Birds
at the National ZooReptiles to Robins
Scientists have long recognized that modern-day birds and reptiles share a common ancestor. Both groups lay shelled eggs and have scales (in birds, confined to the legs), nucleated red blood cells, and a number of skeletal similarities. The famous fossil of the Jurassic bird Archaeopteryx (meaning "ancient wing") was so reptilian in appearance that experts initially mistook it for a dinosaur.
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| Fossil evidence indicates that some non-avian dinosaurs were covered in downy filaments that are structurally similar to the feathers of modern-day birds. (Jessie Cohen/NZP) |
The concept of evolution was still new in 1868, when Thomas Huxley, one of Darwin's most ardent champions, first suggested that Achaeopteryx was a link between dinosaurs and modern birds. Since then, a wealth of fossil evidence has supported Huxley's theory, identifying traits in dinosaurs that earlier had been associated only with birds. Paleontologists' most recent discoveries include a Tyrannosaurus rex predecessor covered with fine down, a dinosaur with four feathered wings, and a small slumbering dinosaur, Mei long, preserved with its head tucked under a front limb in bird-like repose.
Yet despite the striking fossil evidence, there is still no consensus over birds' dinosaur ancestry. A few scientists contend that birds did not evolve from dinosaurs but rather from a group of small, four-legged, arboreal reptiles called the thecodonts. This is the same group from which the dinosaurs probably descended, but thecodont theorists argue that bird ancestors inhabited the Earth in the mid-Triassic Period, about 20 million years before the first dinosaurs appeared in the late Triassic and early Jurassic Periods. These scientists attribute similarities between dinosaurs and birds to convergent evolution—a process by which unrelated organisms evolve similar anatomical structures independently.
However, the prevailing theory proposes that birds descended from a group of dinosaurs called theropods—carnivorous, bipedal dinosaurs that inhabited the Earth from the late Triassic Period until the end of the Cretaceous Period, about 220 to 65 million years ago. The theory traces modern birds to a group of flightless theropods called maniraptorans. These dinosaurs were swift runners with hollow bones, three-clawed toes on each limb, and sharp teeth. In the mid-1990s, scientists discovered one more extraordinary detail: Theropod dinosaurs were feathered. Paleontologists continue to unearth specimens, some entirely covered in downy fluff, and still others sporting feathers that are structurally identical to those of modern birds.
Feathers serve a variety of purposes. Modern birds insulate their bodies with down, flourish their plumage to communicate, blanket feathers over eggs during incubation, and, of course, use feathers to fly. But why did flightless dinosaurs have them? Some scientists speculate that feathers originated in dinosaurs to regulate body temperature. Just as in living birds, dinosaur feathers would have offered protection from cold and prevented overheating in the sun. It follows from this theory that feathers were co-opted for flight further down the evolutionary road.
Precisely how reptiles first took to the air presents yet another enigma. Did they begin with a running start from the ground or by gliding out of ancient treetops? Scientists have debated the ground-up and trees-down theories for a century. Proponents of the ground-up hypothesis posit that the arm motions of small dinosaurs grasping after prey resembled a flight stroke. Eventually, dinosaurs running after prey may have gained enough speed from their hind limbs and thrust from the grasping motion of their forelimbs to become airborne. Alternately, arboreal flight theory proposes that tree-dwelling reptiles initially glided from branch to branch through the forest canopy, and only later flapped themselves into powered flight.
The 2003 discovery of a dinosaur called Microraptor, which had four feathered wings, favors the arboreal flight hypothesis. Long aerodynamic feathers on the hind limbs, such as those covering Microraptor's legs, might have evolved in arboreal dinosaurs to slow their descent should they fall from trees. Furthermore, wings on the hind limbs would have dragged through the dirt, making a terrestrial take-off unlikely. Upon discovering Microraptor, scientists took another look at Archaeopteryx and noticed, for the first time, vestiges of hind wings on its legs. Gradually, intermediate features of this kind lead scientists closer to understanding the evolutionary path traveled by modern birds.
In an 1860 letter to Charles Kingsley, a clergyman and Darwin supporter, Thomas Huxley warned, "… be prepared to give up every preconceived notion, follow humbly wherever and to whatever abysses nature leads, or you shall learn nothing." The debate surrounding bird evolution rages on, leaving scientists to humbly follow the fossils.
—Deborah Press
ZooGoer
34(3) 2005. Copyright 2005 Friends of the National Zoo.
All rights reserved.
