Why humans and apes lack tails

One of the most striking anatomical features of apes, 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 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.

Twenty-five million years ago, after the ape and monkey lineages separated, a chance insertion of an Alu element (Alu Accident) occurred in an important gene in the zygote of an ancient creature.

Humans are primates, and are classified along with all other apes in a primate sub-group known as the hominoids.