Tuesday, 11 June 2013

Embryos and ancestors in deep time

I read Gavin de Beer's monograph while still at school. Scornful of Haeckel's idea that ontogeny recapitulates phylogeny, de Beer details the several ways in which embryonic development is modified during the evolution of new species. The book remains a useful reference source for concepts like neotony and paedomorphosis.

Fifty years on, I was delighted to read this new monograph by Marcelo Sánchez-Villagra. He is a paleontologist and gives fascinating examples from the fossil record. Developmental series of extinct species are relatively rare, but much can be induced from studies of bone and tooth structure. Modern and non-invasive imaging techniques are able to yield new information from rare specimens diligently guarded by museum curators. Phylogenetic bracketing (as of non-avian dinosaurs between crocodiles and birds) is another approach from which we can infer developmental processes in extinct lineages. It then becomes possible to speculate on the role of the Hox family of regulatory genes and draw on other data from molecular biology.

There is of course much here on the evolution of viviparity and placentation and on reproductive strategies. The fossil record has much to teach us here (see my paper here).

De Beer was scathing about Haeckel because his ideas had stood in the way of experimental embryology. Sánchez is kinder, recalling that more Europeans learned about evolution from the writings of Haeckel than of Darwin. His own view is summed up as "ontogeny does not usually recapitulate phylogeny."

The focus of this book is sharpest in the opening chapters and I would have liked to have seen the threads drawn together in a concluding chapter. It is nonetheless a good read and never gets bogged down in technicalities.

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