The idea that birds are descended from dinosaurs is nearly as old as evolution itself, first being proposed by Thomas Huxley in 1868 (Origin of the Species dates from 1859).

The only reason there was a competing evolutionary theory is because it was erroneously thought that birds have a clavicle and dinosaurs don't, so instead it was proposed that birds and dinosaurs have a common ancestor, and that dinosaurs lost the clavicle. Now that they have excavated many more bones paleontologists have since discovered therapod clavicles.