Where do butts come from?

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.

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 buttocks—the 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.

--
Trump won.