Tails.

A horse will use its tail to swat away flies, and birds use their tails to help steer themselves whilst flying. Animals which have evolved with tails use them for important things such as Lemurs will use their tails when climbing. So why do humans develop to not have tails?

1. Humans do not need tails. Because we stand on two legs we are perfectly balanced due to gravity, animals need tails to help stabilise them for the weight of their heads.
2. We also don't need a tail to grip onto things such as tree branches.
When a human is in the womb for 30 days it will have a tail like structure, although most people re-absorb the structure as we develop, some people have been born with tails. This is known as atavism, a trait of distant ancestors that reappears  in modern day. Usually these tails are only a few centimetres long and are usually removed shortly after birth.