Biologists tells us that birds evolved from early reptiles. But how could a land-dwelling animal evolve the ability to fly? Natural selection, creationists argue, could not explain this transition, because it would require intermediate stages in which animals have just the rudiments of a wing. Of what use is half a wing? This would seem more likely to encumber a creature than to give it a selective advantage.
But if you think a bit, it’s not so hard to come up with intermediate stages in the evolution of flight, stages that might have been useful to their possessors. Gliding is the obvious first step. And gliding has evolved independently many times: in placental mammals, marsupials, and even lizards. Flying squirrels do quite well by gliding with flaps of skin that extend along their sides—a good way to get from tree to tree to escape predators or find nuts. …But we no longer have to only imagine this step: we now have the fossils that clearly show how flying birds evolved.
Since the nineteenth century, the similarity between skeletons of birds and some dinosaurs led paleontologists to theorize that they had a common ancestor—in particular the theropods: agile, carnivorous dinosaurs that walked on two legs. Around 200 million years ago, the fossil record shows plenty of theropods but nothing that looks even vaguely birdlike. By 70 million years ago, we see fossils of birds that look fairly modern. If evolution is true, then we should expect to see the reptile-bird transition [“missing link”] in rocks between 70 and 200 million years ago.
And there they are! The first link between birds and reptiles was actually known to Darwin, who, curiously, mentioned it only briefly in later editions of The Origin, and then only as an oddity. It is perhaps the most famous of all transitional forms [“missing links”]: the crow-sized Archaeopteryx lithographica, discovered in a limestone quarry in Germany in 1860. …Archaeopteryx has just the combination of traits one would expect to find in a transitional form [“missing link”]. And its age, about 145 million years, places it where we would expect.
Archaeopteryx is really more reptile than bird. Its skeleton is almost identical to that of some theropod dinosaurs.
Reptilian traits: a jaw with teeth, a long bony tail, claws, separate fingers on the wing (in modern birds, these bones are fused), a neck attached to its skull from behind (as in dinosaurs) instead of from below (as in modern birds).
Birdlike traits: large feathers and an opposable big toe (probably used for perching)
It isn’t clear whether this creature, though fully feathered, could fly. But its asymmetrical feathers—one side of each feather is larger than the other—suggest that it could. Asymmetric feathers, like airplane wings, create the “airfoil” shape necessary for aerodynamic flight.
After the discovery of Archaeopteryx, no other reptile-bird intermediates [“missing links”] were found for many years, leaving a gaping hole between modern birds and their ancestors. Then, in the mid-1990s, a spate of astonishing discoveries from China began to fill in the gap. These fossils, found in lake sediments that preserve the impressions of soft parts, represent a veritable parade of feathered theropod dinosaurs. …the most striking of all of these discoveries is Microraptor gui, “the four-winged dinosaur”. Unlike any modern bird, this bizarre, thirty inch long creature had fully feathered arms and legs, which when spread out were probably used for gliding.
…as these fossils get younger, we see the reptilian tail shrinking, the teeth disappearing, the claws fusing together, and the appearance of a large breastbone to anchor the flight muscles. Put together, the fossils show that the basic skeletal plan of birds, and those essential feathers, evolved before birds could fly.
…The existence of transitional fossils [“missing links”]—and the evolution of birds from reptiles—is fact.
—biologist Jerry Coyne, Why Evolution is True, pp. 39-47
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