Director James Cameron's "Avatar" movies are populated by a species of outsized blue beings resembling humans, except with tails. So why does our species lack a tail, considering that our evolutionary forerunners in the primate lineage had them?

Scientists on Wednesday identified what might be the genetic mechanism behind the tailless condition of us and our ape ancestors — a mutation in a gene instrumental in embryonic development. The tail was a feature of most vertebrates for more than half a billion years and its loss may have offered advantages as our ancestors moved from the trees to the ground, they said.

The researchers compared the DNA of two groups of primates: monkeys, which have tails, and hominoids — humans and apes — which do not. They found a mutation in a gene called TBXT that was present in people and apes but absent in monkeys. To test the effects of this mutation, the researchers genetically modified laboratory mice to have this trait. These mice ended up with either a reduced tail or none at all.