Historically, the origin of birds has always been an argumentative topic in evolutionary biology. But, today most researchers agree that birds belong to a group of beast-footed or theropod dinosaurs which evolved throughout the Mesozoic period.
During the nineteenth century, after the Archeopteryx (primitive bird) was discovered in Germany, the first proposal was made to show the close relationship between dinosaurs and birds. It was revealed that birds and dinosaurs share several unique skeletal features. In addition, fossils of over twenty dinosaur species were collected, preserving feathers. Some small dinosaurs, including the Anchiornis and Microraptor, also had long veined, leg and arm feathers which formed wings. These long footed feathers were also seen on the Jurassic basal avian Pedopenna. The conclusion by Witmer (2009) was that this evidence was enough to show that the avian evolution had undergone a four-winged stage.
Additionally, the fossil evidence demonstrates that the dinosaurs and birds shared certain features such as brooding and nest-building behaviors, hollow (pneumatized) bones and gastroliths inside the digestive system. The findings of the hardened Tyrannosaurus rex soft tissue was a ground-breaking discovery as a molecular comparison could be made between the cellular anatomy and the protein sequencing of the collagen tissue. This showed that birds and T. rex are related more closely to each other than to the alligator.
Another molecular study strongly supported the close relationship of birds and dinosaurs, even though birds where not place within Theropoda. This study used eight extra collagen sequences which were taken from a Brachylophosaurus canadensis thigh bone or femur. A study was done to compare adult and juvenile archosaur skulls and the conclusion was that birds come from dinosaurs by juvenilization or neoteny.
Now, only few scientists are having discussions about whether or not birds originates from dinosaurs, proposing descent from some other kinds of archosaurian reptiles. The sequence of the evolutionary events which led to the early birds in maniraptoran theropods continues to be a hot topic amongst the consensus supporting the dinosaurian ancestry.