The arboreal hypothesis suggests that a ground-running biped first became adapted to life in trees, where it took to leaping from branch to branch, then parachuting. Later it developed flapping flight. Feathers became aerodynamically important at the jumping stage and evolved directly into flight feathers. The arboreal theory is the most favored at the moment, but it does have some difficulties.
Like all little theropods, Archaeopteryx was bipedal, with legs and feet that were well adapted for ground running. Bipedality is a rather poor preadaptation for living in trees, and Archaeopteryx had long, erect legs that were particularly ill-suited to climbing tree trunks (some arboreal proponents suggest that it climbed sloping branches instead!). Archaeopteryx, with long, erect limbs, a comparatively short trunk, and bipedal locomotion, was exactly the opposite in body plan of all living mammals and reptiles that jump and glide from tree to tree.
The claws on the hands of Archaeopteryx were long, thin, and sharp. They look like very effective tearing and slicing weapons, but were far too sharp and pointed to have been useful for climbing either trees or rocks. The claws on its feet have been compared with the claws on the feet of perching birds, but they were also very like the talons of an eagle or a theropod dinosaur, which shows only that they were equally well adapted for clutching branches or prey, or both, and we cannot tell which. Certainly theropod dinosaurs like Velociraptor, with "perching" claws, did not climb trees!
Altogether, the arboreal hypothesis is not unreasonable, but it does require a lot of special conditions. It looks vulnerable to a better suggestion that would explain more of the evidence.
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