Bats are thought to have developed wings as a result of the mutation of a single gene that occurred some 50 million years ago. This genetic variation meant that the knuckle bones-between which the wing membranes stretch today-became greatly elongated, allowing the development of wings. This sudden radical development may also explain why the transition from walking to flying animals occurred so rapidly. It is thought that only a few transitional forms bridged the gap between the small, flightless mammals and the bats. It has also been suggested that these transitional forms probably only lived for a relatively short space of time, which would help to explain the fact that no fossilized remains of any such transitional creature have been found so far.