Birds are members of a specific group of reptiles called archosaurs, which also include the alligators and crocodiles, the dinosaurs, and the pterosaurs. Among living reptiles, birds are most closely related to the alligators and crocodiles.
The earliest known fossil bird is Archaeopteryx, a crow-sized bipedal reptile that bore a mosaic of primitive, reptilian and derived avian features.
Groups of organisms that include all the descendants of a single common ancestor are described as monophyletic because they constitute a single, entire branch on the phylogeny. A monophyletic group is also called a clade. We identify clades by discovering shared, evolutionarily derived character states. A primitive character state cannot tell us which organisms are more closely related.
Birds are living theropod dinosaurs. The more we discover about theropod dinosaurs, the more difficult it has become to distinguish birds from the continuum of evolutionary change within the theropods. The shared derived character states that support the theropod origin of birds can be found throughout the body. Perhaps the most powerful and unexpected support for the theropod ancestry of birds comes from the evidence of feathers on a broad diversity of coelurosaurs. By convention here, Avialae refers to the most exclusive monophyletic group that includes Archaeopteryx and living birds, and Aves refers to the monophyletic group of living birds.
The phylogeny of Mesozoic birds documents that the flight morphology and flight capacity of modern birds evolved in a series of incremental steps. Furthermore, there was substantial ecological and behavior diversity among birds from very early in avian evolution. The Ornithurae ultimately gave rise to modern birds.
Feathers are not unique to birds but rather evolved earlier in theropod dinosaurs and then diversified in form and function. Feathers evolved not as modified, mature elongate scales but as a novel epidermal structure. Feathers did not originate in concert with the evolution of flight. Rather, complex, vaned feathers evolved prior to the origin of birds and prior to the origin of flight in theropod dinosaurs.
Two basic theories of how avian flight evolved have been proposed: an arboreal, or gliding theory, and a cursorial, or running theory. The arboreal theory proposes that the evolution of flight started with gliding and parachuting from elevated perches. The cursorial theory proposes that elongated forelimbs enhanced leaping ability in a small, bipedal theropod dinosaur that ran and jumped to catch prey. The arboreal versus cursorial theories are not clear alternatives. The activities of the avian ancestors may well have mixed these behaviors. The most important issue in the origin of flight is the evolution of the wing stroke that could produce the main components of powered flight: lift and thrust. A powered wing stroke required transformation of the wrist and shoulder from the skeletal wing precursors of theropod or other ancestors. From the phylogeny of theropods, it is clear that many of the anatomical and functional precursors of the avian flight stroke evolved for prey capture in entirely terrestrial theropods with a praying mantis-like forelimb movement and grasping hands. This pattern provides strong evidence of the terrestrial context for many evolutionary events that together facilitated the evolution of avian flight. However, the biggest challenge to the cursorial theory is that the aerodynamic force of lift that makes flight possible is easier to produce at higher speeds. It is easier to produce high airspeeds over the limbs by gliding down from a high perch than it is by running along the ground. It has been suggested that flapping their feathered forelimbs helped early terrestrial ancestors of birds climb steep inclines, such as tree trunks, to escape predators.
—May 2022
Lance Jones 