Origin of avian flight

Somewhere in eumaniraptorans, flight evolved. But how? Two opposing endpoints exist as regards the origin of bird flight (Figure 10.13). The first is the so-called arboreal (or "trees down") hypothesis: that bird flight originated by birds gliding down from trees (Figure 10.13a). In this hypothesis, gliding is a precursor to flapping (powered) flight; as birds became more and more skillful gliders, they extended their range and capability by developing powered flight. Perhaps flapping developed as a modification of the motions used in controlling flight paths.

Antithetical to the arboreal hypothesis is the cursorial (or "ground up") hypothesis for the origin of flight (Figure 10.13b). The cursorial hypothesis states that bird flight originated by an ancestral bird running along the ground. In this scenario, perhaps as obstacles were avoided, the animal became briefly airborne. Flapping (powered) flight appeared early on, as the animal strove to overcome more fully the force of gravity. This idea obviously requires a highly cursorial ancestor, in which feathers were already present. In this hypothesis, the legs, feet, and hands of Archaeopteryx are viewed as an inheritance from a cursorial maniraptoran ancestory.

Which to choose? The arboreal hypothesis is intuitively appealing, and getting airborne is easy. On the other hand, the cursorial hypothesis is strongly supported because ultimately the ancestor of birds had to have been a cursorial creature.

A problem with the cursorial hypothesis is that it has so far proven nearly insurmountable to model a cursorial theropod that developed flight by running along the ground. For this reason, an arboreal stage intermediate in the development of flight has been attractive to many scientists. Yet indications of a cursorial heritage are present in all living birds as, indeed, their limbs are little changed from the non-flying coelurosaurian condition.

Recently an interesting compromise position was proposed. Perhaps flapping wings helped early cursorial theropods to get a purchase on steep slopes, overhangs, or even tree trunks. From this it would not have been a big leap, as it were, to flapping flight. Ultimately, however, the exact scenario by which flight arose may never be known.

How well did Archaeopteryx fly? By all (theoretical) accounts, not particularly well, as compared with living birds. As we have seen, Archaeopteryx lacks many of the skeletal specializations of modern birds. Instead, the creature has a primitively elongate trunk, gastralia, no syn-sacrum, no carpometacarpus, weakly developed coracoids, a small sternum without much of a keel, and none of the supracoracoideus adaptations of living birds.

Recent work suggests that Archaeopteryx could flap its wings, attaining moderately high speeds, but could not perform the kind of slow flight that a running take-off might require. For this reason, some suggest that Archaeopteryx had to have been primarily a tree-dweller. Ultimately we are left with a possibly arboreal animal capable of some powered flight, but not of the kind available to living birds.

The story of Archaeopteryx and the origin of flight reminds us of a fundamental property of evolution. Structures are not commonly invented wholesale in evolution. Evolution modifies existing structures. Here, the feathers and grasping arms of warm-blooded, non-avialan deinonychosaurs were modified - remarkably little - to permit flight. It was a breathtaking evolutionary achievement.

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