That would make life elsewhere in the galaxy even less likely than just the fateful encounter, because it could only occur on those wet, rocky, alkaline hydrothermal vent filled planets that produced at least TWO different lines of free living single celled biochemical machines, er, lifeforms. That last sentence is to highlight that, no matter what religious people (and others) believe, biochemistry is only microminiturized geochemistry, and that life is only different from energetic geochemical systems in that size and free floating status.

Possibly an endosymbiosis between two bacteria (a few I think are known or suspected) or between two archaea (of which I’m not sure if any are known, but of course we currently know less about archaeans than we do about bacteria) would not have produced the same kind of evolutionary opportunity.

Lane has a very thorough and easy to understand description of the basic reasoning behind this theory in his book “Power, Sex, Suicide: Mitochondria and the Meaning of Life”. Further information can be found in Lane’s third book “Life Ascending” in the chapter on the eukaryotic cell.

The thing to remember is that phagocytosis has never, ever (at least to date) been observed in a prokaryote, and is currently considered to be a primitive eukaryotic trait (all eukaryotes either have it or secondarily lost it). It is also intimately associated with a dynamic cytoskeleton, which is also very much a signature eukaryotic trait.

The question at hand is did the mitochondrial endosymbiosis occur before or after phagocytosis evolved. Phagocytosis provides an easy starting mechanism for multiple instances of endosymbiosis, so if the mitochondria were acquired after phagocytosis, then it is just one (perhaps the first) of many similar events, and there is nothing unique of special about it. If on the other hand, the mitochondrial endosymbiosis occurred before phagocytosis, then it must have arisen from some other mechanism, which given the paucity of known endosymbiosis in prokaryotes, might have been a very rare and unlikely event.

Lane and Martin are essentially arguing that due to the energetics involved, phagocytosis can not evolve in cells that do not already have mitochondria, and so the mitochondrial endosymbiosis could not have occurred simply as the result of a phagocytotic event, and must have happened before the evolution of phagocytosis.

In the most extreme version of their theory, the mitochondrial endosymbiosis is not just an “early” event, it is the enabling event. In their theory virtually everything unique about the eukaryotic cell, from large genome, to multiple introns, to straight chromosomes, to large amounts of noncoding “junk” DNA, to dynamic cytoskeleton, to phagocytosis, to large size, to loss of the bacterial cell wall, all of it, arose as a direct consequence of the fateful mitochondrial merger.

Once the fully formed eukaryotic cell evolved, further endosymbiotic events were relatively easy, and occurred multiple times. But the first one was special, unlikely, and unique.

This hypothesis can of course be falsified by a single demonstration of actual phagocytosis in a prokaryote.

Great stuff. Do you have a reference (accessible to a non-biologist) for this:

as far as I know, endosymbiosis does not happen in a single generation – there would be a long period where one cell type would likely be an ectosymbiont or even a parasite (as mitochondria’s closest bacterial relatives are). There is a recent example of an ectosymbiosis between a giant archaea and sulfur-oxidizing bacteria that results in a large multicellular archaeal filament coated in sulfur oxidizing bacteria, putting an interesting twist to how multicellularity may have evolved in its earliest stages.

Also, as far as inbetween forms – as far as I know, endosymbiosis does not happen in a single generation – there would be a long period where one cell type would likely be an ectosymbiont or even a parasite (as mitochondria’s closest bacterial relatives are). There is a recent example of an ectosymbiosis between a giant archaea and sulfur-oxidizing bacteria that results in a large multicellular archaeal filament coated in sulfur oxidizing bacteria, putting an interesting twist to how multicellularity may have evolved in its earliest stages.

I’m not a biologist, but simple logic would dictate that an energy revolution resulting in eukaryotic cells would enable them to multiply exponentially to occupy all available niches for that sort of critter pretty fast, and thus make it difficult (but not impossible) for this type of evolution to happen a second time in a short time frame. So my guess is that it would be slow to evolve an endosymbiosis, but probably not hard, but difficult to do again once the first critter with such an energy advantage got a head start.

All in all, I find the research fascinating and I find the hypothesis about the energy revolution plausible in the main, I only question the assumption that endosymbiosis between two prokaryotes would be rare.

OK. It’s a very good point that energy is essential to maintaining more complex structures. And I like the discussion of the need to have a separate and small genome to process energy. It’s not clear to me how this tells us whether the merger occurred early or late. All it seems to tell us is that the merger occurred before prokaryotes reached the limit of complexity imposed by their pre-merger energy structure.

From my perspective as a non-biologist it’s not really all that interesting when the merger occurred. What’s more interesting (at least to me) is the fact that it was a necessary pre-requisite to further development.

Did Lane and Martin make that point? (That’s a real question; I don’t know.) Did Margulis not know that the merger and a small energy genome was necessary for complexity? (Again, that’s a real question.)

These do seem to be separate issues: (a) that a small genome for energy is required and (b) how it came about. An alternative for (b) — at least in the abstract — might have been that a portion of a prokaryote genome somehow broke off and persisted independently in the cell. In that case it would have been an internal division rather than a merger. (I have no idea whether that’s even biologically possible! It’s a top-of-the-head thought.)

Lane and Martin are essentially using energetics considerations to argue in favor of the “fateful encounter” end of the spectrum.