Seen here looking eerily like several of my relatives, an artist's rendition of Saccorhytus smiles for the camera. Saccorhytus, which were about one millimeter long, lived some 540-million years ago and are believed to be the earliest known ancestor to humans and a bunch of other species. Interestingly, Saccorhytus had a mouth but no anus, meaning the mouth was used for both consuming food and expelling waste. That's nasty. We sure have come a long way, haven't we? 540-million years ago we didn't have one at all, and now it seems like a lot of people are almost entirely anus. Oh, evolution.
Thanks to Mark B, who's convinced 540-million years from now we won't have mouths or anuses.