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It's thought that, shortly after Earth was done being refurbished resurfaced during the Late Heavy Bombardment, life abio-generated almost as soon as it was able to. The short delay led to speculation that: in the right conditions, life arises fairly easily; and, under that assumption, many "lifeforms" arose alongside one another, perhaps some subsisting even today.

Earth, to the standards of the lifeform(s) that won out, is pretty hospitable--perhaps because life made it that way, altering the chemical properties of the atmosphere and terrain. Life thrives in nearly every remote nook and cranny of the planet.

Is there any thinking out there into why abiogenesis may have stopped? Is the process expected/hypothesized to be ongoing concurrently, in the present day? If life came to be so rapidly, why isn't it still coming, especially given Earth's apparent suitability?

(Perhaps our form of life, by its pervasion and alterations to Earth's condition, truly "won out," monopolizing on Earthly resources in such a way that bars newcomers from developing to compete and bars us from detecting their remnants, should they exist?)

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    $\begingroup$ When it comes to the origins of life on Earth I don't think there is theory that has been fully accepted. As you ask, abiogenesis is one theory, panspermia - life seeded from the cosmos via meteor etc., is another. $\endgroup$ – Fred Feb 23 at 20:49
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    $\begingroup$ Please do not accept Michael's answer. He is a problematic member due to his propensity for writing incorrect answers without any support from the scientific media or even from websites. His answer to this question, which someone upvoted, is very outdated and most likely is wrong. You may not like the currently correct answer to your primary question, which is "nobody knows." There are a number of strongly competing and quite different hypotheses regarding when / how life formed. $\endgroup$ – David Hammen Feb 23 at 21:02
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    $\begingroup$ Regarding the Late Heavy Bombardment, there are signs that that concept too is outdated. $\endgroup$ – David Hammen Feb 23 at 21:04
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    $\begingroup$ @BMFForMonica - For example, see nature.com/articles/d41586-018-01074-6 . There are many, many others. $\endgroup$ – David Hammen Feb 23 at 21:29
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    $\begingroup$ @DavidHammen As for liking the answer, I seem to recall an angry mom once telling me "you don't have to like it, you just have to eat it!" $\endgroup$ – BMF For Monica Feb 23 at 21:29
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I think that your question is not really answerable. But perhaps the reasons it's not answerable are themselves interesting. There are two parts to this answer:

  • why do we think abiogenisis is likely?
  • is Earth, today, hospitable for organisms which originate through abiogenesis, if it is likely?

My answer to the first part is, I think, more well-founded than my answer to the second (I'm a physicist not a biologist!).

Is abiogenesis likely?

Well, we have exactly one example of a planet we can study closely with life on it (indeed there is only one example of a planet that we know has life at all at present), and life has been there for such a long time and has modified the environment of that planet so extensively that I think speculation about how life originated on that planet is, well, speculation. That doesn't mean it's not interesting or that there might not be plausible and attractive hypotheses.

As an example it's tempting to argue that, because life originated on Earth almost as soon as it could, and disregarding panspermia, that abiogenesis must therefore be very common. That argument is wrong. There are a huge number of potentially-earthlike planets: this paper (arXiv version) claims that around 10% of stars may have a planet that is about the same size as Earth and gets around the same amount of sunlight as Earth. There may be around $10^{24}$ stars in the observable universe: based on this paper (arXiv) there may be around $2\times 10^{12}$ galaxies and assuming each galaxy has $10^{12}$ stars we get $2\times 10^{23}$ stars. So this means perhaps $2\times 10^{22}$ potentially-earthlike-planets. All we know is that life originated on one of those planets, somehow (and abiogenesis is only one of at least two options), and it did so rather early. What does that tell us? Well, statisticians can play games and those games are probably reasonable, but it doesn't tell us, for sure, that the chance of that happening is higher than $1/10^{22}$.

This argument is related to the anthropic principle which I find rather unappealing in terms of explanatory power: I find the idea that the fine-structure constant must be what it is because otherwise we would not be here to measure it unappealing for instance. But the anthropic principle is nevertheless not an idea that can be ruled out in general: we know life evolved on Earth because we're here to observe it, but this doesn't, yet, tell us anything about the probability of life evolving elsewhere: the universe could have just thrown a $10^{22}$-sided dice $10^{22}$ times and come up with the side that says 'life' once.

This doesn't mean that abiogenisis can't be really common, or even that it not likely that it is, it just means that we don't know that it is. With luck this will change in the relatively near future, as we begin to be able to observe expolanets which convincingly have life. Then we'll be able to use statistics in a more useful way.

How hospitable is Earth, today?

Well, let's assume abiogenisis is common, what can we say about Earth's hospitability to new life generated that way? Well, we know that organisms on Earth are continually under attack by other organisms and things which may be organisms (I'm not a biologist, I'm not sure if viruses count as organisms or not). And we know that these attacks can be pretty successful: for instance when Cortés invaded what is now Mexico the population was around 25-30 million, 50 years later it was around 3 million, and most of this decline happened due to disease (Wikipedia).

So life on Earth exists in this continual arms-race: things try to attack organisms and organisms develop hugely sophisticated immune systems to deal with those attacks. We're living, right now, through an example of what happens when an agent attacks us for which our immune system is not yet prepared: fortunately it's not going to kill that many of us.

So let's propose that abiogenesis happens on Earth, now. There are really two options.

  • Whatever appears is, say, RNA-based, and as such is recognisable by the existing warring factions. It has no defences and gets promptly attacked and eaten.
  • Whatever appears is based on some significantly different chemistry. But whatever it is, it's harvesting energy from somewhere and making it into itself. As soon as it starts harvesting enough energy it becomes interesting to existing organisms who would be quite interested in getting that energy for themselves, who pretty quickly evolve to being able to do so and eat it.

The third option is that whatever it is turns out to be so efficient in some way that it wins the fight with the existing stuff. I suspect, without any kind of proof, that this option is absurdly unlikely to happen.

I think what is more likely to happen is that, any time some new way of chemically stashing energy happens, it becomes attractive to existing organisms, who evolve to be able to harvest that energy and do so, long before whatever thing has happened could be described as 'alive'.

In summary: Earth is almost certainly not currently hospitable to newly-generated life, at all, because the existing life will eat it, one way or the other.

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The Earth was friendly to abiogenesis a little over 4 billion years ago because a) it was chemically different and b) there were no predators to consume whatever virus-like life forms were spontaneously generated. The chemistry of the early atmosphere (methane, ammonia, CO2 etc) lent itself to Urey and Miller type experiments on a massive scale, forming various versions of what is known as 'primeval soup' like the residues in Urey and Miller's laboratory flasks.

These conditions soon vanished, and don't exist anywhere today so far as we know, but if they did, the organic molecules generated would be consumed by micro-organisms. One of those first life forms managed to survive, and over the course of billions of years evolved into the countless life forms which we have today and the equally countless life forms which have become extinct along the way.

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    $\begingroup$ Citation needed, as always, Michael, and very out of date. The Urey - Miller experiment had lots of flaws. Very few scientists now think that that is how life formed. $\endgroup$ – David Hammen Feb 23 at 20:53
  • $\begingroup$ @DavidHammen, the key part of this answer isn't Urey-Miller, but that any new life forms that spontaneously generate, by whatever means, are likely to be eaten by the competition. $\endgroup$ – Mark Feb 28 at 2:28

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