Though the origin of birds has long been a contentious topic, the current scientific consensus is that they evolved from maniraptors, dinosaurs with which they share many anatomical features. One issue that has yet to be resolved, however, is whether the capacity for flight arose in tree-living dinosaurs that glided from branch to branch—the “trees-down” hypothesis—or in fast-running terrestrial dinosaurs—the “ground-up” hypothesis. What other theory about the origin of flight has been proposed? Discuss
Source: The Free Dictionary