Evolution of Cnidarian Giant Axons

Abstract

Speculations about evolution have an irresistable fascination, partly because they are hard to prove wrong, partly because they make us think about origins and look for clues in development. Cnidarians may have been the first metazoans to evolve, although this seems rather unlikely in view of the fact that they are all carnivores, preying on other metazoans. Given, however, that of the surviving phyla they alone retain the presumably ancestral (diploblastic) body plan, it becomes especially interesting to look at their nervous systems for clues as to how nerves evolved. This is not just a selling point for use in grant applications. There really is no better group in which to look for clues. At the same time, given the 700 million years which have elapsed since cnidarians appeared in the fossil record, any clues about nervous origins can only be of the most general nature. Cnidarian nerves are not obviously primitive in functional terms, only in the way they are laid out as center-less nets. Of course the layout can be far more complex than this. Some of them do have centres. At the last count, the jellyfish Aglantha had six, possibly seven, physiologically distinct neuronal subsets running in the marginal nerve rings. My efforts to understand their interactions have aged me prematurely. Other speakers at this meeting have looked at ionic channels, morphogenetic factors and neurochemicals for clues. Molecular biology may (probably will) eventually help us reconstruct phylogeny, and place the cnidarians in their proper place on the many-branched tree of neural evolution, but we are still far from this point. As of now, we do not know in what precursor cell line neurons first arose, through what stages they passed to assume their now familiar form, or what functions they originally served.