[tsimionescu]

> My personal opinion is that life under early Earthlike conditions was/is almost a guarantee. There is compelling evidence that life already existed in and survived the Late Heavy Bombardment- that's very early in our planet's history. If it was truly a rare event that sparked things off, I would expect it to have occurred much later in the planet's history.

That is valid of course, but in the absence of evidence of its existence, the default assumption should be its unlikely.

[rsynnott]

Hmm, I'm not sure that that's correct. If you build an apartment block and it catches fire the week afterwards, then it's perhaps not reasonable to expect that if you build another one to exactly the same design it will never catch fire. At that point you might want to be reviewing your wiring plans.

That life showed up so early says... something. If life showing up was so fantastically difficult, we would arguably expect it to be later in geological time.

[tsimionescu]

Sure, but if you look at an extremely old building and find signs of fire only from its very early age, it's somewhat strange to say that it was very likely to catch fire.

[qes]

Life on earth isn't an apartment building.

What if you could only tell if the building had ever caught fire and how long ago it first caught fire? You can't tell how many times it caught fire, or when the last time was, or how many times it almost caught fire but didn't, or if it would've caught fire later but only didn't because it had already caught fire before.

[tomerico]

Well, if it did show up in the late heavy bombardment, it could have also arrived from outer space.