I speculate that when we look up at the sky at night, it is full of steaming jungles crammed with giant insects.
My theory is it takes a heck of a big whack from a comet or something like that to get a living watery planet from one stable ecosystem to a new more advanced one. The trouble is, it has to be a whack of the right size. Big enough to do the job, not so big as to wipe all the life out completely.
It's easy to think that evolution happens gradually, like drips in a cave building stalectites. Well, it does, but the real action is when there's a huge flood. All those dinosaurs didn't give over gracefully. Their time would've gone on forever if something big hadn't come along to end it.
Imagine building a pyramid, but you can only build the next level if you throw 10 dice and they all come up 6. How many dice throws is it going to take? A lot, but it's mathematically guaranteed to happen and even though it is pretty unlikely, in our case it has already happened and here we are!