, and a legion of marine worms whose digestive tract takes the form of a loose sac. These animals are serially monogamous with their meals, taking food in one glob at a time, then expelling the scraps through the same hole. (Contrary to what you might have read, not everyone poops.) These creatures’ guts operate much like parking lots, subject to strict vacancy quotas that restrict the flow of traffic.

The emergence of a back door transformed those parking lots into highways—the linear “through-guts” that dominate body plans today. Suddenly, animals had the luxury of downing multiple meals without needing to fuss with disposal in between; digestive tracts lengthened and regionalized, partitioning into chambers that could extract different nutrients and host their own communities of microbes. The compartmentalization made it easier for animals to get more out of their meals, Andreas Hejnol, a developmental biologist at the University of Bergen, in Norway, told me. With the lengthening and uncorking of the end of the gut, he said, many creatures grew into longer and larger body forms, and started to move in new ways. (It would take several more eons for true buttocksthe fleshy, fatty accoutrements that flank the anuses of some animals, such as humans—to evolve. Some researchers I talked with are comfortable using butt to mean any anal or anus-adjacent structure; others are purists, and consider the term strict shorthand for buttocks and buttocks alone.)

The benefits of bottoming out the gut are clear; how the back door was excavated isn’t. Soft, squishy, bone-free holes aren’t exactly fixtures of the fossil record, making just about any anus-heritage theory tough to prove. One of the oldest hypotheses holds that the anus and the mouth originated from the same solo opening, which elongated, then caved in at the center and split itself in two. The newly formed anus then moseyed to the animal’s posterior. Claus Nielsen, a developmental biologist at the Natural History Museum of Denmark, is a fan of this theory. It’s both reasonably parsimonious and evolutionarily equitable: In this scenario, neither the mouth nor the anus technically arose first; they emerged as perfect developmental twins.

Hejnol and others favor a different idea, in which the mouth formally preceded the anus, which spontaneously burst through the other end of the body. “It’s a secondary breakthrough,” Hejnol said. “The gut forms, then [makes] a connection to the outer world.” Punching an extra hole in the body is not so difficult: Some worms have managed the feat dozens of times over. One unusually aerated specimen, a type of polyclad flatworm, sports multiple anuses that speckle its backside like feces-spewing freckles. Two others, a pair of sponge parasites called Syllis ramosa and Ramisyllis multicaudata, will twine their body through host tissues like a tapestry of tree roots, with each tip terminating in its own proprietary butthole; they have hundreds, perhaps thousands, in total. (It’s not totally clear why these animals and others spawned an embarrassment of anuses, but in at least some cases, Hejnol thinks it’s a logical outcome of a branched digestive system, which can more easily transport nutrients to a body’s every nook and cranny.)

Hejnol and his colleagues are still amassing support for their hypothesis, but he said there’s already some argument against the hole-splitting idea: Animals don’t generally express the same genes around their mouth and anus, a knock against the notion that the two openings are cut from the same developmental cloth. A better backstory for the orifice, he said, might involve a body plan stolen from the reproductive tract, which already naturally terminates at the animalian posterior.

If that theory pans out, though, it won’t necessarily close the case on the anus’s evolutionary start. A cursory glance at the animal tree of life might at first suggest that anal openings appeared about 550 million years ago, around the time our own bloblike ancestors straightened out into tubes. But Hejnol and many others think that the anus was so useful that animals independently evolved it at least half a dozen times, perhaps many more, and not necessarily in the same way. This timeline has other snags: Some creatures have since lost their anal opening—and some might have made theirs even further back in history.

One of the largest potential wrinkles in the smooth anus narrative takes the shape of a comb jelly—a gelatinous animal that vaguely resembles a translucent Darth Vader helmet and is thought to be at least 700 million years old. As far back as the 1800s, scientists have been puzzling over comb jellies’ back end, and whether they were excreting formal feces from a set of strange-looking pores. More than a century passed before their acts of defecation were finally caught on camera, by the biologist William Browne of the University of Miami and his colleagues, who filmed one of the amorphous creatures taking a big fishy dump in the lab. When the clip debuted at a 2016 conference, “everyone in the hall audibly gasped,” Helm, who attended the lecture, told me. If comb jellies were pooping, that poop had to be coming out of some sort of rear hole. Perhaps, some said, the history of the anus ran far deeper in time than many had thought.

In the months after Browne’s team published its findings, scientists sparred repeatedly over their significance. Some hailed the discovery as revolutionary. But others, Hejnol among them, argued that the now-infamous video didn’t signify all that much dogmatic change, and may not be hard to reconcile with what’s long been known. Comb jellies probably cooked up their anuses independently of other animals, and happened upon a similar blueprint by chance; there’s no telling when exactly that might have occurred. Such a scenario would leave the chronology of our own anus, which emerged out of a different line of creatures at a separate point in time, intact.

The various possibilities aren’t easy to prove or disprove. Just as new apertures can rupture into being, useless ones can disappear, as seems to have been the case with brittle stars and mites, which stitched their ancestral anuses shut. Some ambivalent creatures might even gouge out transient anuses—holes that come and go on an as-needed basis. (A 2019 study by the biologist Sidney Tamm suggested that some comb-jelly anuses could fall into this category of “sometimes-butts,” as Manafzadeh calls them.)

Many of the animals that have managed to keep some version of the anus embellished upon it, and now harbor an organ of immense extravagance. Turtles, like sea cucumbers, breathe through their butt. Young dragonflies suck water into theirs, then spew it out to propel themselves forward. Scorpions jettison their posterior when attacked from behind, evading capture but tragically losing their ability to poop (and eventually dying with their abdomen full of excrement). Lacewing larvae incapacitate termite prey with the toxic flatulence they emit from their end—“they literally KO their enemies with death farts,” Ainsley Seago, an entomologist at the Carnegie Museum of Natural History, told me.

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.