Score another point for Darwin’s bulldog. T.H. Huxley, who argued that birds are descended from dinosaurs nearly 140 years ago (Huxley 1868), would undoubtedly be pleased with himself were he privy to the wealth of data that has accrued in support of his conjecture. Evidence from the rather good fossil records of dinosaurs and birds strongly supports this notion, and now two additional lines of evidence can be added in its favour.
The first relates to my own area of study, that of genome size diversity. In a recent paper in Nature, Organ et al. used the link between genome size and cell size of extant taxa (in this case, osteocyte spaces) to infer genome sizes for extinct dinosaurs from various lineages (see Zimmer 2007 f0r a helpful overview). I have promised to post a detailed discussion of the genome size-flight issue another time, so I will briefly note that the main finding is that saurischian dinosaurs, the lineage from within which birds are thought to be descended, had small genomes relative to other reptiles, including ornithischian dinosaurs. This suggests that genome size reduction began (but probably did not culminate) prior to the origin of avian flight and provides yet another intriguing link between saurischian dinosaurs and birds.
The second was reported in papers published by Asara et al. and Schweitzer et al. in the April 13 issue of Science, and revealed intriguing similarities in the amino acid sequences of collagen proteins from a 68 million year old bone from Tyrannosaurus rex and that of a modern chicken. Specifically, amino acid sequence identity was closer to chicken (and presumably to all other modern birds) (58%) than to frog or newt (51%). Note that collagen sequence is usually quite conserved and that what the authors were dealing with were fragments of proteins. Note also that some other interesting comparisons — especially with other living archosaurs, namely alligators or crocodiles — were not possible based on the currently available data. Plenty more to do as follow up to this study, but a very interesting result. A summary of the studies can be found in National Geographic and the New York Times.
This is a nice example of the way in which independent sources of information converge on a common conclusion in evolutionary biology, and how new discoveries simultaneously can raise novel questions and provide innovative means by which to approach them.
Asara, J.M., M.H. Schweitzer, L.M. Freimark, M. Phillips, and L.C. Cantley. 2007. Protein sequences from mastodon and Tyrannosaurus rex revealed by mass spectrometry. Science 316: 280-285.
Huxley, T.H. 1868. On the animals which are most nearly intermediate between birds and the reptiles. Annals and Magazine of Natural History, Series 4, 2: 66-75.
Organ, C.L., A.M. Shedlock, A. Meade, M. Pagel, and S.V. Edwards. 2007. Origin of avian genome size and structure in non-avian dinosaurs. Nature 446: 180-184.
Schweitzer, M.H., Z. Suo, R. Avci, J.M. Asara, M.A. Allen, F.T. Arce, and J.R. Horner. 2007. Analyses of soft tissue from Tyrannosaurus rex suggest the presence of protein. Science 316: 277-280.
Zimmer, C. 2007. Jurassic genome. Science 315: 1358-1359.