Millions of years ago, our common primate ancestor became too heavy to move on tree branches and begun to spend more time on the ground. The tail, a useful balancing aid in tree dwelling became drag and potential entanglement. It had to be done away with. As these primates continued to evolve, the tail begun to wane, with only a residual stubby tissue remaining as an evolutionary hangover. This, too, petered out as the primates were gradually transformed into terrestrial bipedal humans. In rare cases, human babies do have a diminutive tail which can be surgically removed.