Saturday, June 1, 2013

Evolution In Action

In a paper in the May 30 issue of Current Biology, (1,2) researchers have been able to solve a problem that has perplexed biologists for over two centuries: the evolution of the turtle shell. They have been able to fill what they deem a 35 to 50 million year gap by the study of an extinct South African reptile known as Eunotosaurus africanus. In doing so, they have given us a very good example of evolution in action.

According to the author, Tyler Larson, the turtle shell is a structure whose evolution started over 260 million years ago in the Permian period. The shell is actually composed of approximately 50 bones, the fusion of ribs and vertebrae, the only animals that have evolved their shell in this way. Other animals with shells use bony scales as a protective shell and retain their ribs in order to allow for the mechanics of breathing. The turtles have apparently solved this problem by the evolution of a muscular sling.

Eunotosaurus displays several characteristics—such as a reduced number of elongated trunk vertebrae and reorganization of respiratory muscles to the ventral side of the ribs, among others—which conform to predictions that the initial transformations occurred by the Middle Permian period. The known turtle fossils discovered until recently dated back about 215 million years and had fully developed shells. In 2008 the discovery of Odontochelys semitestacea, a reptile about 220 million years old, showed that it had a fully developed plastron, the belly side of the shell, but only a partial carapace. Eunotosaurus is 40 million years older and it has the broadened ribs and lack of intercostal muscles running between its ribs that are found only in turtles, but lacks broad spines on their vertebrae and other features common to turtles and Odontochelys.

In another study (3) by the Joint International Turtle Genome Consortium, researchers decoded the genome of the green sea turtle and Chinese soft-shell turtle and studied the expression of the genome during development. They found that turtles are not primitive reptiles as previously thought but are related to the group which includes birds and crocodiles. From this data researchers concluded that the turtles must have split from this group around 250 million years ago.

Studying genome expression during development, it was seen that these turtles first develop the basic vertebrae body plan and then turn to the specific turtle development stage. During this late phase, researchers found limb-related gene expression in the embryonic shell, indicating that the turtle shell evolved by using the genetic program used for limbs.

These studies are important not merely for the study of the turtle but to show how evolution works. Over millions of years, evolution of the vertebrae led to their fusion to form the turtle shell. Genes that were originally used to produce limbs were recruited during evolution to create part of the turtle shell. This is how evolution works and what evolution would predict. It is another piece of evidence, if any more were needed, of the validity of evolutionary theory.

     1.     Tyler R. Lyson, Gabe S. Bever, Torsten M. Scheyer, Allison Y. Hsiang, Jacques A. Gauthier. Evolutionary Origin of the Turtle Shell. Current Biology, 2013; DOI: 10.1016/j.cub.2013.05.003
     2.     Cell Press (2013, May 30). How turtles got their shells: Fossil of extinct South African reptile provides clues. ScienceDaily. Retrieved June 1, 2013, from­ /releases/2013/05/130530132431.htm
     3.     Zhuo Wang, et al., The draft genomes of soft-shell turtle and green sea turtle yield insights into the development and evolution of the turtle-specific body plan. Nature Genetics, 2013; DOI: 10.1038/ng.2615