There's an interesting related wrinkle that I happened upon recently: The mystery of mitochondria. Nobody is quite sure how mitochondria came to be in cells, but it appears that they evolved as a separate life form and somehow, improbably, got incorporated into at least one other organism. They exist this way to this date--a reason that their DNA is totally independent from the rest of the DNA in every complex organism on earth. Without mitochondria, life could exist, but it would be very, very simple. It was that way on earth for
2 billion years, and life stayed very simple--single very very small cells. And they could not get bigger or more complex. But, at that point, and by a mechanism not yet explained (and which apparently only occurred
one time in earth's history, based on the existing evidence in every cell), two independent single celled organisms began to exist within a single cell membrane in a mutually beneficial way (perhaps as one was eating the other). What would eventually become the mitochondria provided a tremendous increase in the amount of energy available to the host cell. And at that point, living organisms could suddenly get to be complex and over 10,000 times as large. There was absolutely nothing inevitable about this--life on earth could easily have continued at a very simple level indefinitely.
The fact that both organisms survived, even once, and replicated within each other, was incredibly improbable.
An (overly?) "accessible" version of the story is in this Radiolab podcast:
Cellmates.