For several years now, I have been asking “where are the prokaryotic mice?” Given that the prokaryotic cell plan is the most ancient, abundant, and successful cell plan on earth, why hasn’t the blind watchmaker been able to craft together the convergent equivalent of some metazoan? In fact, as I noted, “had the eukaryotic cell design failed to emerge, the Earth would contain nothing more complex than any extant bacteria in existence today.” Then, back in October 2010, a paper from Lane and Martin was published that supports my contention:
Our considerations reveal why the exploration of protein sequence space en route to eukaryotic complexity required mitochondria. Without mitochondria, prokaryotes—even giant polyploids—cannot pay the energetic price of complexity; the lack of true intermediates in the prokaryote-to-eukaryote transition has a bioenergetic cause.
A few days ago, biologist PZ Myers recently helped to popularize Lane and Martin’s paper and begins by essentially asking…you guessed it…”where are the prokaryotic mice?”
I had to wonder: why have eukaryotes grown so large relative to their prokaryotic cousins, and why haven’t any prokaryotes followed the strategy of multicellularity to build even bigger assemblages? There is a pat answer, of course: it’s because prokaryotes already have the most successful evolutionary strategy of them all and are busily being the best microorganisms they can be. Evolving into a worm would be a step down for them.
That answer doesn’t work, though. Prokaryotes are the most numerous, most diverse, most widely successful organisms on the planet: in all those teeming swarms and multitudinous opportunities, none have exploited this path? I can understand that they’d be rare, but nonexistent? The only big multicellular organisms are all eukaryotic? Why?
So Myers is asking, “Why are there no prokaryotic, big multicellular organisms. In other words, “Where are the prokaryotic mice?” (or better yet, “Where are the prokaryotic squids?”)
Myers also adds:
Eukaryotes have a key innovation that permits the expansion of genome complexity, and it’s the mitochondrion. Without that big powerplant, and most importantly, a dedicated control mechanism, prokaryotes can’t afford to become more complex, so they’ve instead evolved to dominate the small, fast, efficient niche, leaving the eukaryotes to occupy the grandly inefficient, elaborate sloppy niche.
Another way of saying this is that the prokaryotic cell plan constitutes an “edge” to evolution, guiding prokaryotes to the small, fast, efficient niche. In contrast, the eukaryotic cell plan allows these cells to circumvent this edge, guiding them to terrain where the emergence of metazoan-type complexity is not only possible, but is just a matter of time. I explained the significance of this crucial step before:
This is not about whether prokaryotes could ultimately spawn eukaryotes, as I accept that. As Steve notes, there is a solid hypothesis about that transition and I have previously explored some of the ways this symbiotic union may have been front-loaded. This is about whether the cell design – the composition and architecture of the prokaryotic cell – is capable of generating something as structurally complex as a mouse (for a mouse, like all animals, is an assembly of cells). Seen from this angle, the endosymbiotic hypothesis supports my position. That is, in order for prokaryotes to ultimately spawn eukaryotes, they first had to go through a radical re-design of cell structure.
Prokaryotic cells can be viewed as the highest expression of mutation and selection, for there is no better cellular candidate for a “self-replicator.” Yet after billions of years, the prokaryotic cell plan has failed to achieve anything near the level of structural complexity as exhibited by the eukaryotic cell plan. To reach such structural complexity, the cell design had to be radically retooled, partly through endosymbiotic union, a one-time event given the widely accepted monophyly of eukaryotes. Once the eukaryotic cell design was established, prior to the radiation of all extant eukaryotes, the basic cell design was now capable of supporting the emergence of complex, metazoan life. The evolution of metazoa did not require further extensive retooling of the eukaryotic cell plan, given that metazoan cells are so similar to protozoan cells; it was more like the natural outflow of the potential inherent in the eukaryotic cell plan.
Myers also adds:
So, what Lane and Martin argue is that the segregation of energy production into functional modules with an independent and minimal genetic control mechanism, mitochondria with mitochondrial DNA, was the essential precursor to the evolution of multicellular complexity — it’s what gave the cell the energy surplus to expand the genome and explore large-scale innovation.
In other words, had the eukaryotic cell design failed to emerge, the Earth would contain nothing more complex than any extant bacteria in existence today.
So this all leads to the next level of analysis – why haven’t any bacteria evolved a mitochondria-like structure to open up this niche to them? In the next posting, we’ll build on some of Myers explanation, but take it a bit deeper.