Remnants from a conversation that I had at Confusion:
There are days that I think that the number of intelligent species in the galaxy may be one or less. (On average. There are a lot of galaxies out there.)
I think there may be more life than that. But I frequently suspect that there is very little in the way of advanced life. Microscopic things, sure. Larger organisms, maybe not so much.
Without mitochondria, it would be very difficult to have advanced life forms as we know them on Earth. Maybe there's something else that could somehow take the place of mitochondria in other systems, I suppose. Maybe not.
But it looks like mitochondria are the result of a developed symbiosis between a tiny bacterium that another bacteria engulfed, but ended up not digesting. And analysis of mitochondrial DNA suggests that capture happened once in the history of life on Earth.
It could be that mitochondrial capture is "easy", but you'd expect there to be distinctive lines of mitochondria then and that doesn't appear to be the case. It could also be that mitochondrial capture gives you such a big advantage that you outcompete all of the other organisms on the planet before they can manage their own mitochondrial capture.
Or it could just be that mitochondrial capture is really unlikely.
It's a theory, anyway.