Some animals identified as being dinosaurs may have evolved from birds, according to a recent paper published in the Proceedings of the National Academy of Sciences.

Keep in mind that animals can evolve similar traits independently. The accepted transition from dinosaur to bird, or in this proposed case—bird to dinosaur—didn't necessary follow a simple path from large beast to tiny, feathered flier. For example, some dinosaurs are thought to have had feathers and beaks, traits we now tend to associate with birds. It's also believed that some dinosaurs increased in size, shrunk, and then became large again. The evolutionary paths, in other words, don't always follow certain, predictable courses, since animals are constantly adapting to ever-changing habitats and climates.

The new PNAS paper doesn't entirely surprise me, because there have been recent discoveries of very bird-like dinosaurs that weren't even very closely related to birds. Check out our story on Haplocheirus sollers, for example. I tend to agree with Jonah Choiniere, lead author of that Science paper, who believes the first birds emerged out of the Maniraptora, aka "hand snatcher," clade, but birds and dinosaurs from that point on then went down different evolutionary paths.

John Ruben, a professor of zoology at Oregon State University, authored a commentary on the PNAS paper. Ruben doesn't dispute that birds and dinosaurs likely shared a common ancestor. Per the study, however, he suggests that once birds started down their own evolutionary path they may have given rise to raptors. This is where the debate heats up because he and others contend that very bird-like 'dinosaurs,' such as Velociraptor, may have actually been more bird than dinosaur.