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Birds are believed to have evolved from reptiles, specifically small carnivorous dinosaurs known as theropods that lived in the Late Jurassic period. A recent discovery of fossils from South America, China, and other countries as well as an examination of museum specimens with new methods have made possible the discovery of the origin of birds. The discovery of the origin of birds commenced with a specimen of Archaeopteryx, a dinosaur that had feathers on its arm and tail (Rashid et al. 20). However, unlike living birds, it had a long tail and teeth. Because Archaeopteryx shared unique features with theropods, scientists were convinced that these reptiles were the ancestors of birds.
The evolution of birds from theropod dinosaurs began with the development of short, hair-like feathers on the necks, heads, and bodies (Chatterjee 90). These feathers had different color patterns and provided insulation. Theropods that are more closely related to birds had feathers such as the branched downy and the asymmetrical flight feather that is identical to that of extant birds. In later theropods, feathers may have been for brooding purposes (keeping eggs warm and protecting them). This is suggested by a fossil of an oviraptorid theropod dinosaur in which the skeleton is bent up on a nest of eggs similar to a brooding chicken. The hands are spread out over the eggs as if they shelter them, suggesting the purpose of the feathers.
Besides the development of feathers, the evolution of the theropods’ digits also took place. The hands of initial theropod dinosaurs had a long second digit and small fourth and fifth digits. As these evolved to bird-like, the fifth and fourth digits were completely lost. The wrist bones beneath the first and second digits took a semi-circular form that allowed rotation of the hand sideways against the forearms. This ultimately allowed movement of birds’ wings in a way that created thrust for flight (Rashid et al. 34).
As the evolution went on, most bones in the theropod’s skeleton were reduced, perhaps to make flight easier (Lee et al. 56). In addition, bone walls became thinner, and feathers were lengthened and their vanes became asymmetrical, so as to improve flight. The long bony tail was condensed to a stump, on which feathers developed to improve maneuverability and stability (Lee et al. 60). The wishbone became stronger, and bones evolved on the shoulder girdle to anchor the flight apparatus on the forelimb. The breastbone was also enlarged and on it, a central keel that anchored the flight muscles developed. Because the main locomotive form was to be flight, arms became longer than the legs. Teeth were repeatedly lost in several lineages of early birds, after which the beak fully developed.
Research on the DNA of birds and reptiles shows astonishing similarity that tends to associate them with a common origin. DNA suggests that birds are descendants of reptiles shared with lizards, turtles, and crocodiles. Moreover, birds have DNA that is more similar to crocodilians than turtles and lizards do to crocodilians.
The evolution theory of birds from reptiles can also be supported by the numerous similarities between the two organisms. To begin with, both birds and reptiles lay eggs. However, the eggshells of birds are hard as compared to the leathery eggshells of reptiles. Birds and reptiles also have scales on their bodies. Birds have scales on their feet, beaks, and claws whereas reptiles have scales covering their entire bodies. Finally, birds and reptiles both belong to kingdom Animalia and phylum Chordata, indicating common ancestry.
 
 
Works Cited
Chatterjee, Sankar. The rise of birds: 225 million years of evolution. JHU Press, 2015.
Lee, Michael SY, et al. “Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds.” Science 345.6196 (2014): 562-566.
Rashid, Dana J., et al. “From dinosaurs to birds: a tail of evolution.” EvoDevo 5.1 (2014): 25.