Though it’s currently useless, the human coccyx—commonly referred to as the tailbone—remains nestled at the bottom of the spine, a remnant of our tailed ancestors. Long before human tail-lessness, our early fish relatives had two: A fleshy one and a more flexible fin. As animals took to land, they lost the back fin and kept the fleshier appendage.
Eventually, apes ditched them. These primates no longer required tails to maintain balance or send social signals.
Meanwhile, we humans have held onto a tailbone—several fused caudal vertebrae of the same sort found in actual tails—for no apparent reason. Neither harmful nor beneficial, it simply sits attached to the sacrum.
Scientists call these evolutionary holdovers “vestiges”: The term comes from the Latin word vestigium, meaning footprint. Vestigial features can range from anatomical structures like bones and organs to physiological phenomena like goosebumps; when our furrier ancestors got cold, their hairs stood up to provide insulation.
Though scientists disagree on the specifics, the standard definition claims that vestiges lack a present function.