"The moment you say 'anus,' you can hear a pin drop in the room." So a jellyfish biologist at the University of North Carolina at Asheville tells Katherine Wu, who has written a fascinating piece for the Atlantic about this "evolutionary marvel" that also happens to be "the body's most embarrassing organ." It's worth reading her piece for the lead alone (a brief ode to the sea cucumber's multi-duty anus), and the high school biology class refresher you'll get. The dawn of the anus was a huge step in animal evolution, transforming "a one-hole digestive sac into an open-ended tunnel." Eating and defecation no longer happened out of the same orifice, which is good for fairly obvious reasons, and creatures no longer had to fully digest a meal before having another—a twist of events that spurred massive growth.
But for as much as we know about how the anus has benefitted the animal kingdom, we know relatively little about how and when the anus came to be. Wu digs in. Theories are split between whether the mouth and anus formed at the same time ("originat[ing] from the same solo opening, which elongated, then caved in at the center and split itself in two") or whether the mouth came first, and the anus sort of magically appeared, with the gut forming and then the hole to the outside following. Just as hotly debated is when they arose (the significance of a 2016 video of pooping comb jellies is still being argued). But what is pretty plain is that our own anuses are ... pretty plain, and Wu builds her case. "Young dragonflies," for instance, "suck water into theirs, then spew it out to propel themselves forward." (Read for full piece for many more fascinating details on the organ whose name we're loathe to speak.)