Over at Evolution News and Views (ENV), the blog for the Discovery Institute, Ann Gauger has a new article about Volvox (“A Simple Transition to Multicellularity — Not!”). This isn’t the first time ENV has weighed in on the evolution of multicellularity (see here and here, for example), but since their website doesn’t allow comments, this is the first time I’ve had a platform from which to respond.
The article starts out with a mostly accurate description of Volvox biology, although the description as
…among the simplest animals to have more than one cell type
is cringe-worthy: Volvox is no more an animal than is a tomato:
She then summarizes David Kirk’s excellent “12-step” paper, which synthesizes a quarter century of developmental genetics carried out mostly by Kirk and his numerous students and postdocs (note: I have criticized some details of the 12-step paper myself, but it was foundational for much of my PhD work). Not surprisingly, Dr. Gauger is not impressed by Kirk’s paper:
Missing from this story, though, are the details necessary for this 12-step progression to occur.
Missing if you haven’t actually read the 12-step paper or any of the relevant literature from the subsequent decade. I plan to take this article apart, but I also plan to go camping this weekend, so for now I’ll have to settle for one substantive criticism, on the topic of inversion. Dr. Gauger correctly points out that Volvox embryos begin life inside-out, with their flagella on the inside, and must undergo a process of inversion to get their flagella on the outside where they can do some good. She notes that this process requires the motor protein kinesin, and
Neither microtubules or [sic] kinesin are [sic] present in bacteria so their origin must be accounted for.
What she fails to mention is that microtubules, kinesins, and kinesin homologues are present in Chlamydomonas reinhardtii, a close unicellular relative of Volvox. It might be argued that this only pushes back the problem; kinesins still had to originate somewhere (though the eukaryotic nuclear genome is thought to have descended from an Archaean, not a bacterium). However, this is an article about multicellular life evolving from unicellular (eukaryotic) life. If the common ancestor of Chlamydomonas and Volvox had kinesins, which it most certainly did, then where is the problem? Also not mentioned (but clearly described in the 12-step paper), Gonium pectorale, a small colony more closely related to Volvox, undergoes partial inversion during development. Whether or not partial inversion was an intermediate step on the way to complete inversion, its presence in Gonium certainly demonstrates that it is a plausible intermediate.
Stay tuned for Part 2; there is plenty more wrong with Dr. Gauger’s article.