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I'm very sceptical of this. It's a distinct possibility, and it's even pretty likely, but I don't think Earth based abiogenesis is even close to being seen as u

I'm very sceptical of this. It's a distinct possibility, and it's even pretty likely, but I don't think Earth based abiogenesis is even close to being seen as unlikely. It's still the simpler explanation in my opinion. The lack of fossils is not too convincing since microbes don't fossilise well. And it's entirely possible that life from the RNA world was just outcompeted by prokaryotic, DNA-based life. There's also the question of RNA viruses and viroids, which could be descendants of the RNA world.

mjan22640 4y ago
Parasites in general undergo simplification, they reduce and eventually lose the functionality that the host provides for them. In my opinion this is the case in viruses, that they are so simple not because they are remnants of ancient life, but because that they are parasites that underwent secondary simplification.
- mtlmtlmtlmtl 4y ago
  That's one of the 3 most popular hypotheses, mine is another. But the jury is still very much out on this one.
adastra22 4y ago
Microbial life doesn't fossilize well, but the earliest secondary fossils we do have are of microbial mats. They exactly match in all respects certain microbial mat living fossils we see on the ocean today, produced by relatively complex life forms. Most hypothecated pathways for abiogenesis would have left similar secondary fossils in the geologic record. Yet we don't see them. There's not a lot of rock left from that long ago, so maybe it just didn't get preserved. But it is striking and unexpected how (1) the earliest fossils are of quite complex organisms; (2) they pop into existence very soon after the Earth cools; yet (3) they don't evolve into any more complex patterns until hundreds of millions of years later. So life evolved quickly (10's of millions of years), rapidly developed complex features, then whatever evolutionary force drove that suddenly stopped and they stayed stable and unchanged for half a billion years? It is actually a simpler supposition that life evolved on Mars (or Venus, but panspermia from Mars is more likely), and a relatively complex LUCA was transferred to newly reformed Earth via an impact event. This explains all of the oddities around LUCA and the fossil record of the origin of life on Earth. Mars would have had a vibrant early-life ecosystem that provided an evolutionary push for complexity. LUCA, on the other hand, colonized a lifeless Eden and was able to spend 100's of millions of years without evolutionary pressures. And as the science surrounding ALH-84001 shows, panspermia is not an unlikely event at all. If there was life on Mars at all, it absolutely would have been transferred.
- mtlmtlmtlmtl 4y ago
  >Microbial life fossils But since the RNA world is still poorly understood, and the nature of the lifeforms is highly speculative, there are many possible explanations for this consistent with an Earth origin. For instance it could be that RNA life didn't form microbial mats, or was geographically much more limited(perhaps only existing in deep sea vents, much more dependant on chemistry from the environment) until colonising the rest of the ocean later, leading to a much stronger signal in the fossil record. The RNA world might have been much shorter lived too for reason I'll get back to. >popping into existence and only complexifying after hundreds of My One very plausible explanation for this apparent lag is that the evolution of eukaryotes was a very unlikely event like endosymbiosis. Couple with the fact that RNA based life would be much more genetically unstable, allowing to evolve faster, and might have been much more fragile, representing a stronger selective pressure. Whereas DNA based prokaryotes are extremely versatile, which is clear from the fact that they've colonised the entire planet and lived through more drastic changes in climate than any other life form on earth. There's also no reason that I know of to think Mars was a more(or less) likely location for abiogenesis to have occured than Earth was. More and more evidence is suggesting that the building blocks for RNA are quite abundant even sans planets, and that abiogenesis itself might actually be very likely, but later complexification might not be. I think both hypotheses have a lot of merit. Although I will say if we find life on Mars and it turns out we are Martians I will be disappointed, because it would mean we still only have one sample of life. There's also a possibility that there was life on mars seeded from Earth, some of which came back to us fossilised, though I'm still undecided on whether ALH-84001 contains any fossils or not. There doesn't seem to be much consensus there.
  - adastra22 4y ago
    The situation with possible fossil life on ALH-84001 is one where if you are seeking consensus you probably have to wait until all the existing astrobiologists retire or die off. Having met and talked through the evidence with some of the principle people involved in that investigation, I'm personally convinced that the evidence for fossil life is very strong and the present criticisms miss the mark. But the announcement was so mishandled and politicized that continuing to fight that battle would be career-ending for all but the most senior scientists. Regardless though, the fossils are not what I was referring to, but rather the simulations confirmed with mineral analysis which showed that the interior or ALH-84001 was never heated to more than 40 deg C (iirc) during its entire journey from Mars to the antarctic, and that had life been in the rock it would have survived the journey.

mjan22640 4y ago

Parasites in general undergo simplification, they reduce and eventually lose the functionality that the host provides for them. In my opinion this is the case in viruses, that they are so simple not because they are remnants of ancient life, but because that they are parasites that underwent secondary simplification.

mtlmtlmtlmtl 4y ago
That's one of the 3 most popular hypotheses, mine is another. But the jury is still very much out on this one.

adastra22 4y ago

Microbial life doesn't fossilize well, but the earliest secondary fossils we do have are of microbial mats. They exactly match in all respects certain microbial mat living fossils we see on the ocean today, produced by relatively complex life forms. Most hypothecated pathways for abiogenesis would have left similar secondary fossils in the geologic record. Yet we don't see them. There's not a lot of rock left from that long ago, so maybe it just didn't get preserved. But it is striking and unexpected how (1) the earliest fossils are of quite complex organisms; (2) they pop into existence very soon after the Earth cools; yet (3) they don't evolve into any more complex patterns until hundreds of millions of years later. So life evolved quickly (10's of millions of years), rapidly developed complex features, then whatever evolutionary force drove that suddenly stopped and they stayed stable and unchanged for half a billion years? It is actually a simpler supposition that life evolved on Mars (or Venus, but panspermia from Mars is more likely), and a relatively complex LUCA was transferred to newly reformed Earth via an impact event. This explains all of the oddities around LUCA and the fossil record of the origin of life on Earth. Mars would have had a vibrant early-life ecosystem that provided an evolutionary push for complexity. LUCA, on the other hand, colonized a lifeless Eden and was able to spend 100's of millions of years without evolutionary pressures. And as the science surrounding ALH-84001 shows, panspermia is not an unlikely event at all. If there was life on Mars at all, it absolutely would have been transferred.

mtlmtlmtlmtl 4y ago
>Microbial life fossils But since the RNA world is still poorly understood, and the nature of the lifeforms is highly speculative, there are many possible explanations for this consistent with an Earth origin. For instance it could be that RNA life didn't form microbial mats, or was geographically much more limited(perhaps only existing in deep sea vents, much more dependant on chemistry from the environment) until colonising the rest of the ocean later, leading to a much stronger signal in the fossil record. The RNA world might have been much shorter lived too for reason I'll get back to. >popping into existence and only complexifying after hundreds of My One very plausible explanation for this apparent lag is that the evolution of eukaryotes was a very unlikely event like endosymbiosis. Couple with the fact that RNA based life would be much more genetically unstable, allowing to evolve faster, and might have been much more fragile, representing a stronger selective pressure. Whereas DNA based prokaryotes are extremely versatile, which is clear from the fact that they've colonised the entire planet and lived through more drastic changes in climate than any other life form on earth. There's also no reason that I know of to think Mars was a more(or less) likely location for abiogenesis to have occured than Earth was. More and more evidence is suggesting that the building blocks for RNA are quite abundant even sans planets, and that abiogenesis itself might actually be very likely, but later complexification might not be. I think both hypotheses have a lot of merit. Although I will say if we find life on Mars and it turns out we are Martians I will be disappointed, because it would mean we still only have one sample of life. There's also a possibility that there was life on mars seeded from Earth, some of which came back to us fossilised, though I'm still undecided on whether ALH-84001 contains any fossils or not. There doesn't seem to be much consensus there.
- adastra22 4y ago
  The situation with possible fossil life on ALH-84001 is one where if you are seeking consensus you probably have to wait until all the existing astrobiologists retire or die off. Having met and talked through the evidence with some of the principle people involved in that investigation, I'm personally convinced that the evidence for fossil life is very strong and the present criticisms miss the mark. But the announcement was so mishandled and politicized that continuing to fight that battle would be career-ending for all but the most senior scientists. Regardless though, the fossils are not what I was referring to, but rather the simulations confirmed with mineral analysis which showed that the interior or ALH-84001 was never heated to more than 40 deg C (iirc) during its entire journey from Mars to the antarctic, and that had life been in the rock it would have survived the journey.