What evolved first birds?

The earliest birds evolved in the Late Jurassic and Early Cretaceous periods. The origin and evolution of birds was a subject of much debate amongst scientists. Now, much evidence suggests that they evolved from small theropod dinosaurs. Modern birds emerged and diversified in the Cenozoic Era.

Although their direct lineage is not clearly known, birds have been shown to have a close relation to the dinosaurian theropods, a group of bipedal carnivorous dinosaurs which include every carnivore from the tiny Compsognathus to the massive Tyrannosaurus. It is likely that they evolved from any group of small theropods, such as the dromaeosaurids (commonly known as raptors) but it is not clear exactly which. In any case, these dinos were small, light and fast, and began to develop feathers. Feathers themselves evolved from keratinous filaments. At first these feathers may have been used either for display or warmth, but eventually they became adapted for flight.

The origin of bird flight itself is a debated issue. Some scientists believe the "from the ground up" idea, in which proto-birds ran on the ground and learned to use their feathers to lift them up. Others believe the "from the trees down" idea, in which proto-birds lived in trees and instead jumped down, using their feathers to help them glide.

As evidence that birds evolved from dinosaurs, there have been many transitional fossils found, such Archaeopteryx, Sinosauropteryx and Microraptor, which share features of both dinosaurs and birds. For example, they had teeth, claws, and bony tails, but also had feathers, birdlike skeletons, and were capable of short-term flight. Many small theropods have been found with primitive feather structures, such as Velociraptor.