Butts - An Evolutionary Marvel

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.

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