"What I would say with some degree of certainty from the example of life on earth, is that if you simply have a population of bacteria... the chances of it giving rise to the kind of morphological complexity... we see in eukariotic cells, and we do not see in bacteria, is remote... because bacteria and archaea, if you look at the amount of , they dwarf the genetic variation that we see in Eukaryotes. They have explored genetic sequence space to orders of magnitude greater that Eukaryotes did, and despite exploring all of that space, they haven't come up with morphological complexity. ...[T]hey did through an endosymbiosis. ...It's rare between prokaryotes, rare to the point that we know of one example of free-living bacteria with bacterial cells living inside it. We know of two other examples where, there's a for example, which has inside its own cells... some gamma protein bacteria, with beta protein bacteria living inside them. It's a little bit of a strange system and it's hard to know, again, can you generalize from this, because it's all inside a Russian doll?"