How, if at all, could one tell that abiogenesis happened several times on a planet?

Question

I'm wondering if life could arise again on a world which already has a fully developed biosphere and be successful.

Ignoring how life does arise, in the first place and again (black smokers, clay crystals, warm ponds, geyser pits, RNA-world or metabolism first). Could the second abiogenesis event's children compete with the first one's children?

I'm wondering about several possible scenarios :

• both abiogenesis events are geologically speaking simultaneous (within a few 100.000 years of each other) and come to interact early on.

• long independent development (imagine a world where life forms in several of its small, separated oceans independently and comes into contact after tens or hundreds of millions of years).

• abiogenesis in a well established biosphere (imagine abiogenesis happened on Earth 200 million years ago (or today, it doesn't matter)).

Assuming that the two biochemistries can coexist (water-loving and carbon based) but aren't compatible (different chirality, different basic amino acids, different structures and molecules), what would happen and what traces are biologists likely to find in the biosphere?

I suspect that game theory might be as important to this question as biology. Is this likely a winner takes-it-all scenario, where the loser is utterly annihilated? Might one of the trees of life survive in nice habitats like the crust, seafloor and so on? Could the biologies mix, possibly through horizontal gene transfer? Could an equal coexistence be possible?

Answer 1

Life on Earth is syncretic. That is, every cell in your body is the descendant of several very different forms of life that symbiotically merged into a single unicellular organism (mitochondria for you, chloroplasts for plants, and quite possibly others still).

There is some evidence that these may even have been separate abiogenesis events. Thus, abiogenesis might tend to occur multiple times everywhere. If this were the case, we can also posit that most such abiogenesis events are unsuccessful -- the organisms become extinct soon afterwards. Only with the multiple events do some become lucky enough to persist.

This of course means that this phenomenon likely has persisted into recent times. It might even be happening somewhere this very moment, in some deep hydrothermal vent or deep in Earth's crust! These will of course either become extinct in short order, or find a niche that (so far) avoids detection by science.

If the latter, then there are only so many possibilities:

1. The new tree of life avoids symbiosis with other organism, and manages some tenuous coexistence with the existing biology. Eventually it will be discovered.
2. It becomes symbiotic, and a few million years later may become integrated enough that science will be unable to distinguish it as being a separately-evolved biology.
3. Its niche is so narrow and extreme, or it is so unlike any biology we can imagine that it is never detected.

What (probably) isn't possible:

1. It utilizes an incompatible chirality. Once the dominant chirality is firmly established, the new tree of life will find out just how difficult it is to buck this trend, as most existing nutrients are just wrong. Imagine growing up left-handed in a world where they only make right-handed scissors... that explode into boiling mercury if you use them poorly.
2. Two different sets of base pairs. If they both use the same polymer (DNA, RNA, or some analog) but different base pairs, in all probability they will converge to use the same set of base pairs through lateral gene transmission.

Answer 2

How did THAT happen...?

The conditions at the start of life on Earth were radically different than those of our current world. There were free amino acids, radically differing chemistries, and little if any free oxygen. What evolved there wouldn't survive 10 minutes today. Further, life has to have a pile of resources and abundant time to work out the bugs of their systems, as well as propagating that system for generations in a world full of organisms always willing to take any resources away from it.

From a biologist's standpoint, all life on Earth really ISN'T very different from each other. It is, in fact shockingly similar given the sheer amount of biology and different environments/strategies life has used to achieve success.

I can't say it COULDN'T happen, but the nature of it would have to have some kind of extraordinary advantage over existing life for it to work. If, for example, this life could directly utilize the heat difference in lava and the rest of the world, it would have an amazing energy resource.

Further, the organisms would need to be at least in part non-competitive (at least to start) so in the lava example, their biochemistry would be based on reactions that could tolerate temperatures and molecules that regular life simply couldn't use.

But that's not to say they couldn't take advantage of our biochemistry. While I can't QUITE envision how a biochemistry so different from ours might work, it could (for example) use oxidation reactions at high temperatures, or consume carbon in some way we wouldn't normally associate with life. It would need to be VERY different. I'd guess in the lava example that it might be silicon-based (yes, that's a whole different argument).

If it wasn't very different, then it would be almost completely the same. To get around the competition issues, the life would need to wholeheartedly adopt the existing strategies (minus whatever specific advantage this organism processed). If a life form was extremely similar to ours, it would end up using OUR biomolecules out of efficiency, and likely looking more and more like our kind of life with each passing generation.

So I think that it is extremely unlikely for a true new biogenesis to happen on Earth. If it did, it would either be VERY alien (and likely assumed to actually BE alien), or it would be so similar after a 'short' period of time that people would likely assume it evolved differences from our kind of life, rather than vice versa.

• An exception could exist if the two biogenesis events occurred in a similar time-frame. If that happened, you could have two 'competing' biologies struggling for dominance in a primitive world. They might separately exist for a time, but eventually one or the other would get relegated to niche environments due to changing conditions, or they would somehow achieve a symbiosis where the advantages of each were shared. Look for totally alien life in places like the bottom of the ocean, thermal vents, or the like. Sustained existence of such biology would mean the other ecosystem would at least in part share biomolecules as they are part of the environment.

The most likely tell of such life would be in genetics (which might be the wrong word, since the other organism might not use genes). Your organisms would have different functional ways to pass on heredity. Even mitochondria use DNA, but if organisms used two completely different systems of heredity, then they would be clearly from two different biogenesis events. This is the most conserved facet of life, and they would cling to the basics of inheritance even if the biochemistry otherwise became essentially identical. So I doubt there would be gene transfer (although functional sharing in symbiosis could be deeply profound)

Answer 3

We have the idea of Last Universal Common Ancestor now. We are able to track down our tree of life (albeit not really precisely) just by doing statistics on genomes, and the tree seems singular so far. Anything so alien as a different set of amino acids should be immediately obvious (unless it becomes some spectrum rather than separate groups... see next paragraph).

If there is the slightest possibility of eating each other, parasitism or symbiosis, there may be convergent evolution. One may want to eat the other since more food opportunities is better. The other may want to eat the first one for the same reason - you could call it Nash equilibrium, or maybe prisoner's dilemma (they will find out eventually that eating means being eaten, but it will be too late). This may lead to more similarity, eventually muddling the evidence for separate origin. In reality, this can sometimes do wonders like mimicking a foreign DNA sequence rather precisely, without an actual horizontal transfer. A different scenario is one tree of life trying to eat the other, the other TOL trying to remain inedible... This may lead to some interesting evolutionary dynamic, if it can last, which I am not sure. Does not sound like any kind of Nash equilibrium, but, you know, the tree of life is green.

If there is any horizontal gene transfer, I think that implies the level of similarity for convergent evolution to kick in.

OTOH, if they are hopelessly incompatible, there still may be competition for basic resources (carbon, sunlight... unless the other is silicon and geothermal). Competition is enough to drive one side to extinction without any single individual being eaten. Also, not being able to eat each other does not mean not being able to poison each other, for example, so the war can be fought with more direct means. Note that this is not much different from what's happening inside our singular TOL.

In your scenario 3, competition for resources happens even before one of the competitors is around. In an established biosphere, vast majority of resources is locked in some kind of bacterium-bacteriophage cycle (or even in higher forms of life), instead of being available for experiments. A second, delayed abiogenesis may be much more difficult.

Even if incompatible, there still may be high-level convergent evolution, like various harmless snakes mimicking the coral snake. These snakes are not able to fool biologists, though (as long as they are not fossil-only, maybe).

Population size, and probably some evolution rate, affect population stability. A zillion bacteria are more stable than a million dinosaurs. We still do have both bacteria and archaea after all, but no (real) dinosaurs. Both populations starting large and diverse may help; Even if one is clearly superior, the other may find its niche. Again, that happens within out TOL: Diversity persists, no single optimum lifeform dominates. (No, we do not.) If both (all) TOLs have a good start, they all have a decent chance to make it to our biologists, even if it's far from every single species what survives.