We know this much from planet Earth, and therefore this should be our starting point when considering life elsewhere: The Earth is 4.5 billion years old. Life

We know this much from planet Earth, and therefore this should be our starting point when considering life elsewhere: The Earth is 4.5 billion years old. Life existed by roughly 3.5 billion years ago, possibly earlier. Multicellar life only got going about 0.7 billion years ago. So, roughly speaking, 1 billion years to get to life, but 4 billion years to get to multicellular life. So we know that multicellular life is 4 times more difficult than life itself. So for every 4 planets where we find life, we should expect 3 of those planets to only have single-celled life. Of course, it is possible that life arose on Earth as early as 4 billion years ago. We have no record of that, partly because the Earth was still getting hit by asteroids that left craters 1,000 kilometers in diameter (we know this because we can see that the moon was hit with these asteroids, back at that time). Since such events would sterilize the planet, that would also imply that life re-started many, many times. But it would also mean that the single-cell/multicell ratio is more like 1 in 8.