Wow. Flashback to all my zoology classes. “One of the defining characteristics of a vertebrate is a post-anal tail.” Along with a notochord and gill slits and a couple of others that have slipped my mind. Notice that this means we are not vertebrates. Damn.
Actually, we slip by thanks to our embryonic stages, in which we have all of the above.
Why do most land vertebrates have tails? To keep their pants up. (Ba-dump-bump)