Origin of birds

The scientific question of which larger group of animals birds evolved within has traditionally been called the "origin of birds". The present scientific consensus is that birds are a group of maniraptoran theropod dinosaurs that originated during the Mesozoic era.

A close relationship between birds and dinosaurs was first proposed in the nineteenth century after the discovery of the primitive bird Archaeopteryx in Germany. Birds and extinct non-avian dinosaurs share many unique skeletal traits. Moreover, fossils of more than thirty species of non-avian dinosaur with preserved feathers have been collected. There are even very small dinosaurs, such as Microraptor and Anchiornis, which have long, vaned arm and leg feathers forming wings. The Jurassic basal avialan Pedopenna also shows these long foot feathers. Paleontologist Lawrence Witmer concluded in 2009 that this evidence is sufficient to demonstrate that avian evolution went through a four-winged stage. Fossil evidence also demonstrates that birds and dinosaurs shared features such as hollow, pneumatized bones, gastroliths in the digestive system, nest-building, and brooding behaviors.

Although the origin of birds has historically been a contentious topic within evolutionary biology, only a few scientists still dispute the dinosaurian origin of birds, suggesting descent from other types of archosaurian reptiles. Within the consensus that supports dinosaurian ancestry, the exact sequence of evolutionary events that gave rise to the early birds within maniraptoran theropods is disputed. The origin of bird flight is a separate but related question for which there are also several proposed answers.

Scientific investigation into the origin of birds began shortly after the 1859 publication of Charles Darwin's On the Origin of Species. In 1860, a fossilized feather was discovered in Germany's Late Jurassic Solnhofen limestone. Christian Erich Hermann von Meyer described this feather as Archaeopteryx lithographica the next year. Richard Owen described a nearly complete skeleton in 1863, recognizing it as a bird despite many features reminiscent of reptiles, including clawed forelimbs and a long, bony tail.

Biologist Thomas Henry Huxley, known as "Darwin's Bulldog" for his tenacious support of the new theory of evolution by means of natural selection, almost immediately seized upon Archaeopteryx as a transitional fossil between birds and reptiles. Starting in 1868, and following earlier suggestions by Carl Gegenbaur, and Edward Drinker Cope, Huxley made detailed comparisons of Archaeopteryx with various prehistoric reptiles and found that it was most similar to dinosaurs like Hypsilophodon and Compsognathus. The discovery in the late 1870s of the iconic "Berlin specimen" of Archaeopteryx, complete with a set of reptilian teeth, provided further evidence. Like Cope, Huxley proposed an evolutionary relationship between birds and dinosaurs. Although Huxley was opposed by the very influential Owen, his conclusions were accepted by many biologists, including Baron Franz Nopcsa, while others, notably Harry Seeley, argued that the similarities were due to convergent evolution.

A turning point came in the early twentieth century with the writings of Gerhard Heilmann of Denmark. An artist by trade, Heilmann had a scholarly interest in birds and from 1913 to 1916, expanding on earlier work by Othenio Abel, published the results of his research in several parts, dealing with the anatomy, embryology, behavior, paleontology, and evolution of birds. His work, originally written in Danish as Vor Nuvaerende Viden om Fuglenes Afstamning, was compiled, translated into English, and published in 1926 as The Origin of Birds.

Like Huxley, Heilmann compared Archaeopteryx and other birds to an exhaustive list of prehistoric reptiles, and also came to the conclusion that theropod dinosaurs like Compsognathus were the most similar. However, Heilmann noted that birds had clavicles (collar bones) fused to form a bone called the furcula ("wishbone"), and while clavicles were known in more primitive reptiles, they had not yet been recognized in dinosaurs. Since he was a firm believer in an interpretation of Dollo's law that stated that evolution was not "reversible", Heilmann could not accept that clavicles were lost in dinosaurs and re-evolved in birds. He was therefore forced to rule out dinosaurs as bird ancestors and ascribe all of their similarities to convergent evolution. Heilmann stated that bird ancestors would instead be found among the more primitive "thecodont" grade of reptiles. Heilmann's extremely thorough approach ensured that his book became a classic in the field, and its conclusions on bird origins, as with most other topics, were accepted by nearly all evolutionary biologists for the next four decades.

Clavicles are relatively delicate bones and therefore in danger of being destroyed or at least damaged beyond recognition. Nevertheless, some fossil theropod clavicles had actually been excavated before Heilmann wrote his book, but these had been misidentified. The absence of clavicles in dinosaurs became the orthodox view despite the discovery of clavicles in the primitive theropod Segisaurus in 1936. The next report of clavicles in a dinosaur was in a Russian article in 1983, where Mongolian paleontologist Rinchen Barsbold argued that one or several lineages of theropods may have evolved into birds.