One of the most striking anatomical features in apes, and which sets them apart from monkeys, is the absence of a tail. All mammals have a tail at some point during their development, but apes – including humans, chimpanzees, bonobos, gorillas, orangutans, and gibbons – lose them in utero, leaving behind three to five vestigial vertebrae called the coccyx, or the tailbone.
Apes started to lose their tails in this way around 25 million years ago when the ape and monkey lineages split from a common ancestor. And until recently, nobody knew why apes started to do this.
