John Wilkins points me to an article by Carl Zimmer in the NYT on complexity in cnidarians, and tells me I have to discuss it. When Wilkins tells me to do something, I don't ask when, and I don't just hop to it instantly—I hop into my time machine and get it done six months before he asks me, with an update a month before. Jeez, but I'm good.

The question is about the validity of germ layers as a marker of phylogenetic history. I think it still holds up; gastrulation was a watershed event. What the cnidarian story is telling us, though, is that molecular complexity preceded the step in tissue diversification. Here, I'll really be full of myself and quote me:

The kernel of this idea is already in evolutionary theory, that evolution may proceed by duplication and expansion of elements of the genome, followed by fine-tuning, specialization, and paring down. In that sense, as a general observation, this result isn't surprising at all. What is unexpected is how far back in time we have to push the period of expansion of complexity. The original bilaterian was equipped with a fairly elaborate set of molecular tools.

The observation that genomic complexity is not tightly coupled to morphological complexity is also important. What that is saying is that the diversification of complex forms observed in the Cambrian was not going on concurrently with an elaboration of genetic systems, but was instead built on a foundation of rich genomic resources accumulated during the previous few billion years.

I think growing genetic complexity is only one part of the story, and that what the evolution of gastrulation did was provide novel epigenetic circumstances that amplified genomic potential. Interactions between tissue layers are still a key distinction.