The phosphotyrosine signaling pathway was long thought to be a metazoan innovation. This is because the pathway was once observed to be restricted to metazoans and was devoted to communication roles in processes crucial to the metazoan way of life – immune response, hormonal control, embryogenesis, etc. But ever since the genome of a single-celled choanoflagellate was sequenced, we now know this pathway was not a metazoan innovation – it predated the appearance of metazoans. It thus represents a perfect candidate for exploring evolution as a front-loaded phenomenon. So far, we on are pretty solid ground.
If you’re willing, allow me to stretch your thinking a bit.
Now let me turn your attention to something scientists have noticed with all this genomic information in hand:
The tyrosine kinase signaling networks of choanoflagellates and metazoans are very distinct, with only a few genes clearly orthologous between the two systems. Yet, both systems seem to have come up with similar or identical mechanisms and architectures. This apparent convergent evolution hints at global constraints within the phosphotyrosine signaling networks.
The author then lists a sample of such convergence:
Same solution, different mechanism.
And both metazoans and Monosiga have independently developed kinase proteins with dual catalytic domains, one of which is inactive.
This suggests that choanoflagellates have found new ways to wire the phosphotyrosine signaling circuitry, but that the overall themes are still similar.
Okay, so it seems we have yet another example of convergent evolution, a phenomenon that adds yet more support to the plausibility of front-loading. But we may have something even more significant here.
How do we conventionally define and explain convergent evolution? Well, we can’t get more mainstream than PBS’s site on evolution, so let’s let them explain it:
This illustration shows an example of convergent evolution in four different animals from around the globe. They may look similar, but it’s not because they’re close relatives. Instead, they’ve evolved similar adaptations because they occupy similar niches — dining on ants, hunting in the high grass, or swimming in the dark — although their evolutionary origins are quite different.
A niche is defined as “The ecological role of a species; the set of resources it consumes and habitats it occupies.”
PBS also adds:
This is a dramatic example of convergent evolution, when organisms that aren’t closely related evolve similar traits as they both adapt to similar environments. There are a finite number of effective solutions to some challenges, and some of them emerge independently again and again.
Yep, that is the mainstream explanation. And notice that it is the environment that is supposed to be responsible for crafting the similar traits. Similar niche – similar trait. Similar environment – similar trait.
So here’s the problem. There is no compelling evidence that the lineage that led to complex, metazoan life forms (including humans) and the lineage that led to the single-celled Monosiga independently experienced significantly similar niches or were exposed to similar environments. On the contrary, one might reasonably think that metazoan life, by definition, opened up a whole new world of niches and environments that cannot be experienced from a unicellular context.
Yet the convergence apparently occurred. Ready for the mind-stretch?
If both metazoans and Monosiga have independently come up with similar or identical mechanisms and architectures in the tyrosine kinase circuits, it would seem we might trace this to an intrinsic, rather than environmental, cause. That is, there was some kind of inherent molecular inertia built into the basic design plan of the TK circuit to cause it to evolve similarly in different creatures experiencing different niches.
The emergent complexity of this circuitry inside the choanoflagellate remains mysterious, as it is not clear why such a simple life form needs it. I would hypothesize that while these protozoans exist as single-celled organisms, they also exist, in nature, as a community that communicates and coordinates. But as long as they remained single-celled, the full functional potential of the system could not be exploited.
But when we turn to metazoa, as the circuitry unfolded because of the molecular inertia, a ready-made communication system was there “in waiting” to facilitate the rapid emergence and spread of metazoan life.
If the two systems are indeed convergent, and if they became so similar for intrinsic, rather than niche-related reasons, we would have an excellent example where two expectations of front-loading, intrinsic control and deep homology, fuse. And this wold lead us to ask just how many examples of convergence are due to intrinsic, rather than environmental reasons?