[saas_sam]

Is "life only appeared once on earth" an accepted fact at this point? There was clearly at least one occurrence that led to a subsequent successful reproductive pattern from which we evolved, but prior to that, or alongside that, it strikes me as pretty unlikely that we would be able to discover other fleeting abiogenesis events.

We just discovered new salivary glands in human heads. Organs. Not microscopic ones. They are in our actual heads that we are walking around with and we just found out about it like a week ago.

So, forgive me if I am a little skeptical of our ability to find hard-to-find things. Some stuff is... hard to find.

[Baeocystin]

No. We have a Last Universal Common Ancestor, but that says nothing about whether or not there were other independently-formed branches that were competing at the time.

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.

https://en.wikipedia.org/wiki/Late_Heavy_Bombardment#Geologi...

[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.

[mod]

Not to mention it could have started and been scarfed up by already established life. It didn't have to be long-lasting to have happened.

[undersuit]

An abiogenesis event being snuffed out by another abiogenesis event... that's like alien invasions.

[tsimionescu]

Sure, that's not impossible at all.

Still, until we find such life, the default assumption should be that it doesn't exist, and so that abiogenesis is an extremely unlikely even, even on Earth.