A NEW look at a fossil discovered over a decade ago has strengthened the theory that bats evolved from flightless creatures that hung from tree branches by all four legs. Most paleontologists now believe that birds evolved from small ground-dwelling dinosaurs that had developed feathers for another purpose. The origins of bats, however, have always been puzzling. The earliest bat fossils, dating back to some 50 million years, have wings that closely resemble those of modern bats. Their anatomy suggests kinship with the cat-sized flying lemurs found in southeast Asia which,'like most species of bats, hang upside down from trees by all fours.

Three years ago, Nancy Simmons of the American Museum of Natural History in New York proposed a theory that related the bat and the flying lemurs through a common ancestor that hung from branches by all four legs, before evolving the ability to glide and eventually to fly. However, there was no fossil evidence to support this theory. Now Mark Hamrick of Kent State University in Ohio, USA, may have found the missing link.

Hamrick was studying the Phenacolemur, a mouse-sized flightless animal that, say scientists, coexisted with the early bats. He found that bones, originally thought to come from its hand, actually came from the rear foot and resembled those that the flying lemurs use to hang from trees.

"Nobody had realised this was hanging from its hind feet before," says Hamrick, who reported the results at the recently-concluded meeting of the Society for Vertebrate Paleontology in Utah, USA. He believes the animal climbed by hanging underneath branches from all fours because it was the easiest way to reach the fruit, nectar and insects at their tips.

Hamrick concludes that bats and Phenacolemur share a common ancestry indeed. However, it may be difficult to find an intermediate form that shows how bats evolved their wings. Fossils of small mammals rarely survive, Simmons says, and the intermediate form would have existed only briefly. "Once powered flight began to evolve, it was such an important feature that evolution proceeded very rapidly," she says.