I recently wondered, in response to an “ideas” paper in BioEssays, whether animals, fungi, slime moulds etc. actually had a multicellular common ancestor. Dickinson and colleagues’ argument (partly) hinged on the shared presence of epithelia, “barrier” cell layers with distinct insides and outsides, in animals and the social amoeba Dictyostelium discoideum. The most recent crop from BioEssays includes a short letter by František Baluška of the botany department at the University of Bonn that challenges this argument.
Plants, Baluška reminds us, also have epithelia. These epithelia are functionally more similar to animals’ than the one Dickinson’s team found in the amoebae. While there may be doubts about amoebae, plants almost certainly became multicellular independently of animals. Ergo, convergent evolution can clearly produce similar tissues in two distant lineages. So why would we take the possession of an epithelium as evidence for a multicellular common ancestor?
Which is a perfectly valid argument, but it misses the point in my opinion.
The botanist writes,
[Plants] evolved their own plant-specific epithelia 3–5, obviously via convergent evolution. This fact alone not only continues to make plausible the traditional independent origin of multicellularity in the metazoa and social amoebae, but it also indicates that the power of convergent evolution should not be underestimated.
Of course it shouldn’t, but Dickinson’s team wasn’t arguing that “the traditional independent origin of multicelluarity” in animals and amoebae was not plausible any more. They find it unlikely that the functional and molecular similarity (does the latter exist between plants and animals?) between animal and amoeba epithelia is convergent, but they are suggesting that we investigate their new hypothesis, not that we summarily throw out the old one. Baluška is attacking a straw man.
Furthermore, he only addresses this one argument, but the thing in the Dickinson article that made me think the most was phylogeny. According to the traditional scenario, it seemed more likely that all those different unikont groups evolved multicellularity independently. But multicellularity is very widespread among unikonts, so precisely what makes the traditional scenario more likely? (Incidentally, has anyone done any actual statistics on this?)
As far as I’m concerned, the letter said nothing to change my mind. Dickinson et al. presented an interesting idea that’s definitely worth a closer look. I don’t think the evidence is currently strong enough to upset the consensus, but the proposal is not at all daft. I have to say I agree that plants should not be ignored, though. Because we can assume that any similarity between them and animals when it comes to being multicellular is the result of convergence, they’d be a wonderful “control group” when people start testing Dickinson et al.‘s hypothesis.
I think that’s something students of evolution should always keep in mind. Plants and animals have little reason to do things in the same way – they diverged very long ago, adapted to completely different lifestyles, etc. If they do so anyway, that might tell us something deeper about the way living things work. A limitation imposed by physics, a very ancient genetic predisposition, or simply the best way to do something – either way, finding the reason will enrich our knowledge of life and evolution. Animal scientists would be well advised to remember that.
***
Reference:
Baluška F (2012) Rethinking origins of multicellularity: Convergent evolution of epithelia in plants. BioEssays, available online 26/10/2012, doi: 10.1002/bies.201200134
The Cambrian Mammal 