[tsimionescu]

Given that life only appeared once on earth in 3.5 billion years on Earth, the Milky Way is around 14 billion years old, we don't really have a good reason to believe there were more than a handful of abiogenesis events in the history of the galaxy.

Of course, its possible this is completely wrong, perhaps the Earth was completely unlucky, or perhaps there have been many abiogenesis events and they have just failed for whatever reasons.

[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.

[rosywoozlechan]

I imagine it's much harder for life to develop a second time once there's already life present. So I'm not sure how that affects your 3.5 billion time window. Prior to life appearing on Earth, the planet's conditions may not have been suitable. It may be that life appeared as soon as possible once the conditions allowed for it and that life developed to become so good at taking up available resources needed for life that new instances of life could not come about.

[tsimionescu]

That only makes sense if we imagine life appearing in a primordial pool. But if abiogenesis were a common event on Earth, life should have formed all around, at distances far too large to immediately consume other proto-life forms. I also find it hard to imagine that, had this been a common event, a single strain of life should have resisted all the way to today.

Of course, until we manage to reproduce abiogenesis or find other examples of life, we won't know for sure. But I would say the theory with the least amount of assumptions right now is that life only appeared once in Earth's history or at least in one single relatively small place.

[baja_blast]

Why would there be multiple abiogenesis events? It seems once life evolves it can survive a lot and has all extinction events. The fact life occurred almost immediately after it formed suggests abiogenesis is common and more the rule as opposed to the exception. Multicellular life on the other hand could be much rarer since it took billions of years to happen.

[tsimionescu]

If abiogenesis is common, we should expect it to keep happening everywhere around us. We should have noticed at least a few instances by now, given that we have been trying explicitly to produce it.

So at the very least, we can say that the conditions for the apparition of life and the conditions for the proliferation of life seem to be quite different.

Also, given the amount of isolated biomes on earth, if abiogenesis had ever been a common occurrence, i think we should expect for there to still exist some survivors of those other events.

[swebs]

The entire planet is coated with bacteria and bacteriaphages. It's entirely possible that any new life created through abiogenesis is almost immediately outcompeted by evolved microbes.

[tsimionescu]

In today's world, perhaps, but it must have taken vast amounts of time until the first bacteria covered the planet. If abiogenesis were a frequent event on the early earth, there should have been plenty of space for more than one lifeform to survive before life became ubiquitous. And of course it's possible, but it shouldn't be the default assumption either.

[delecti]

What would abiogenesis even look like today? Some random molecules bouncing together in just the right way and spontaneously forming some organic compounds. And then a bacteria comes along and eats it, or it encounters the huge concentration of environmental oxygen and reacts.

Going from inorganic sludge to life isn't "an event", it's multiple events over millions of years until the right chemicals get together and form something that can reasonably be called "a thing" that can even be alive. It only makes sense for that process to have happened once, because the resulting life would have outcompeted any future proto-life.

[tsimionescu]

Sure, that's a plausible account. However, they would only compete if they appeared in a pretty small geographical area. That still leaves it as being a relatively singular event. Maybe much more than once in a few billion years, but still much rarer than many seem to assume.

I would also note that we don't really know of any mechanism that would require millions of years for chemical reactions, or any equivalent of the theory of evolution that would work for hypothetical complex organic substances. Not to say that either is impossible!

[rsynnott]

> Given that life only appeared once on earth in 3.5 billion years on Earth

So, one theory is that once life shows up it tends to suppress protolife.

If it is hard for life to show up, then it is arguably very suspicious that it apparently did so so soon (in geological time) after conditions were suitable for it.