[tsimionescu]

They meant, it occurred once on Earth, or at the very least it has stopped occurring for a few billion years now. That is still staggeringly low odds of a living planet seeing this event.

The same is true of abiogenesis - here we have a planet teeming with life absolutely everywhere we've looked, and yet 0 abiogenesis to be seen anywhere today. So, in general, when seeing a planet like the Earth today, we should expect to find NO life on it, as we already know teh Earth today can't sustain abiogenesis.

[YeGoblynQueenne]

I'm out of my depth here (I know a few things about computers but - biology? Not so much) but I suspect that once life gets going it kind of hogs all of the resources necessary for life to begin again.

So perhaps abiogenesis hasn't happened again on Earth because it's already happened and there's no space for it to happen again until the current batch of living things has been extinguished.

If that holds any water, then life could have started any nymber of times in the past and ended soon after, without leaving any trace we can detect.