Humans is the only species who uses the bipedal walk ability all the time, which changed the structures of our body, on of those structures is the gluteus maximus aka ass cheek muscles, they're evolved to be bigger and bulkier, which means tighter hole that the shit can go in between. This causes for a dirty exit when you take a dump. Which also explains why we clean our ass(wipe) every time we take a shit. Plus it's just common sense, dirty ass = bad bacteria, bad bacteria=sickness, sickness eventually leads to death if not treated.
