This mutation may explain how humans lost their tails
Some lucky ape-like ancestor got it 20 million years ago.

The oldest known primate fossils are around 66 million years old, around the same time the dinosaurs went extinct. These ancestors had full-fledged tails that were likely handy when living in canopies. The utility of this flexible body part can be attested to by the fact that even after all these years, most living primates and the vast majority of monkeys still sport a tail.

On the other hand, by the time Proconsul, the most primitive ape that is well-known from a fossil skeleton, appeared some 20 million years ago, it had no tail at all.

Why exactly humans and their closest relatives lost their tails has been a matter of debate for some time. What biologists have noticed is that apes can walk with upright stances thanks to a shorter lumbar region enabled by the absence of the tail. Meanwhile, new world monkeys use all fours.

As such, the loss of the tail can be seen as an adaptation to a particular environment, which allowed our ape ancestors to leave trees and walk on the ground. With some adjustments, early humans could not only walk but jog over the grassland.

zmescience.com