Birds originated from a group of small dinosaurs called maniraptorans about 150 million years ago, and recent findings show many maniraptorans were already very bird-like, with feathers, hollow bones, small body sizes and high metabolic rates, researchers at McGill University in Montreal said.
But true birds began to evolve when both the forelimbs and hind limbs underwent a dramatic decoupling from body size, they said, allowing early birds to accomplish flight.
Forelimbs lengthened and served as airfoils, while hind legs shrank, reducing drag in flight -- the reason modern birds tuck their legs as they fly -- and aiding perching and moving around on tree branches, they said.
A combination of better wings and more compact legs would have been critical for the survival of birds in a time when another group of flying reptiles, the pterosaurs, dominated the skies and competed for food, the researchers said.
"Our findings suggest that birds underwent an abrupt change in their developmental mechanisms, such that their forelimbs and hind limbs became subject to different length controls," Hans Larsson, a McGill macroevolutionist, said.
Deviations from the rules of how an animal's limbs normally scale with changes in body size usually indicate some major shift in function or behavior, the researchers said.
"The origin of birds and powered flight is a classic major evolutionary transition," said Alexander Dececchi, who participated in the study as a McGill graduate student and is now a postdoctoral researcher at the University of South Dakota. "Our findings suggest that the limb lengths of birds had to be dissociated from general body size before they could radiate so successfully.
"It may be that this fact is what allowed them to become more than just another lineage of maniraptorans and led them to expand to the wide range of limb shapes and sizes present in today's birds."
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