Birds have evolved from reptiles.
There is still no consensus among the scientists about the exact pathway of evolution of birds. The prevailing theory proposes that birds descended from a group of dinosaurs called theropods. Others contend that birds did not evolve from dinosaurs but rather from a group of small, four-legged, arboreal reptiles called the thecodonts. Still others feel that birds may not have originated from reptiles at all.
There are various speculations on the ways in which reptilian features gradually changed to avian features.
Let us look at evolution of 2 features in birds - feathers and flight.
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. 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.
In addition, connecting links also serve to understand the process of evolution. While scientists continue to debate exactly where birds came from, nobody denies that their subsequent success in colonising the planet has been immense. In future also, birds are expected to continue to adapt to habitats and changing conditions.
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