One of the perks of working for a school... I'll try to summarize as best I can:
The main calculation in the Nature paper is the energy budget of a cell. They give the example of E. coli: "to raise gene number tenfold, E. coli must also increase its energy budget by close to tenfold; and therein lies the problem." The reason so much more energy is required is due to protein synthesis (DNA codes to RNA which codes proteins, if you're unfamiliar with genetics). So obviously with a larger genome, there's more proteins to synthesize, and this cannot be accomplished by simply making fewer copies of the old proteins. If anything, you need more of some of those original proteins as infrastructure support for this new Genome 2.0.
So (and this is my wording) prokaryotes are at a local maximum for genome and cell size. Chucking mitochondria into the picture provides the wattage (literally) for them to reach a much greater genome and cell size, and therefore organismic complexity.
I am unclear about why the acquisition of mitochondria by a cell is considered to be such an unlikely event, as I lean towards the view that given enough cells/space/time, unlikely short term possibilities become distinct probabilities eventually. Also, I can't help wondering if the reason that simple cells are not observed to absorb free mitochondria is that eukaryotic cells and the organisms they evolved into have somehow poisoned the well, or attacked newly-complex competitors so aggressively that they've suppressed the population of omnivorous simple cells.
But I'm no biologist. Am I missing something obvious, or is there a particular paper explaining why this mitochondrial upgrade is considered such an unlikely event?
I would guess if one prokaryote engulfs another, it's highly unlikely that both of them are going to survive the experience, much less that both will thrive and evolve a symbiotic relationship, much less that this relationship will persist through cell divisions into subsequent generations. Both organisms would have to be uniquely suited to the relationship in the first place.
The main calculation in the Nature paper is the energy budget of a cell. They give the example of E. coli: "to raise gene number tenfold, E. coli must also increase its energy budget by close to tenfold; and therein lies the problem." The reason so much more energy is required is due to protein synthesis (DNA codes to RNA which codes proteins, if you're unfamiliar with genetics). So obviously with a larger genome, there's more proteins to synthesize, and this cannot be accomplished by simply making fewer copies of the old proteins. If anything, you need more of some of those original proteins as infrastructure support for this new Genome 2.0.
So (and this is my wording) prokaryotes are at a local maximum for genome and cell size. Chucking mitochondria into the picture provides the wattage (literally) for them to reach a much greater genome and cell size, and therefore organismic complexity.