Nick Lane

Well, the easiest way to understand that is to say, well, it wasn't adaptation to an external environment, to a way of life.

It was adaptation to an internal selection pressure.

If you think about it in terms of a kind of a battle between the host cell and the endosymbiont for finding a way of living together, you can argue for the nucleus arising that there's all kinds of genetic problems.

parasites coming out of the mitochondria, forcing you to do something to protect your own genome.

So you can construct a lot of this history of eukaryogenesis, it's called.

So you start with simple cells with a cell inside and you end up with the same cell structure everywhere, all these endomembrane systems and everything else.

So to have a multicellular organism where effectively you're deriving from a single cell, and that restricts the chances of effectively all the cells having a fight.

There's plenty of examples of multicellular slime molds, for example, where the cells come together

And they can form structures like a stalk, for example, which loosens spores into the environment.

But they basically fight because they're genetically different to each other.

So you start with a single cell and you develop... So there's less genetic fighting going on between the cells than there would be if they come together.

But that means then if you want to have...

complex functions if you want to have a liver doing one thing and kidneys doing something else and the brain doing something else all of the cells have to have the same genes but you you express this lot in the liver and that lot in the brain so you must have a large genome the only way you can have a large genome is by having mitochondria and having a eukaryotic cell there is no examples of this level of sophistication of a multicellular bacterium

Yeah, I know where you're coming from.

There's no other way to solve that.

Well, maybe there is, but I think we have to look at the probability of certain things happening.

So if you want to have a giant bacterium, there are a bunch of giant bacteria around on Earth.

There's at least six or seven different quite unrelated species that have evolved giant size.

And the thing that they all have in common is they have what's called extreme polyploidy, which is to say they have literally tens of thousands of copies of their complete genome.

So it may be a small genome, but we're talking a three megabase genome, so kind of 3,000 genes in it.