We evolved from monkeys, and most monkeys have tails. But when the ape line split from the monkey line in Africa around 25 million years ago, our tails vanished, leaving behind only a vestigial trace in our coccyx, aka tailbone. This is a simple bottom line for a complicated story, as Ewen Callaway describes in an accompanying article. The team deduced that more than 100 genes are involved in tail development in the mouse.