[NineStarPoint]

We don’t, but we do know that there over the about 4 billion years of life, the first 1 to 1.5 billion years were purely prokaryotic. If it was a common thing to have work out, you wouldn’t expect such a massive amount of time before it occurred.

What I’m less confident on is that it’s the only path to complex multicellular life. It’s the step our history took, but it’s possible there are multiple rare pathways to the next step (or possibly different initial conditions could make the eucharyotic union or other options more probable? Just because it was unlikely here doesn’t mean that’s true on all planets).

[morsch]

> We don’t, but we do know that there over the about 4 billion years of life, the first 1 to 1.5 billion years were purely prokaryotic. If it was a common thing to have work out, you wouldn’t expect such a massive amount of time before it occurred.

Based on that evidence you could also conclude that it was impossible for it to happen for 1 to 1.5 billion years (because it didn't) and then it was suddenly very likely (because it did).