The earth is around 4.5 billion years old. Life showed up within the first 1/2 billion years right around the time it cooled off enough for life to be possible. But let's say it's 50 / 50 every 500 million years (I think it's a lot higher than that.) well that's 99.8% after 4.5 billion years.
Note: If that seems high it's about the same as 1 chance in 1000 per million years for any form of life to show up.
It might seem odd for a random process to end up with a life form but there are a lot of possible life forms that could work. AKA picking a single winning lottery ticket is hard but winning the lottery is not that hard if you could buy a million tickets that never expire.
Note: If that seems high it's about the same as 1 chance in 1000 per million years for any form of life to show up.