Final answer:
Birds evolved from theropod dinosaurs, which are cladistically categorized as Saurischia within the archosaurs. Fossils such as Archaeopteryx show a mix of dinosaur and avian features, illustrating their evolutionary link. Today's birds are classified into three groups: Paleognathae, Galloanserae, and Neoaves.
Step-by-step explanation:
The ancestry of birds lies among the theropod dinosaurs. Birds are a part of the diapsid group known as the archosaurs, which also includes living crocodilians, pterosaurs, and dinosaurs. The evidence is clear that birds evolved from within the clade of Dinosauria, particularly from the group known as Saurischia.
Although Saurischia is divided into 'lizard-hipped' dinosaurs and 'bird-hipped' dinosaurs, it is the former category—specifically the theropods—that led to the evolution of birds. Theropods are bipedal predators, and many notable similarities exist between later theropods, such as maniraptorans, and birds, in areas such as hip and wrist bone structures, as well as the presence of the wishbone. Distinctive examples include fossils of creatures like Archaeopteryx, which showcase a combination of dinosaur and avian features.
Over 10,000 species of birds exist today, and they have been categorized into three major groups: Paleognathae, Galloanserae, and Neoaves. Birds have undergone significant evolutionary radiation following the extinction of the dinosaurs in the Cenozoic era, leading to the vast diversity we see today.