Monday, October 03, 2005

How the metazoan got its body plan....   posted by Razib @ 10/03/2005 08:48:00 PM

Folk science tells us that the Cambrian Explosion was the source of all the body plans that we see around us, an evolutionary miracle where morphological diversity emerged fully formed from the brain of Gaia (i.e.; bilateral vs. radial symmetry). Some Creationists use this to argue that God created all these creatures specially, and the famous Hox developmental gene family were the tools which aided God in his design. Of course, geological time isn't the same as our own intuitive gestalt perceptions of what an instantaneous "explosion" is like. A few years ago the paleontologist Andrew Parker, in his In the Blink of an Eye, argued that the development of vision was the crucial prod that resulted in the proliferation of multicellular (hard-bodied) life. My post about the Pax gene family suggests that we should be cautious about such an explanation, since the smoking gun of a shared derived character doesn't seem to fit well in the case of vision (synapomorphies vs. symplesiomorphies).

If any of these issues strike your fancy, I will point you to two papers, The dawn of bilaterian animals: the case of acoelomorph flatworms and The Hox gene complement of acoel flatworms, a basal bilaterian clade. The short of it is that the Hox gene family itself, that most wonderous of toolkits, has likely been tinkered with by evolution. The action in these papers centers on the animal node of the tree of life, it seems that the 8-10 Hox gene complement characteristic of vertebrates and most of those we colloquially know as invertebrates developed in stages, likely via gene duplication, from the common ancestor which the Bilateria share with the less complex taxa. In particular, the papers above suggest that one group of bilateria, a branch of the Platyhelminthes termed the Acoela, might be misclassified, and that their true cladistic position is basal, the outgroup, to the two great families of deuterostomes and protostomes. This group exhibits only some of the Hox genes characteristic of the bilateria. The normal explanation, the most parsimonious one, would be that these genes were lost over time (this is known to have occurred among the nematodes), assuming the old cladistic position. But the authors of the papers above point out that multiple loci now point to a misclassification and a basal location in the phylogenetic tree vis-a-vi other Bilateria. In the second paper they assay two distantly related species of Acoela and offer that the loss of genes hypothesis is implausible, and the minimal set of Hox is likely an ancestral characteristic. Additionally, they point out that that non-Bilaterian animals tend to exhibit this minimal set as well. They conclude that the Hox genes went through duplications during the evolution of bilaterians. There are many assumptions that the authors make, and certainly this is not a well established field (i.e.; evo-devo), so findings and analyses are extremely provisional, but the message is that the revolution that occurred in the minds of evolutionary biologists when the conservation of Hox genes was discovered should not blinker us to the reality that change is still a constant when it comes to natural history, and even tools get tinkered with.