Some of you may have heard of a controversy among scientists: Did birds evolve from birds, or did they evolve from an older reptilian ancestor?
Creationists, of course, have seized this argument and tried to somehow contort disagreement in a single area into a way to show that evolution as a whole is unstable (keep in mind that birds don't fossilize well, so this leaves scientists a lot of room for imagination and speculation. Since different folks have different speculations, a couple of different ideas have been left to duke it out).
I want to state in advance that I am not a scientist, and there may very well be more to this than I know. Most scientists support the view that I will present, but very smart (and very educated) people disagree with the most prevalent view of bird ancestry. There are many arguments launched toward the dinobird theory: A few scientists have claimed that the 'downy feathers' belonging to the dinosaur Sinosauropteryx were really just collagen fibers (these resemble hair and have been known to form on fossils of sharks); there are lots of scientists who think that flight could evolve only by graceful glides from the trees, not hopping up and down on the ground the way theropod dinosaurs must have done.
The first claim that is usually presented against dino bird ancestry is this:
"Most scientists believe that birds descended from theropod dinosaurs. But theropod dinosaurs have lost digits IV and V from a primitively five-fingered hand, leaving them with digits I, II, and III. Birds, on the other hand, have digits II, III, and IV, having lost digits I and V. It is almost impossible for the two groups to be closely related with such a significant anatomical difference."
PZ Myers wrote a wonderful article that explains why this argument is wrong:
Anatomists initially assigned digit numbers I, II, and III to bird limbs on the basis of their form, but later had to revise that to II, III, and IV on the basis of embryology. Dinosaur digits are assigned numbers I, II, and III on the basis of their adult form (which is admittedly much less ambiguous than adult bird digits!)…but what about their embryology? If we had access to information about expression of molecular markers and early condensations in the dinosaur limb, would we have to revise their digit numbers?
Another argument against birds evolving from dinos is that it requires a ground up origin of flight. This is viewed as being less plausible because in order to fly the dinosaurs would have had to work against the force of gravity (The alternative is that the ancestors of birds glided down from trees). This argument is easily answered by simply viewing the latest theories on the evolution of flight (See chapter 3 of "The Four Winged Dinosaur").
In order to find out more about the subject, I emailed Paleontologist Greg Paul:
Hi Greg, I have two questions about bird evolution:
1) What about Dr. Alan Feduccia's claim that bird teeth are not homologous to theropod teeth? (Indicating they are unrelated)
2) Does Sinosauropteryx really have "protofeathers" or is this simply collagen fiber?
Thanks and I hope to hear your response soon,
Feduccia & company used to argue that dromaeosaurs could not be bird relatives because the formers teeth were too different from those of toothed birds. Because dromaeosaurs are now known to have well developed feathers, Feduccia etc. now argue that dromaeosaurs are birds, not dinosaurs.
New small, short armed theropods from the Yixian show that the feathers formed a soft tuft at the tip of the tail, and came off the upper foot. It is not possible for those to be collagen fibers.
(By the way, velociraptors are dromaeosaurs and they had feathers, as well as others)
One final argument that is often brought up is that it would be impossible for the avian lung system to evolve. To counter that one, simply take a look at how similar the avian and dinosaurian air systems are.