Tangent: RNA-like replicators either arose by chance combination of more primitive (or non-) replicators somewhere on Earth, or arose elsewhere and came to earth on spaceships or as spores on inert projectiles.

Let's assume that Earth's first interesting replicator spontaneously arose somewhere in a very low probability mixing of chemicals, followed by exponential growth (it just had to happen once). If this happened relatively soon after conditions permitted, does that suggest that the filter for life given Earthlike planet is pretty easy to pass? I'd guess it does - for example, if life first arose a billion years later than it actually did, the filter is probably harder. But of course I'd hesitate to just perform the obvious calculation; I think you have to compute the probability given that we exist, and it's hard to figure out how likely that (something like) we exist if the first life arises earlier or later.

Does anyone know the best estimate for when some interesting replicator populated a significant part of Earth?

Do we have evidence of any pre-RNA replicator of significance? (another tangent, sorry).