This issue is actually still hotly debated among biologists and other scientists. While birds definitely evolved from reptiles, exactly which group of reptiles gave rise to birds remains a mystery and a motive for discussion. Current belief is that birds are descended from thecodonts, a reptile that existed 150 million years ago. Birds have been around for a pretty long time!
In 1861, a bird-like fossil named Archaeopteryx lithographica was discovered, providing the missing “link” needed to show this relationship between birds and reptiles. The first birds probably did not have feathers, but these structures actually developed pretty quickly. While they are of unknown origin, it is thought that birds first used them to keep a warm body temperature, for mating rituals, or to glide from one tree to the other.
If you would like to learn more about the origin of birds and their feathers, click here.