A new study by researchers at McGill University in Montreal suggests that birds may have evolved from a group of theropods called maniraptorans, according to Science World Report.
The Maniraptorans were small, meat-eating dinosaurs, and lived approximately 150 million years ago. Theropods first appeared 230 million years ago and possessed many early vestiges of birds, including hollow, air-filled bones, wishbones, and feathers.
According to the McGill study (first published in the September issue of Evolution), the birds who evolved from Maniraptorans share all these traits, plus a high rate of metabolism. As their limbs changed size vis-à-vis their bodies, they were able to take flight.
"The origin of birds and powered flight is a classic major evolutionary transition," researcher Alexander Dececchi told Science World Report. "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."
The biggest change, of course, was the lengthening of the forearms. With longer forearms, the birds were able to utilize lift and basic aerodynamic principles to take to the skies. Meanwhile, the hindquarters - previously integral for ground movement - got shorter, to minimize drag in flight and to maximize efficiency in movement.
It's somewhat ironic and counterintuitive that birds evolved from land dinosaurs. After all, ancient reptiles also stumbled across flight. The Pterosaurs (from Greek, meaning "winged lizard") also flew, and - considering that they co-existed with Maniraptorans - likely competed with early birds for food. However the pterosaurs died out, while birds evolved into one of the most diverse groups of vertebrates in the animal kingdom. Today, there are over 10,000 living species of birds in the world.