7.8 Dinosaurs among us: further evidence is for the birds
Dinosaurs among us: further evidence is for the birds
By John Timmer | Published: April 24, 2008 - 01:00PM CT
Building phylogenies that
sort out complicated evolutionary histories typically forces scientists to take
an either/or approach. They can use DNA sequences of existing organisms, which
are relatively easy to resolve into quantitative measures of relationships, or
they can work with morphology, the shape of fossilized bones, which is prone to
arguments over what constitutes an objective measure. A brief report in today's
issue of Science manages to
have the best of both worlds: it uses protein sequences of two extinct
organisms to tell us that our inferences from morphology were spot on.
Tyrannosaurus chicken?
The key to the study was the discovery of some exceptionally well-preserved bones from extinct organisms: the dinosaur Tyrannosaurus rex and the mastodon, a North American mammal that vanished in the Younger Dryas extinction. Most biological molecules slowly decay over time, but do so with a half life, which means that a small population of proteins will persist for long periods of time. The scaffolding of bones contains a lot of a single type of protein called collagen, and collagen itself consists of a large series of similar repeats. The combination of factors allowed researchers to successfully sequence a few of these repeated collagen protein structures that survived in these bones.
The data from the extinct organisms was combined with collagen sequences from living vertebrates, including fish, amphibians, reptiles, and mammals. Many of these sequences were available in databases, and the researchers determined a few more using recent genome sequencing efforts. The results were built into a phylogenetic tree by four methods, all of which were in substantial agreement.
As the tree here shows, the ancient sequences are in complete agreement with what scientists had predicted based on morphology. The mastodon (Mammut in the diagram) lies next to the elephant (Loxodonta), which is an obvious candidate for a family member. But both group next to the tenrec (Echinops), a small animal that resembles a hedgehog. This relationship was known from previous evolutionary studies but certainly wouldn't be obvious to a casual observer.
Meanwhile, the study provided further support for another conclusion: dinosaurs are still around, persisting as birds. The similarities are so strong that the study was unable to resolve the relationship among T. rex, the ostrich, and the chicken. In contrast, the alligator and the anole (a small lizard) fell well outside this group, really driving home how separate birds and dinosaurs are from other reptiles.
The authors wrap the paper up by making the argument that we can trust the conclusions we reach based on an analysis of the morphology of extinct species. It will continue to be a rare case when we have DNA or protein sequences from extinct animals to work with but, in those cases that we do, they clearly indicate that we can get things right without the sequences.
Images courtesy of Science.
Science, 2008. DOI: 10.1126/science.1154284
