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But if feathers didn’t arise as adaptations for flying, what on earth were they for? Again, we don’t know. They could have been used for ornamentation or display—perhaps to attract mates. It seems more likely, though, that they were used for insulation. Unlike modern reptiles, theropods may have been partially warm-blooded; and even if they weren’t, feathers would have helped maintain body temperature. And what feathers evolved from is even more mysterious. The best guess is that they derive from the same cells that give rise to reptilian scales, but not everyone agrees.
Despite the unknowns, we can make some guesses about how natural selection fashioned modern birds. Early carnivorous dinosaurs evolved longer forelimbs and hands, which probably helped them grab and handle their prey. That kind of grabbing would favor evolution of muscles that would quickly extend the front legs and pull them inward: exactly the motion used for the downward stroke in true flight. Then followed the feathery covering, probably for insulation. Given these innovations, there are at least two ways flight could have evolved. The first is called the “trees down” scenario. There is evidence that some theropods lived at least partly in trees. Feathery forelimbs would help these reptiles glide from tree to tree, or from tree to ground, which would help them escape predators, find food more readily, or cushion their falls.
A different—and more likely—scenario is called the “ground up” theory, which sees flight evolving as an outgrowth of open-armed runs and leaps that feathered dinosaurs might have made to catch their prey. Longer wings could also have evolved as running aids. The chukar partridge, a game bird studied by Kenneth Dial at the University of Montana, represents a living example of this step. These partridges almost never fly, and flap their wings mainly to help them run uphill. The flapping gives them not only extra propulsion, but also more traction against the ground. Newborn chicks can run up 45-degree slopes, and adults can ascent 105-degree slopes—overhangs more than vertical!—solely by running and flapping their wings. The obvious advantage is that uphill scrambling helps these birds escape predators. The next step in evolving flight would be very short airborne hops, like those made by turkeys and quail fleeing from danger.
In either the “trees down” or “ground up” scenario, natural selection could begin to favor individuals who could fly farther instead of merely gliding, leaping, or flying for short bursts. Then would come the other innovations shared by modern birds, including hollow bones for lightness and that large breastbone.
While we may speculate about the details, the existence of transitional fossils—and the evolution of birds from reptiles—is fact. 10
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