Few subjects in evolutionary theory have posed such intriguing puzzles for so long as the origin of birds. Evidence of avian beginnings has been elusive in the fossil record because birds' light, hollow bones rapidly decompose. So far, the oldest-known bird fossil is the famous Archaeopteryx lithographica, discovered in 1861 just two years after the publication of Darwin's On the Origin of Species, but Archaeopteryx leaves many questions unanswered.
This odd, crow-sized creature had long legs and three toes tipped with claws; its jawbone and teeth were like those of a small dinosaur, and its extended spine formed a tail, another reptilian feature found in small dinosaurs too. But the creature also had wings and bore feathers -- certainly birdlike traits.
Scientists now view Archaeopteryx, which lived about 150 million years ago, as the earliest known (or most basal) member of the lineage of modern birds, but it still retained many features of small dinosaurs. These small, two-legged dinosaurs called theropods scurried around something like today's roadrunners. Many characteristics that typify birds were present in the theropods before birds evolved, including hollow bones, a wishbone, a backward-pointing pelvis, and a three-toed foot. In the course of theropod evolution, the forelimbs and hands became progressively longer. In some theropods, the bones of the wrist took on a shape that allowed the joint to flex sideways. This would have allowed these animals to whip their long hands forward in a swift snatching motion, perhaps to catch prey. The wishbone in theropods served to anchor the muscles that pulled the forelimb forward in this grabbing movement -- a motion that functional analysis shows to be almost identical to the flight stroke of modern birds. Theropods, though, probably remained largely on the ground.
Despite the increasingly clear picture of the evolution of birds from theropod dinosaurs that has emerged, a few scientists are still unconvinced. No alternative hypothesis has been offered to explain the multiple similarities between birds and theropods, however, and there is scant evidence to support a link to any of the other animals that have been suggested as possible ancestors or relatives. Meanwhile, the evidence connecting birds and theropods continues to accumulate.
For a long time, feathers were regarded as a uniquely avian feature. Bur recent fossil evidence suggests that feathers, too, evolved in theropods before birds. Whether they evolved for warmth, for display, or served some other function is not yet known. But in a small, lightly built bipedal predator leaping into the air to catch insect prey, even primitive feathers could have given a small amount of lift. Larger feathers would have increased lift until it was possible to stay airborne for short distances. The evolution of feathers with an asymmetrical shape, like those of Archaeopteryx, further enhanced the flight capabilities of early birds.
After Archaeopteryx, the fossil record suggests that birds diversified rapidly, though some of these Cretaceous early birds would have looked quite strange to our eyes, with their toothed beaks and clawed fingers. Our knowledge of this period of bird evolution is growing rapidly. Since 1990, more than three times as many bird fossils dating from the Cretaceous have been discovered than were found in the previous two centuries. While most of the bird lineages that arose during the Cretaceous died out, some of them survived to gave rise to the wonderful diversity of birds we see today.