Birds are believed to be descended from dinosaurs, but some significant changes must have happened as they evolved from their ancestors. A new study involving baby chicks may help clear up a mystery of how one of those changes occurred -- how birds got their wing "fingers."

All four-limbed creatures, including dinosaurs, evolved from an ancestor that had five digits at the end of its limbs. These became flippers, wings, hands or paws, and some or all of the digits disappeared altogether in some cases.

Scientists think birds evolved from a group of meat-eating dinosaurs called maniraptors some 150 million years ago, during the Jurassic period. Modern birds have three digits in each of their wings, which means two digits in the forelimbs of these dinosaurs would have had to be lost during evolution.

A question of numbering

But which three digits survived? Paleontologists and developmental biologists have disagreed heartily on this. A proposal made in 1999, called the “frame shift” hypothesis, explained a discrepancy in the evidence, but not everyone accepted it.

The new study, conducted by researchers who transplanted cells from one part of a chick’s body to another, adds to the support for the hypothesis. The results of their study appear today (Feb. 10) in the journal Science.

If you number the digits so that digit 1 corresponds with our thumbs, digit 2 with our index fingers and so on, the fossil record shows that birds' wings evolved using digits 1, 2 and 3 of the dinosaur’s forelimbs.

However, in a bird embryo, the digits arise from the places on the limb bud associated with digits 2, 3 and 4. This conflict supported those who challenged whether birds were directly descended from dinosaurs.

In 1999, Günter Wagner and Jacques Gauthier of Yale University bridged the two factions by proposing that during development, digit 1 actually arose from the second position (where digit 2 should have arisen), and so on -- a frame shift.

"The great thing about the frame shift theory is it makes both things correct," said Ann Burke, an evolutionary morphologist at Wesleyan University in Connecticut, who was not involved in the current study. "Birds are dinosaurs, but developmentally the digits are 2, 3 and 4."