Final answer:
Birds evolved from theropod dinosaurs, as reflected in fossils such as Deinonychus and Archaeopteryx, which exhibit bird-like characteristics, including similar respiratory systems, bone structures, and the presence of feathers.
Step-by-step explanation:
Theropod Dinosaurs and the Evolution of Birds
Birds are considered to have evolved from theropod dinosaurs due to numerous physical and genetic similarities. Over 150 million years ago, the ancestors of modern birds likely resembled theropods like Deinonychus, as suggested by the fossil record. Substantial evidence pinpoints that birds evolved within the clade Dinosauria, a group that includes theropods, known for having features such as a similar flow-through respiratory system with lungs and air sacs, along with other morphological similarities. For example, Archaeopteryx and Xiaotingia, which are considered to be early bird precursors, exhibit such traits. These similarities are further supported by the alignment of features such as the hip and wrist bone structures and the wishbone, which is common in both theropods and modern birds. Moreover, birds are part of the diapsid group named the archosaurs, alongside dinosaurs, pterosaurs, and crocodilians, with whom they share a common ancestor. Pterosaurs, despite having wings like birds, are not dinosaurs and represent a case of convergent evolution with their hollow bones.
In summary, the link between theropod dinosaurs and birds is well-established through fossil evidence and the continuation of certain anatomical features such as feathers. These features highlight the evolutionary bridge between these ancient reptiles and modern avian species. The fossil Archaeopteryx underscores this connection, showcasing both reptilian characteristics and feathers indicative of its place in the evolutionary lineage of birds.