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.