Humans lost their tails for a fragment of DNA - Biotech. If today we are able to walk elegantly in an upright position without having to use a long tail as a balance wheel, we probably owe it to a small fragment of wandering DNA.
25 million years ago was randomly inserted into a gene responsible for the development of the tail, making it disappear. The change would have been very useful in the transition from life in trees to life on the ground and would have maintained by evolution despite increasing the risk of congenital defects.