Collagen is the protein that holds our bodies together. It constitutes our connective tissues. Lampreys, sharks and skates use it instead of bone. It is such a tough protein that some small fraction may have survived 68 million years in a T. rex skeleton excavated a few years ago. A team of six scientists recently analyzed the amino acid sequence in this T. rex collagen, as well as that in a half-million-year-old Mastodon (Chris Organ and others, Science, April 25, 2008). A computer was used to compare these fossil sequences with those of living species in order to determine their most likely evolutionary relationships.
I found the resulting phylogenetic tree so interesting that I decided to share it with you. My contribution was to translate obscure names, such as Raja, Cynops, Loxodonta, Monodelphis, Struthio, etc., into the more familiar Skate, Newt, Elephant, Opossum, Ostrich, etc. The (horizontal) lengths of the branches in the phylogenetic tree are proportional to the minimum number of mutations required to produce the observed diversity in amino acid sequences.
The T. rex sequence, however, is disputed in a rebuttal authored by 27 scientists who present evidence that collagen older than a million years must be too degraded to be useful (Mike Buckley, et al., Science, May 15, 2008). They accept the mastodon sequence as being valid. It was preserved for a shorter period under cooler conditions.
Human collagen is arbitrarily placed near the center of the diagram, for the benefit of our egos.