Avian Origin of Flight

We had yet another successful discussion section this past week. We discussed a paper describing both the support for and limitations of the "ground up" and "trees down" hypotheses in the evolution of avian flight (paper cited below). The basics of the ground up hypothesis are that theropod dinosaurs utilized proto-wings as a means of lengthening a predatory strike, leaping into the air to catch insects, or to assist them in incline running. The trees down hypothesis suggests that bird ancestors were arboreal and used proto-wings for gliding from tree to tree before the evolution of flapping flight. The authors clearly support the latter hypothesis. They cite several reasons for the physical impossibility of the ground up method including the fact that theropods could not have run fast enough to gain lift from proto-wings and that there is no obvious intermediate form that would have been beneficial to these ancestral forms.

Besides the obvious fact that many discoveries have been made since this paper was published, there are several other questions that were brought up during the discussion. One concerned the usefulness of wings that have been adapted for gliding (and thus likely stiffened) in developing the dynamic flapping flight we see in birds today. In general, gliding structures are characterized by stability more than maneuverability (save flying squirrels whose "wings" are much different in form). We also had a difficult time imagining full flapping flight evolving from a completely terrestrial ancestor. We therefore imagined that a terrestrial ancestor that could use it's wings in incline running (say up the trunk of a tree) could then have used the same flapping mechanism from greater heights. In general, we feel that neither argument is mutually exclusive. However, the paper does clearly indicate that the authors do not recognize the relationship between birds and theropods. Given the mountain of evidence continuing to gather (feathered dinosaurs, morphology, frameshift mutations etc.) we feel that the relationship is well supported.