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Why aren't there any competing biologies on Earth? I read sci-books about life based on silicon and I've read an article that said that other structures than DNA can encode genetic information.

So does physics allow for many competing biologies? What makes DNA-based biology the dominant biology on earth according to known laws of physics?

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    $\begingroup$ Silicon-based biochemistry is pretty unlikely, at least at the temperatures and pressures present at the surface of the Earth. While carbon-hydrogen compounds of any length are effectively stable and can include complex branching and cyclization, silicon-hydrogen compounds don't get much longer than trisilane or cyclopentasilane without falling apart. Also, Si-Si double bonds are rare, while there are a wide variety of relatively stable alkenes (C-C double bonds), alkynes (C-C triple bonds), and aromatics. We haven't even included oxygen and nitrogen yet! There is no replacement for carbon. $\endgroup$ Apr 1 at 15:42
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    $\begingroup$ CO2 is an easy waste product to get rid of. SiO2... less so. Imagine having to excrete a kilo of quartz every day. $\endgroup$
    – J...
    Apr 1 at 18:45
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    $\begingroup$ Chemistry is a bigger factor here than physics (though I guess chemistry is just applied physics at it’s core...). As a simple example, the oxidizing environment that (most) life on earth lives in strongly selects for certain biochemistries because it makes certain types of reactions much easier than others (and, in fact, you can find some really ‘exotic’ metabolic processes in things found in strongly reducing environments like geothermal vents in the oceanic benthic zones). $\endgroup$ Apr 2 at 11:55
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There are indeed almost certainly other potential alternatives to DNA-based biology and the RNA-based biology that may have predated it, which could be used to form viable organisms.

Many of them likely have some energetic disadvantage relative to DNA and RNA (e.g., requiring a higher bond formation energy, having a less flexible backbone) and those would naturally be expected to be outcompeted. But some probably aren't so different from DNA and could indeed have formed a viable alternative---if nothing else, the opposite chirality molecules are certainly candidates. Why aren't alternatives still around?

Surprisingly, it turns out that when time scales are long enough, we should expect a natural loss in the number of different independent lineages. Another more recent example of this is the fact that all modern humans are descended from Mitochondrial Eve and Y-chromosome Adam, despite the fact that these two individuals were members of thriving human populations at entirely unrelated times. Biodiversity is constantly being gained through divergence of existing lines and lost through lines that fail to reproduce. Over many generations of reproduction, this produces a random walk pattern that converges toward elimination of all but a single lineage in a given population even if all lineages are equally fit.

Now back to the origin of DNA-based life. If abiogenesis is a rare and difficult event, then we would expect only one or a small number of life models to have emerged billions of years ago. Random fluctuations over all that time will tend to drive all but one toward extinction. With evolution at play as well, any lineage that randomly gains an advantage will be even more likely to outcompete the others.

And once there is life of any sort around? Well, potential alternatives to DNA are still nutrients, and therefore will simply be eaten by existing life forms before they have the chance to evolve into something new and interesting.

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    $\begingroup$ One mistake: Y chromosome Adam wasn't necessarily (and probably wasn't) human. Some estimates for his age predate modern humans by a long time. I can't remember off the top of my head if the same is true for MT Eve but it probably is. $\endgroup$ Apr 2 at 7:59
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    $\begingroup$ @KonradRudolph From what I have read, the current estimates are consistent with species emergence estimates. The science is still unsettled, though. $\endgroup$
    – jakebeal
    Apr 2 at 10:42
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    $\begingroup$ @Dunois It perhaps a surprising conclusion, but consider the fact that nearly every species that has ever existed is extinct, including entire massive and long-lasting taxa such as trilobites, and mass extinctions are frequent events on geological time scales. I think a stronger claim would be to assert that there would not be large-scale mass extinctions in the billions of years of difficult to observe microbial prehistory. $\endgroup$
    – jakebeal
    Apr 2 at 15:21
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    $\begingroup$ @Dunois The question is focused on life on Earth, and so is my answer and my comment. $\endgroup$
    – jakebeal
    Apr 2 at 15:53
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    $\begingroup$ @KonradRudolph: Non-Homo sapiens sapiens ≠ non-human. Even with the earliest estimates, both Y-A and Mt-E would still indisputably have belonged to the genus Homo. $\endgroup$
    – Vikki
    Apr 3 at 21:51
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Jack Szostak's research group has looked at alternatives for RNA as replicators, see e.g. here, and they found that RNA is a far better replicator than the alternatives. So, it seems that RNA-World was preceded by a prebiotic world where RNA won the competition from rival nucleic acids. Then during the RNA-world period, the RNA-world evolved ever more complex ribozymes. More complex proteins were being made by primitive ribosomes, which in turn led to the evolution of DNA as a stable storage platform for information.

But this does not fully address the problem, because as @uhoh has commented on another answer, the mirror image of RNA would, of course, have the same performance. Life arose quite fast after the conditions on Earth were suitable for life, which is inconsistent with the idea that abiogenesis is a very rare event. The relevant processes that would lead to life appearing somewhere on Earth are very localized in nature. We're talking molecules interacting with each other in some tiny confined space rich certain energy-rich environment. The precursors RNA molecules cannot get diluted to much.

So, this process being a very localized processes on micron to millimeter-scales cannot possibly have led to a world-wide competition where alternative, equally good molecular systems such as the mirror image of RNA would have lost out. Given that life appeared very fast somewhere on Earth, it would have had to appear fast on many places on Earth with at least random chiralities and perhaps also implemented by totally different biochemical systems.

It's then hard to see why the different chiral versions of life would en up wiping each other out much later when they would come into contact with each other. Living organisms tend to not wipe each other out due to competition if they are not closely related, and the mirror version of life would not interact strongly with ordinary life. As pointed out here, a photosynthesizing mirror organism would cause problems for us precisely because it would convert CO2 into inedible compounds:

After doing some rough calculations on the effects of a mirror cyanobacteria invasion, Kasting isn't sure which would kill us first: the global famine or the ice age. "It would quickly consume all available nutrients," he says. "This would leave fewer or perhaps no nutrients for normal organisms." It would wipe out the global ocean ecology and starve a significant portion of the human population. As the CO2 in the ocean was incorporated into inedible mirror cells, they'd "draw down" CO2 from the atmosphere, Kasting says. For a decade or two, you'd have a cure for global warming.

But Kasting predicts that in about 300 years, the bugs would suck down half of Earth's atmospheric CO2. Photosynthesis of most land plants would fail. "All agricultural crops, other than corn and sugar cane, would die," he says (they do photosynthesis a little differently). "People might be able to subsist for a few hundred years, but things would get pretty grim much more quickly than that." After 600 years, we'd be in the midst of a global ice age.

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    $\begingroup$ The plot of the chirality disaster depends on the assumption that the wrong-chirality organics will be inedible. This is plain wrong, at least for sugars and there is a lot of xxx-racemase enzymes for other things. $\endgroup$
    – fraxinus
    Apr 3 at 18:01
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While the discussion of heterochirality was squelched I think it deserves to be addressed explicitly, because unlike most "competing biologies" that one might imagine complementary chiral biologies have zero competitive advantage over each other.

The abstract of Emergence of homochirality in large molecular systems (Laurent, Lacoste and Gaspard (2021) PNAS January 19, 2021 118 (3) e2012741118) sums it up nicely, albeit based on a complex mathematical model based on cheminformatics and random matrix theory:

The selection of a single molecular handedness, or homochirality across all living matter, is a mystery in the origin of life. Frank’s seminal model showed in the ’50s how chiral symmetry breaking can occur in nonequilibrium chemical networks. However, an important shortcoming in this classic model is that it considers a small number of species, while there is no reason for the prebiotic system, in which homochirality first appeared, to have had such a simple composition. Furthermore, this model does not provide information on what could have been the size of the molecules involved in this homochiral prebiotic system. Here, we show that large molecular systems are likely to undergo a phase transition toward a homochiral state, as a consequence of the fact that they contain a large number of chiral species. Using chemoinformatics tools, we quantify how abundant chiral species are in the chemical universe of all possible molecules of a given length. Then, we propose that Frank’s model should be extended to include a large number of species, in order to possess the transition toward homochirality, as confirmed by numerical simulations. Finally, using random matrix theory, we prove that large nonequilibrium reaction networks possess a generic and robust phase transition toward a homochiral state.

It's a stretch from cheminformatics and random matrix theory to what may have actually happened, but it is helpful to know that nature provides a way for one member of a chiral biology pair to completely annihilate the other despite any specific competitive advantage.

This is important because today we see exclusively one chirality on Earth, and use it as a way to differentiate between processes involving life and those that don't.

That's discussed nicely, and at length in D. G. Blackmond (2010) The Origin of Biological Homochirality (Cold Spring Harb Perspect Biol. 2010 May; 2(5): a002147.)

And exobiologists may be tempted to apply this beyond Earth as well which means they need to consider non-life-based chiral selectivity carefully!

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    $\begingroup$ +1 Some things in evolution are just historical accidents and don't correspond to optimization against any selection pressure. Chirality is an obvious one, since it so clearly could have gone either way. But what else is just a historical accident? $\endgroup$ Apr 2 at 18:05
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Carbon

jakebeal's answer is very good, but the Wikipedia article on DNA alternatives is also very thorough. In particular, when astrobiologists debate whether alien life is likely to be carbon based, the answer is often "yes", because no other element has the chemical flexibility of carbon. While silicon is in the same group and can form long-chain molecules, it simply lacks the diversity of total molecular options, and the chains it can produce are much shorter on average than carbon (because it's a much larger atom and makes weaker double-bonds). Note that any carbon replacement suffers the same problem as silicon: much larger and less chemically flexible.

Note that your article simply assumes a carbon-based backbone, and asks what other molecules could replace the common nucleic acids.

Crystals

Some folks talk about the possibility of radically different life forms, such as crystals. Unfortunately, crystals are good at growing, but not good at adapting. It is hard to imagine any kind of metabolism in a crystal-based system.

Energy

Way out in the woo-woo space, non-scientific folks like to play with the idea of "energy-based life forms". While I fully confess to a failure of imagination here, I would just like to point out that all the life we know about works hard to maintain homeostasis, which gets increasingly difficult in high-energy environments. This is because it takes energy to maintain the low-entropy configurations of living bodies, and all systems must emit heat to do work. Of course, dumping heat gets increasingly difficult the hotter one's environment.

The most literal way to describe an "energy-based being" is one that has no normal matter as part of its body. A being made entirely of photons doesn't make any sense at all. That's because most photons simply don't interact with each other, so it would be like having a body in which none of the cells could communicate. Planck-scale photons could use pair production to "interact" with each other, in some kind of ephemeral body, but it is hard to imagine how this could remain coherent over any kind of timescale. Also, the electron/positron pairs would push the definition of "no normal matter".

Of course, we could just be talking about beings whose base metabolic rate is so high that they emit blackbody radiation in the visible spectrum (so that we see them as natural sources of light). They would be made of ordinary matter, but such beings would consume pretty significant quantities of energy relative to terrestrial life. We have a good example of a blackbody that emits strongly in the visible spectrum: the sun. It has a blackbody temperature of about 6000k, and requires a massive scale of thermonuclear fusion to maintain that temp. Needless to say, any beings operating in that energy regime would either be much, much larger than human-sized, or would require physics that is well beyond our current understanding (perhaps they are powered by hearts of micro-black holes?).

Obviously, such a biology would not naturally arise on earth. ;)

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