THE EVOLUTION OF BIRDS

Did birds evolve from dinosaurs? An evolutionary link between birds and reptiles in general has long been suspected. Early workers considered the similarities between the two groups, such as scaly feet, and shelled eggs, to indicate a remote link between the two groups. Over the years, the controversy has centred on which reptiles were closest relatives to early birds, such as the well-known Archaeopteryx, the first feather of which was found in 1861. In the 1870's Thomas Huxley and others had discussed a possible dinosaur origin for birds, pointing out that some dinosaurs, such as the small turkey-sized Compsognathus, were very lightly and delicately built in a similar way to birds. It was therefore either possible that birds had evolved from dinosaurs and had inherited similar charcteristics, or that the similarities in the skeletons had come about as a result of similar lifestyles (what evolutionary biologists call convergent evolution).

In 1926, Gerhard Heilmann published a book, The Origin Of Birds, in which he suggested convergent evolution was the result of the similarities, rathe than common ancestry. His opinion carried great influence, as workers then turned away from dinosaurs and looked to more primitive reptiles such as the archosaurs as bird ancestors. It has been suggested that this 'turning away' from dinosaurs as the ancestors of modern birds may have led to the widespread (and largely mistaken) belief that dinosaurs were sluggish and dim-witted, despite the occurrence of small and clearly highly active forms such as Compsognathus and Hypsilophodon. This opinion dominated until quite recently.

Since the 1960's and 1970's, however, the evidence from the skeletons has been reexamined, and there is now more evidence supporting common ancestry than convergent evolution. This evidence points to the fact that modern birds and theropod dinosaurs such as Deinonychus (of the Cretaceous period) shared a common ancestor in the Triassic, and that birds living today are in fact highly evolved theropod dinosaurs.

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