The molecular ancestry of segmentation mechanisms.

I n 1830 a very important debate on natural history took place in the French Academy of Sciences. As retold in enjoyable detail by T. A. Appel (1), the adversaries were Georges Cuvier and Etienne Geoffroy Saint-Hilaire. These were pre-Darwinian times—the Origin of Species was published in 1859—so their arguments sound antiquated today, but their reverberations among biological ideas have continued to the present day. Cuvier, the discoverer of extinction, held the view that animal anatomy was determined by the functional purpose of each organ, which he called the ‘‘conditions of existence.’’ Geoffroy, the discoverer of ‘‘analogies’’ (now called homologies), held that animal anatomy had a ‘‘unity of plan’’ upon which thousands of variations were imposed. He compared vertebrates to arthropods, arguing that they shared antero–posterior (A–P) characteristics, such as head, thorax, and abdomen, as well as dorsal– ventral (D–V) landmarks such as a CNS, gastrointestinal tract, and heart, except that a D-V inversion of the body plan had occurred. Geoffroy even had the temerity of comparing the segmentation of arthropods to that of vertebrates. Cuvier presented the better arguments and was considered the victor of the debate. However, modern evolution and development studies have discovered that common genetic networks pattern the A–P and D–V axes of very distantly related bilateral animals (2, 3). This realization has provided a measure of molecular support for Geoffroy’s unity of plan hypothesis and for the idea that the last common ancestor of the invertebrate and vertebrate lineages was a rather complex animal, called Urbilateria (which means primeval bilateral animal), which predated the Cambrian explosion that took place 535–525 million years ago (3). In this issue of PNAS, Pueyo et al. (4) make an important contribution to the question of whether common mechanisms of segmentation are shared by insects and vertebrates (5, 6).