[rrmm]

Does it seem that way? It happened at least once (but could have happened many times without "taking over"), and certainly one sort of life seemed to successfully out-compete all others. But none of that says single-origin to me.

Early on I would expect a whole lot of "horizontal gene transfer" sort of things to have taken place. So for example in addition to actual horizontal gene transfer, there are mechanisms like one organism enveloping another to eventually become organelles, co-opting products from each other, etc. All of which would act to homogenize life and make certain process ubiquitous.

Finally, there's an outside chance that "there's only one way to do it".

[roncesvalles]

I think single origin event is highly likely because, for example, it's wholly conceivable that a slightly different variant of AUCG (or just one of the molecules) could've emerged and it would have similar characteristics, but not differentiated enough that one would have a very strong selective advantage over the other.

Diversity could exist in harmony and the lack of any diversity is a pretty strong signal that the only extant version is either very rare or the only to ever emerge.

Everything in nature is diverse except RNA/DNA and this fact alone is a sort of evidence.

[vlovich123]

Or the basic life that forms is going to look the same regardless of where it starts on Earth, meaning that you’d never have evidence of two origins.

Or RNA was just a winning virus that infected all other life or killed all competition to make it seem like there was only one origin.

Absence of evidence is not evidence of absence.

[roncesvalles]

I'm pretty sure we've checked every lifeform to see if they follow the universal genetic code, and they do. RNA is too fundamental to be a virus infection.

When you should see evidence but don't, "absence of evidence is not evidence of absence" doesn't apply. Otherwise absence becomes unprovable à la Russell's teapot.

[vlovich123]

Except for that you’d have to have a reasonable belief you know what to look for and where to look.

By comparison, we’ve looked nowhere (Earth is big and mostly inaccessible to humans) and don’t know what we should be looking for, since we don’t have any testable models for the origin of life yet.

And re RNA I was just giving one example of why all life might have it (ie RNA gave an evolutionary advantage so anything without RNA died). There’s all sorts of reasons why we don’t have evidence of multiple origins, not least of which is that we just haven’t looked in the right place and there could easily be life being created from scratch on this planet without even knowing about it. That’s what the entire field of synthetic biology is even about; just doing it in a lab instead of out in nature so that we can understand the conditions better.

[d1sxeyes]

Absence of evidence is of course not evidence of absence, but all life on earth today seems to be descended from a single organism, 3-4 billion years ago: https://en.m.wikipedia.org/wiki/Last_universal_common_ancest...

This is about a quarter of the lifetime of the universe ago, and we don’t have any evidence at all that life has ever occurred in any other way. We’ve only really been looking for a hundred years or so, but we’ve not found any “fountain of life” where life is being created, we’ve not found evidence of any type of life that isn’t broadly related.

I absolutely agree that it’s not evidence, but I believe that on balance, it makes more sense to take our working hypothesis to be something that fits the evidence we do have, rather than believing the evidence must exist we just don’t have it.

To be clear, I’m not advocating that we don’t investigate both possibilities, and I wouldn’t put much weight behind my own guess here.

[dgroshev]

An important nuance here is that LUCA doesn't really imply there was only that one organism. There could be more organisms, it's just the one that we can trace, similar to mitochondrial Eve (see the "popular reception" section): https://en.m.wikipedia.org/wiki/Mitochondrial_Eve