Once upon a time, in Paris in 1830, Etienne Geoffroy St. Hilaire debated Georges Léopole Chrétien Frédéric Dagobert, Baron Cuvier on the subject of the unity of organismal form. Geoffroy favored the idea of a deep homology, that all animals shared a common archetype: invertebrates with their ventral nerve cord and dorsal hearts were inverted vertebrates, which have a dorsal nerve cord and ventral hearts, and that both were built around or within an idealized vertebra. While a thought-provoking idea, Geoffroy lacked the substantial evidence to make a persuasive case—he had to rely on fairly superficial similarities to argue for something that, to those familiar with the details, appeared contrary to reason and was therefore unconvincing. Evolutionary biology has changed that — the identification of relationships and the theory of common descent has made it unreasonable to argue against origins in a common ancestor — but that difficult problem of homology remains. How does one argue that particular structures in organisms divided by 600 million years of change are, in some way, based on the same ancient organ?
One way is sheer brute force. Characterize every single element of the structures, right down to the molecules of which they are made, and make a quantitative argument that the weight of the evidence makes the conclusion that they are not related highly improbable. I'll summarize here a recent paper that strongly supports the idea of homology of the vertebrate and arthropod heart and vascular systems.
Continue reading "Evolution of vascular systems" (on Pharyngula)