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If the "primordial soup" theory of abiogenesis is to be believed, self-reproducing organisms spontaneously arose on Earth at least 3.5 billion years ago, surprisingly soon after the Earth cooled down enough to potentially harbor life.

It may never have happened again, however, since all lifeforms on Earth today are similar on a molecular level (DNA), suggesting a common origin.

This appears to imply at least one of the following:

  • Immediately after the first spontaneous abiogenesis, environmental conditions on Earth changed dramatically, making a repeat impossible.
  • The first organisms that arose consumed any subsequent organisms that came into being. This was suggested by Alexander Oparin, but I find it very difficult to believe since it would have to happen everywhere on Earth. Also, most single-celled organisms found today do not consume other organisms, and even where they do that does not usually lead to their complete extinction.
  • The common origin theory is in fact false, despite the similarities between organisms, and abiogenesis did occur multiple times in the same way.
  • Abiogenesis happened more than once, but only the descendants of one occurrence survived till today.
  • Spontaneous abiogenesis never occurred on Earth after all because the conditions never allowed it; instead, a proto-organism arrived on Earth from a planet where the conditions do (Panspermia).
  • The standard theory of geological history is wrong (i.e. the Earth was in a "fertile" state for much longer before the eventual origin of life than commonly thought, due to some unknown mechanism causing radiometric dating to give wrong results).

None of these seem likely to me, yet even less likely I find the idea that life arose spontaneously almost immediately (within a few hundred million years, possibly even faster) after the Earth's crust solidified, and never again afterwards.

What are the currently held theories on that matter? Can poeple provide references to relevant publications.

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    $\begingroup$ Are we sure that all life on Earth shares a common origin/ancestor? Even if all multi-celled life does, there may be single-celled organisms which are not related to any of these related species. Plus, we have not witnessed every location on Earth, like the bottoms of some oceans... $\endgroup$
    – trysis
    Aug 3, 2014 at 21:37
  • $\begingroup$ This question shows how any new life would have to compete for resources with existing life: biology.stackexchange.com/questions/59892/… $\endgroup$
    – cowlinator
    Feb 13, 2020 at 21:52
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    $\begingroup$ It is so strange to hear human say "few hundred million years" are "almost immediate". Sure, it is short compared to age of earth but it is also almost incomprehensibly long time compared to anything else. Seventy years is an eon to microorganisms. What is "few hundred million years" to microorganisms? $\endgroup$
    – BagiM
    Aug 1, 2020 at 9:05
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    $\begingroup$ @trysis yes, we are sure, because all organisms have the same DNA code and chirality. $\endgroup$
    – Anixx
    Sep 6, 2021 at 6:48
  • $\begingroup$ As @BagIM has correctly noted, quantitative qualifiers in science are meaninful only in comparison to something else: e.g., a million years is a long/huge time compared to human lifetime, but it is of the same order as the existence of humans as species, and it is a very small time period compared to the existence of life or Earth. $\endgroup$ Sep 6, 2021 at 9:28

6 Answers 6

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It may never have happened again, however, since all lifeforms on Earth today are similar on a molecular level (DNA), suggesting a common origin.

An important distinction to make here is that all extant life on Earth has a common origin.

It's completely possible that abiogenesis occurred many times, but whatever organisms emerged as a result became extinct early on or are not preserved in the fossil record.

Immediately after the first spontaneous abiogenesis, environmental conditions on Earth changed dramatically, making a repeat impossible.

Nope. The red bands and subsequent dating techniques suggest it took a very long time for a change to occur.

The first organisms that arose consumed any subsequent organisms...

Possible, but we have solid data suggesting the first functional organisms were autotrophs. That is, they made their own food from whatever they were exposed to, and the predator/prey relationship had yet to play a major role.

The common origin theory is in fact false, despite the similarities between organisms, and abiogenesis did occur multiple times in the same way.

The Common Origin Theory - as you've defined it - is almost certainly true. DNA is a constant, as is RNA, mitochondria, etc.

That, however, doesn't mean abiogenesis couldn't have happened more than once. As I said above, the Common Origin Theory applies to extant (living) species, not necessarily all organisms that have ever existed. Species go extinct all the time.

Spontaneous abiogenesis never occurred on Earth after all because the conditions never allowed it; instead, a proto-organism arrived on Earth from a planet where the conditions do (Panspermia).

That merely pushes the question of abiogensis back a bit. Why did it evolve on another world and not Earth, then? How did it survive in space? Why was it so well suited to Earth's environment if Earth-like planets are (relatively speaking) rare?

The standard theory of geological history is wrong (i.e. the Earth was in a "fertile" state for much longer before the eventual origin of life than commonly thought, due to some unknown mechanism causing radiometric dating to give wrong results).

Estimations and conclusions have come from more than radiometric dating. We have fossil records, geological records spanning continents, and even ice-core records from the poles that both confirm radiometric dating and establish themselves as independent evidence.

None of these seem likely to me, yet even less likely I find the idea that life arose spontaneously almost immediately (within a few hundred million years, possibly even faster) after the Earth's crust solidified, and never again afterwards.

Why?

What are the currently held theories on that matter? Both random speculations and references to relevant publications are welcome.

This SE is not the place for random speculations. I don't have publications handy, so I may return and edit my answer when I have the impetus.

The most widely held theory is that all extant life, with a few possible exceptions, has evolved from a single organism that existed billions of years ago. That organism may or may not have been the only one to arise. If it was one among many variations of progenitors, we have yet to find the progenitors in the fossil or biological records. Given the extremely long time span between now and the beginnings of life, we don't expect fossil records to exist of all the organisms that existed some 3 B.Y.A. so the best we can do is speculate on what might have existed to account for what we see today.

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    $\begingroup$ Great answer, but what "few possible exceptions" are you talking about? I'm genuinely interested. $\endgroup$ Dec 30, 2013 at 2:22
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    $\begingroup$ @ChinmayKanchi - I remember a few years back that some researchers found some (assumed) Archae members that had some surprisingly different inheritable material. Not completely new genetic material, but odd nucleotides IIRC. I haven't heard about it since, so I don't know if it was ever verified. $\endgroup$
    – MCM
    Dec 31, 2013 at 0:40
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    $\begingroup$ Right. I seem to remember that vaguely as well. Wasn't it something like a tetrose sugar instead of a pentose? $\endgroup$ Dec 31, 2013 at 14:53
  • $\begingroup$ The narrative that life arose once a few hundred million years after the crust solidified and then never again doesn't add up from a probabilistic perspective. If it took a relatively short time for abiogenesis to happen once, it means it isn't an extremely rare event. But if that is the case, why have we never seen it happen again in billions of years? $\endgroup$ Aug 15, 2021 at 21:36
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    $\begingroup$ @RohitPandey -- A brief of the theory of "why doesn't life arise these days" is that early Earth presented a completely different environment to what exists now. Conditions some 4 BYA were extremely different, and so different chemistry was taking place. Another possibility: New variations of life exist, but the populations are very small or well hidden and we haven't found them. $\endgroup$
    – MCM
    Aug 16, 2021 at 22:58
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It may never have happened again, however, since all lifeforms on Earth today are similar on a molecular level

Actually, the similarity is at the chemical level where the chirality of all known-life's amino acids are "left-handed" (see http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Alternative-chirality_biomolecules). Since there is no advantage to having left-handed proteins over right handed, we would expect the chemistry in a spontaneously organism to randomly chose one or the other. So, it's 99% likely that life didn't arise more then 7 times: (1/2)^7 < .01.

Or, all the descendants of the right-handed protein life could have died-off, but this seems unlikely if there is no inherent efficiency difference.

The first organisms that arose consumed any subsequent organisms that came into being. This was suggested by Alexander Oparin, but I find it very difficult to believe since it would have to happen everywhere on Earth

This is where your missing a big point:

  • It's not necessarily eating them, or even outcompeting them once they arise, it's about cutting the legs out of the environment - eating the primordial soup of any loose complex organic matter. The places where the minimal materials for self replication machinery would have been available (since crystal-based origin of life was rejected in the 60's) would be very rare, and only on very specfic parts of the Earth...
  • Two of the current theories for this location, were in seafloor thermal vents, or on beaches with radioactive sand. Once life arise once in one of these niches, it will gain a strong foothold in these exact locations, crowding out those places orginal "ideal conditions" that led to the generation of life in the first place.

Ultimately, we'll probably be better able to answer this question by looking out to other planets than by un-earthing any satisfying evidence that could be found intact from 3 billion years ago.

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Some people think that abiogenesis did occur more than once, and that the results of those occurrences are still with us. Paul Davies, for instance, is a proponent of what he calls 'the shadow biosphere' (here's a link to a popular article on his views: http://www.theguardian.com/science/2013/apr/14/shadow-biosphere-alien-life-on-earth).

I've seen him talk about it, and while he has pretty much zero evidence for the idea, it's an intriguing thought. His argument is basically that we're not finding the organisms in the shadow biosphere because we're just not looking in the right places or in the right way; our discovery techniques are all biased towards our particular RNA/DNA-based life.

Note that this doesn't really put common origin in the ground, it would just mean that there's more than one common origin for each 'type' of life.

Also, it isn't necessary for early organisms to consume each other in order for there to have been multiple origins that subsequently went extinct. Our brand of life could simply have out-replicated the competition.

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There are essentially two ways this can be interpreted:

  1. Why did only one type of life evolve at the beginning, as opposed to multiple types coexisting?
  2. Why have no other forms of life evolved much later than the one we know about (i.e. when relatively "modern" cells already existed?

I think the answer to 1) is "we don't actually know if it didn't". During the "RNA world" and earlier, it's quite possible that many self-replicating molecular machines evolved totally independently from each other and coexisted much like separate species today. However, once whole cells came along, any "free agent" self-replicating molecules that hadn't joined forces with others to make cells (or didn't evolve to exploit cells, like viruses) were likely left in the dust.

As for the answer to 2), similarly to the last comment above, once the familiar type of life was on a solid footing, it would have probably eaten or starved any competing form of life before it could get off to any kind of start. A crude new self-replicating molecule could never outgrow a highly evolved cell containing multiple already highly evolved self-replicating molecules within it.

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The OP mentioned this possibility: "Abiogenesis happened more than once, but only the descendants of one occurrence survived till today."

This seems to be a viable hypothesis. Suppose that around 4 billion years ago life did arise at multiple locations on the Earth. Most theories invoke very small localized environments where very complex organic chemistry would lead to life via an intermediary RNA-World scenario. The origin of life could not have been a global phenomenon, it would take place in some miniscule pore in a rock, perhaps in a hot-spring environment. It's then hard to see why this happening at one location on the Earth would preclude the same sort of thing happening on the other side of the Earth.

So, if we assume that life originated here on Earth, then given that it started so fast after the environmental conditions would allow for it, instead of billions of years later, it follows that it must have started at many different locations on the Earth.

So, the fact that all life that exists today descends from one common ancestor, would then suggest that all the other lifeforms went extinct. But this does not have to have happened soon after the origin of life. As pointed out in Winawar's answer, Paul Davies has suggested that alternative lifeforms could exist even today and have escaped detection because we haven't been looking for them using the right methods. While that's a rather far-out hypothesis, the possibility that multiple lifeforms existed until, say, 2.8 billion years ago is a lot less far-out hypothesis, and certainly shouldn't be dismissed out of hand.

Assuming that Paul Davies is wrong about different types of life still existing today, the question is then why they would eventually have died out. After all, lifeforms compete mostly with their closest related versions, life today is very diverse and the more unrelated living organisms are the less they compete over resources.

The answer could be mass extinctions, particularly those caused by living organisms. Take e.g. the great oxygenation event. Oxygen is a rather basic chemical compound that could reasonably have been toxic to most lifeforms that would all have evolved under hypoxic conditions. So, it's entirely possible that until the great oxygenation event there existed multiple weakly interacting parallel biospheres. One of these biospheres started to produce large amounts of oxygen, destroying all other biospheres, only narrowly escaping killing itself.

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None of these seem likely to me, yet even less likely I find the idea that life arose spontaneously almost immediately (within a few hundred million years, possibly even faster) after the Earth's crust solidified, and never again afterwards.

I think that's exactly what happened and it isn't even surprising.

All the answers here assume a-priori that the rate of abiogenesis is high (since we saw life start early here). However, this isn't consistent with two facts:

  1. We haven't observed abiogenesis again, either here or anywhere else in the universe. Granted, it would be hard to observe it if the resulting life didn't progress to a certain extent, but if one doesn't find fairies one must seriously consider at least the possibility that there are no fairies (or they're exceedingly rare).
  2. We tried to replicate abiogenesis in the lab and to date haven't succeeded. Unlikely we will anytime soon. So, it can't be that easy.

The observations made by the OP can also be explained in the context of a very low rate of abiogenesis by invoking survivorship bias and the anthropic principal. Here, one doesn't have to assume a wildly fluctuating rate of abiogenesis or extreme extinction events (the argument still works even with those assumptions, but doesn't need them).

Note that we really are in a sweet spot where we've evolved intelligence and started thinking about expanding beyond our planet when a relatively short time is left (a few hundred million years; according to [2], all life might be wiped out in a billion years, and conditions for intelligent life will be gone sooner; also see [3]). So, if the whole process had started a few hundred million years later (not such a long time compared to the total span of time the planet has been suitable for life), no one would be around asking such questions.

Essentially, this argument is invoking survivorship bias and the Anthropic principal [1]. The idea is that abiogenesis is indeed very rare, but if it doesn't happen soon enough, intelligent life doesn't get a chance to evolve. We have one sample where intelligent life did evolve (or at least, intelligent enough to ask such questions) and are making observations on this one sample. So we have to condition on the fact that intelligence did evolve in that sample. When we do that, the probability of life evolving "early" becomes much higher (whereas it would be close to uniform over the time-line of the planet being "fertile" for life without the conditioning and a low rate of abiogenesis). The question merely isn't taking this conditioning into account.

Another thing to note - intelligence took about 4 billion years from abiogenesis to evolve here. In reality, there will be some random distribution for the "time till intelligence". Did we get a typical sample from this "time till intelligence" distribution or did we draw a short straw? Conditioning on the fact that intelligence evolved here before it was wiped out by an expanding Sun tells us its likely we did draw a shorter straw. The typical timeline from Abiogenesis -> Intelligence is likely longer than what we saw here.


This argument addresses the Fermi paradox as well, as a bonus. And it paints an optimistic view of the future since the "great filter" becomes life starting soon enough for intelligence to evolve and would therefore be behind us.


References:

[1] https://en.wikipedia.org/wiki/Anthropic_principle

[2] https://theconversation.com/a-billion-years-from-now-a-lack-of-oxygen-will-wipe-out-life-on-earth-156241

[3] https://image.gsfc.nasa.gov/poetry/venus/q79.html

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    $\begingroup$ The poster may welcome speculation, but this list does not. It is specifically a question and answer list about biology. I have edited his question. $\endgroup$
    – David
    Dec 10, 2017 at 18:12
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    $\begingroup$ In my view, the best we can do with questions we don't know the answer to is speculate (probably why he solicited it in the first place). Indeed, that is a very good (only?) way to find the answer. So banning speculation in all contexts might be unwise, even for a question and answer forum. $\endgroup$ Dec 10, 2017 at 19:59
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    $\begingroup$ I don't see BTW why invoking selection bias is any more speculative than the other arguments here. To me, its the most likely explanation, frankly. It's a very old argument applied to other questions like "why is the universe so fine tuned for life". $\endgroup$ Sep 5, 2021 at 21:11
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    $\begingroup$ @David Every reply to this is speculation, I don't see how this is any different $\endgroup$
    – 0x777C
    Dec 1, 2021 at 10:57
  • $\begingroup$ @0x777C — You may be right. Or not. But as I recall, at the time I edited the question it already had too many answers for there to be a hope of closing it. My aim was not to encourage speculation that was not based, at least in part, on scientific arguments. But, whereas I have contributed answers to speculative questions on the relative times of origin of RNA, DNA and protein, I had nothing other than the obvious to contribute to this, and was only involved in site hygiene. $\endgroup$
    – David
    Dec 1, 2021 at 12:50

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