Today, birds are one of the most diverse groups of vertebrates, and they've had a long time to evolve. The first "bird" in the fossil record is largely considered to be Archaeopteryx, which existed 155 million years ago. Around 55 million years ago, the first hummingbirds and parrots began to appear.
The more researchers learn, the more they realize that flight has evolved multiple times
across animals, helping them reach the same goal of flight in a variety of ways.
Birds weren't the only creatures taking to the skies back then. Flying reptiles called pterosaurs, which reached the size of small planes, dominated the skies as early as 215 million years ago. The fossil record has also shown evidence of flying dinosaurs, like microraptors, as well as other creatures that could glide from tree to tree.
The story of the origin and evolution of flight is a long and complicated one, made even more tangled and complex as new research restructures the roots and branches of this family tree.
Within the last 20 years, advances in technology, such as conducting computerized tomography, or CT scans of fossils, as well as the discovery of a wealth of fossils in China, are helping fill the gaps in the story of how animals transitioned from crawling on the ground to flying in the air.