11 ms·

An interesting phenomenon I rarely see brought up in support of panspermia is the apparent universality of the genetic code. The fact that all known life shares

An interesting phenomenon I rarely see brought up in support of panspermia is the apparent universality of the genetic code. The fact that all known life shares a common genetic structure implies a common ancestor. The lack of competing genetic codes could be interpreted to imply that life only started on earth once, or that it only made it to earth once. Natural selection is a constant force, presumably new genetic codes could offer an advantage, e.g., robustness against damaging mutation, resulting in multiple competing genetic codes. The notion of abiogenesis occurring only once on earth seems improbable considering complex organs like eyes have evolved independently several different times. Or the fact that convergent evolution "reinvents" similar species in response to similar selective pressures. Selective pressures constantly "push" life in the same directions, so it would seem that whatever forces gave rise to life would have happened so repeatedly, but there doesn't seem to be any evidence for that. A clean explanation for this would be that life didn't start on earth, and earth may have never had the right conditions to originate life, hence no competing genetic codes. But it was still hospitable enough to maintain life, so one universal genetic code flourishes with no emergent competition.

rjurney 7y ago
Yeah, it seems to have been incredibly unlikely. It took a long time.
hdfbdtbcdg 7y ago
Or life started multiple times but the results of one of those times outcompeted all the others.
- rectangletangle 7y ago
  This is true, but if that did happen presumably we could find some evidence for it, since it would be a relatively common occurrence during the period where earth was capable of abiogenesis. Because if it occurred at least twice, it probably happened many more times. The counterpoint is that the fossil record is very incomplete when going that far back, seeing as tectonic subduction literally wipes the slate clean.
  - yarg 7y ago
    When lifeforms out compete each other, the winner eats the loser.
    - blorb 7y ago
      Only if the loser in nutritious and not toxic poison.
- yarg 7y ago
  Or that the self replicating systems of molecules merged into one (organic life on earth is base 4, containing two sets of two complimentary pairs - it potentially could've done it with just one of those sets), Or that the same self replicating systems were the most (or only) stable systems that could be forged from the locally available inorganic molecules.
- bloak 7y ago
  Or there might have been a catastrophe that killed off almost everything. In which case the organisms that survived might not have been very widespread before the catastrophe, or typical of what existed before the catastrophe. If a large asteroid hit the Earth today it might be only a few subterranean extremophile microorganisms that survive. Asteroids were more frequent during the early history of the solar system.
tsimionescu 7y ago
Doesn't that just beg the question? If life originated on some other planet and made it to Earth, why did only the DNA-based lifeform(s) make it here? If we assume that different kinds of life based on different nucleic acids can co-exist, then it seems unlikely that a single kind would have happened to be carried to Earth by a natural process. If we assume DNA has some inherent fitness advantage (so that only DNA-based life survived the journey), than we don't need the additional assumption of panspermia: perhaps only the DNA-based life survived some early period in Earth's history. Also, for abiogenesis, given that we still can't reliably reproduce something like that, and that we have never seen new kinds of life spontaneously appearing on Earth, we do have some circumstantial evidence for the possibility that it is an incredibly unlikely event, such that it might have only happened once in Earth's history. Edit: There is also another simple possibility: perhaps given the resources available on Earth, either forever or in the period where abiogenesis was possible, perhaps DNA(+RNA) is the only molecule with the right properties to act as genetic material. Perhaps there is no common ancestor to all life on Earth, but a "forest" of inter-breeding, spotnaneously arisen original organisms, but they were all DNA/RNA-based simply because of the available resources.
- rkagerer 7y ago
  Another possibility often overlooked is that there's life out there on completely different timescales than we can recognize. We're only likely to notice those within a few orders of magnitude to our own. Maybe DNA is simply geared well compared to other mechanisms for the timescales we're interested in.
  - rectangletangle 7y ago
    This is an interesting thought. Plants are actually very animated at their own timescale, adapting to their environment through phenotypic plasticity. However this happens far too slow for humans to notice without a time lapse.
    - gremlinsinc 7y ago
      This is why it takes so long to hold a meeting of the Ents.
schoen 7y ago
Probably, although a not-widely-accepted but interesting theory is that there might be microorganisms that we can't detect because we're not looking for the right things (because the things we look for are based on macroscopic life and things that interact biologically with it). https://en.wikipedia.org/wiki/Shadow_life https://en.wikipedia.org/wiki/Shadow_life xkcd just ran a strip with a joke about this theory last week https://xkcd.com/2171/ https://xkcd.com/2171/ I think it's unlikely because it seems hard to imagine no interaction between the kinds of life ever after billions of years of coevolution. On the other hand, there are tons of microorganisms that do share our biochemistry that we haven't noticed or identified yet, so maybe it's not impossible to imagine some that don't that we also haven't noticed or identified (since it would be more difficult for us to recognize them).
- orwin 7y ago
  It is interesting, but following Popper's epistemology rules, this hypothesis is not scientific. So saying it is a not-widely-accepted theory is right, but with reasons: this is not a theory. Not that just because a hypothesis is not scientific, it doesn't mean that people can not research or fine-tune it until it become a scientific theory, but in this state, spending public research credit on it is not a very good choice (especially in the current state of funding for biology). (it can be read wrong, my command of english is not great: i'm just adding thing to your comment, this is not a critisism of you or what you said)
  - mannykannot 7y ago
    Popper's view is neither the last word on what science is, nor the final arbiter as to whether something is scientific: what he did was to add some powerful insights to a description of how science works. Having said that, it is neither unscientific nor against Popper to postulate something for which there is currently no evidence (the search for exoplanets was started before there was evidence of them) - you would have to postulate something that cannot, in principle, be empirically determined (such as p-zombies) to violate the falsifiability principle (if that's what your specific issue is here.) Such hypotheses are not outside of science; they are an essential part of it, and some unspecified part of scientific spending is effectively spent on generating them. On the other hand, as Hossenfelder has pointed out [1], falsifiability is a low bar. In the case of shadow life, it is reasonable to ask both how it could remain isolated and why have we not noticed it yet. [1] https://backreaction.blogspot.com/2019/01/just-because-its-falsifiable-doesnt.html https://backreaction.blogspot.com/2019/01/just-because-its-f...
dalbasal 7y ago
An alternative explanation (s) is that "there's only one way to do life" or that the origin of life is extremely rare/unlikely. But panspermia too.
teekert 7y ago
Interesting but your thought experiment only "proves" that the likelihood for something that can replicate itself to emerge is far smaller than the likelihood that that something eventually evolves into something extremely complex. Nothing more imho.
ozy 7y ago
> The fact that all known life shares a common genetic > structure implies a common ancestor. Or a (local) optimum.
- tandr 7y ago
  Prions have a more optimal optimum, but they are not exactly "life" from what I have read.
crimsonalucard 7y ago
You can't quantify the complexity of something like an eye. The eye could actually be very very simple to form, due to circumstances and variables outside of our knowledge. Scientists have discovered that the animal with the most genes--about 31,000--is the near-microscopic freshwater crustacean Daphnia pulex, or water flea. By comparison, humans have about 23,000 genes. One of the most astonishing features of the D. pulex genome is its compactness: despite being around 200 Mb in size (around 16-fold smaller than the human genome which is 3,200 Mb in size); its 12 chromosomes contain a minimum set of 30,907 predicted protein-coding genes, more than the 20,000–25,000 contained in the human counterpart. The unexpected complexity of the water flea genome compared with a human genome illustrates unknown dimensions of complexity that we cannot fully comprehend. What this means is this. You do not know how complicated the original template of life is... the complexity for the first genome to produce instructions that allow for self replication to kick off evolution may be so complex that it could only happen once on our planet, once in this galaxy or once in the universe. We simply do not have the knowledge to know the probability space of life on earth or life as we know it.
joycian 7y ago
Why are there no other human species (anymore)? Because there was no room for them anymore and they could not compete. In the same way, it it very well possible that life utilizing other genetic code got stamped out (early on).
ivanhoe 7y ago
well, if you look on viruses and bacteria situation stops looking that much uniform, there're some odd bugs there https://upload.wikimedia.org/wikipedia/commons/9/98/A_Novel_Representation_Of_The_Tree_Of_Life.png https://upload.wikimedia.org/wikipedia/commons/9/98/A_Novel_...
kitd 7y ago
OK, I'm not any kind of expert in this area, but is RNA not considered an early form of genetic coding? And DNA evolved from it? Also, don't viruses replicate by "borrowing" the mechanism of their hosts? My understanding is that life visible to the naked eye is a minority of life forms on the planet. Maybe other forms of genetic coding than DNA are more prevalent than we anthropocentrists think.
- orwin 7y ago
  Yes. DNA probably "evolved" from RNA. Keep im mind that the time scale is huge and that at the time, the earth was really hot and pressurized, so we can only have speculations. A posituve thing is that in Miller experiment, he found out that those conditions create aminate acids. That also indicate that panspermia theory is unlikely (not impossible, but not Occam's razor choice at least).
- gomijacogeo 7y ago
  My own tin-hat theory on that is DNA probably co-evolved with RNA pretty quickly. The main difference between RNA and DNA is that a single RNA strand can fold up and bond with itself to make interesting() 3d shapes while DNA can't self-fold. I think single-stranded DNA initially served as a durable, robust structure for RNA to assemble against before it went off and folded up and did whatever it did chemically. This might help explain why RNA polymerase only runs 3'->5'; in fact one of the early functions of one of the RNA polymerase components was maybe to keep the single DNA strand "tidy" and free of random base-pairings. Also, it might help explain why the 3'->5' DNA strand has to jump through so many extra hoops to get replicated - double-stranded DNA came later. It might also explain the reason reverse transcriptase exists at all. Again, I am probably provably a crank on this. () This is the whole appeal of the "RNA world" hypothesis, you have one molecule that can serve both as "code" and catalyst for other chemical reactions.
nabla9 7y ago
On the contrary. If life started multiple times on earth competitive exclusion probably took care of all other primitive forms rapidly. Even small differences, energy efficiency and reproduction lead to situation where only one form of primitive life survives. Once the life really started going, it changed the chemistry of the earth rapidly. Similar chemical evolution starting from scratch was not possible once the life started. Eventually when photosynthesis started, biologically induced molecular oxygen caused so called Oxygen Catastrophe where Earth's chemistry turned from reducing into oxidizing. Oxygen also changed the chemistry in the seas and rocks. Molecular oxygen is great disinfectant.
- amelius 7y ago
  How about different chirality? Different chiral forms would have the same advantages.
  - gomijacogeo 7y ago
    Most precursor molecules (e.g. CH4, NH2, etc) aren't chiral, so differently-chiral'd proto-life would still compete for the basic building blocks. Each time one set of complexes managed to get more efficient at something, it probably drove more than half of its competitors to extinction. Repeat a few times and you wind up with essentially a single global consensus and the arms-race moves on to higher levels of abstraction.
    - amelius 7y ago
      > Most precursor molecules (e.g. CH4, NH2, etc) aren't chiral, so differently-chiral'd proto-life would still compete for the basic building blocks. Yes, but horses and cows also compete for the same building blocks. And yet, we have horses and cows. Your argument could be right if there wasn't an abundance of these building blocks.
      - gomijacogeo 7y ago
        And yet we have no more mastodons, dinosaurs, trilobites, ... Stuff still goes extinct, it's just a lot slower with cows than with molecules. I think you're just failing to grasp the sheer numbers involved here: there are probably around 10^9 cow-like critters and they interact with other, competing critters every, let's say, 10^4 seconds, with the odds of any one of these interactions being fatal being, again, let's say 1 in 10^4. By comparison, there were probably something like 10^37 (if I counted my orders of magnitude right) biomolecules interacting every 10^-6 seconds where every time some new chemical "A-ha!" moment occurred, that reaction now became 1000x more favorable than any other competing reaction. That's like, what, 41 orders of magnitude; even if I'm off by 10+ orders of magnitude, there was just a whole lot more selective pressure going on during the pre-biotic era. Less favorable chemical pathways were outcompeted on scales so vast you'd need trillions of cow-years to approximate how de-de-de-deselected-for they were.
TeMPOraL 7y ago
> The fact that all known life shares a common genetic structure implies a common ancestor. The lack of competing genetic codes could be interpreted to imply that life only started on earth once, or that it only made it to earth once. The more I read about DNA and RNA, the more it seems to me they're somewhat fundamentally grounded in chemistry - you can't just make codons mean arbitrary things by messing with the replication mechanisms. There's less path dependence, and more of "one optimal way of doing things". That said, I think it's reasonable to assume we all have common lineage[0] - it's arguably a winner-takes-all scenario, as there's a huge first-mover advantage for the first self-replicating family of things that got loose on the world. -- [0] - causally, not genetically; the history of life is not a tree structure due to constant lateral gene transfer that bacteria and archaea engage in.
Angostura 7y ago
The genetic code isn't quite as universal as you might think. Here's an interesting article on the peculiarities of encoding in the mitochrondial genome, and what it tells us about evolution. https://whyevolutionistrue.wordpress.com/2014/09/28/guest-post-why-the-genetic-code-is-not-universal/ https://whyevolutionistrue.wordpress.com/2014/09/28/guest-po...
- gomijacogeo 7y ago
  Also, https://en.wikipedia.org/wiki/Genetic_code#Alternative_genetic_codes https://en.wikipedia.org/wiki/Genetic_code#Alternative_genet... Although most (all?) of the variability is in the one place you can make it without almost-certainly killing your host organism - in repurposing the STOP codons. There are usually three (and likely UGG was a fourth before it got hijacked by Tryptophan in the before time) and all you need is one.
doctorstupid 7y ago
Perhaps genetic replication did emerge multiple times on Earth, but always to the same solution which had the highest probability?
ifrancium 7y ago
Drew Endy gives a pretty fun talk along the same lines: https://www.youtube.com/watch?v=GY28A0tBNPU https://www.youtube.com/watch?v=GY28A0tBNPU
2a96eb7d685a49c 7y ago
Perhaps the exact conditions that allowed for abiogensis were only present for a relatively short period of time. That would explain why it only happened once. The fact that we haven't been able to replicate it leads one to think that the conditions must be very unlikely. Also, your argument suggest the reason there are not different genetic structures on Earth is because the one we have came from outer space. If one genetic pattern can come from outer space why haven't others?
Balgair 7y ago
The panspermania hypothesis has one very large hurdle to get over: the half-life of DNA is ~500 years [0]. If life came from outside our planet via an asteroid or something, then it had to be a LOT of DNA that came over. More exotic theories can be concieved, but the jist is that it is really really hard for DNA to travel the stars and between planets intact. The simplier explination is that it started here. [0] https://royalsocietypublishing.org/doi/full/10.1098/rspb.2012.1745 https://royalsocietypublishing.org/doi/full/10.1098/rspb.201...
- asfarley 7y ago
  I get the impression that this article is mostly about dating bones. When people talk about DNA-based-panspermia, I've always assumed that they meant either a living or dormant organism (with the potential for DNA replication/error-correction), rather than a completely inert blob of DNA.
  - Balgair 7y ago
    I always figured that the extremes of interstellar space automatically precluded anything that was even remotely alive. The temperature swings, vacuum, radiation, and ejection and re-entry violence always seemed to me to be so crazy that I assumed that nothing could survive it; just bits of genetric material, a dead ameoboid carcass, etc. I'll have to ponder on the idea that anything could get past all those 'gates'. If it did, woah, what a hardy little thing!
liberte82 7y ago
There's one possibility that explains it: that the probability of a genetic code evolving naturally is really, really, really low. Like, less likely to happen than once per universe. Then, once this barrier is attained, other forms of evolution occur more readily. As we don't know the mechanisms for how it happens, it's a possibility. The anthropic principle allows for it, but it likely requires multiverse theory and/or infinite universes coming and going over time. There's just no consciousness in all the infinite universes where it never occurred, so there's no one there wondering why there's no life.
jhedwards 7y ago
> The notion of abiogenesis occurring only once on earth seems improbable considering complex organs like eyes have evolved independently several different times. Not quite. Simple life forms probably existed for far longer than DNA. The fact that we see convergent evolution in complex life forms, which emerged very late in the history of life, only suggests that there are recurring patterns in the system that enabled the formation of higher life. It doesn't suggest anything about the origin of cellular life itself, which existed for hundreds of millions of years before eyes or lungs.
dragonwriter 7y ago
> The notion of abiogenesis occurring only once on earth seems improbable considering complex organs like eyes have evolved independently several different times. Evolution of eyes and abiogenesis aren't obviously similar enough for this analogy to be compelling without some narrative as to why what occurred with one should shape expectations of the other. The conditions in which life first emerged in either the Hadean or Eoarchean eons weren't around for much more of the history of the Earth; if the average expected frequency of abiogenesis under the conditions it actually emerged was once every quarter billion years or so, then—no matter which point in the possible range of time it first occurred—conditions wouldn't have been right for more than one to a very few instances, and if there were more than one, still one would likely be much more established by time available to spread alone, and then there were be massive environmental changes likely producing extinctions. Only one lineage surviving that bottleneck isn't surprising even if there were more before, and while we have evidence of early life it's far from complete either in coverage or ability to determine biochemistry and lineage. It's really not necessary to invoke extraterrestrial origin to explain only one lineage on Earth (and it actually may make the problem worse: because then you have to explain why we haven't gotten alternate lineages from the same or other extraplanetary sources.)
la_barba 7y ago
>Natural selection is a constant force, presumably new genetic codes could offer an advantage, e.g., robustness against damaging mutation, resulting in multiple competing genetic codes. But what would the advantage look like? Isn't successfully surviving and propagating for billions of years, the ultimate advantage? It would seem like our current state is the best one then..

rjurney 7y ago

Yeah, it seems to have been incredibly unlikely. It took a long time.

hdfbdtbcdg 7y ago

Or life started multiple times but the results of one of those times outcompeted all the others.

rectangletangle 7y ago
This is true, but if that did happen presumably we could find some evidence for it, since it would be a relatively common occurrence during the period where earth was capable of abiogenesis. Because if it occurred at least twice, it probably happened many more times. The counterpoint is that the fossil record is very incomplete when going that far back, seeing as tectonic subduction literally wipes the slate clean.
- yarg 7y ago
  When lifeforms out compete each other, the winner eats the loser.
  - blorb 7y ago
    Only if the loser in nutritious and not toxic poison.
yarg 7y ago
Or that the self replicating systems of molecules merged into one (organic life on earth is base 4, containing two sets of two complimentary pairs - it potentially could've done it with just one of those sets), Or that the same self replicating systems were the most (or only) stable systems that could be forged from the locally available inorganic molecules.
bloak 7y ago
Or there might have been a catastrophe that killed off almost everything. In which case the organisms that survived might not have been very widespread before the catastrophe, or typical of what existed before the catastrophe. If a large asteroid hit the Earth today it might be only a few subterranean extremophile microorganisms that survive. Asteroids were more frequent during the early history of the solar system.

tsimionescu 7y ago

Doesn't that just beg the question? If life originated on some other planet and made it to Earth, why did only the DNA-based lifeform(s) make it here? If we assume that different kinds of life based on different nucleic acids can co-exist, then it seems unlikely that a single kind would have happened to be carried to Earth by a natural process. If we assume DNA has some inherent fitness advantage (so that only DNA-based life survived the journey), than we don't need the additional assumption of panspermia: perhaps only the DNA-based life survived some early period in Earth's history. Also, for abiogenesis, given that we still can't reliably reproduce something like that, and that we have never seen new kinds of life spontaneously appearing on Earth, we do have some circumstantial evidence for the possibility that it is an incredibly unlikely event, such that it might have only happened once in Earth's history. Edit: There is also another simple possibility: perhaps given the resources available on Earth, either forever or in the period where abiogenesis was possible, perhaps DNA(+RNA) is the only molecule with the right properties to act as genetic material. Perhaps there is no common ancestor to all life on Earth, but a "forest" of inter-breeding, spotnaneously arisen original organisms, but they were all DNA/RNA-based simply because of the available resources.

rkagerer 7y ago
Another possibility often overlooked is that there's life out there on completely different timescales than we can recognize. We're only likely to notice those within a few orders of magnitude to our own. Maybe DNA is simply geared well compared to other mechanisms for the timescales we're interested in.
- rectangletangle 7y ago
  This is an interesting thought. Plants are actually very animated at their own timescale, adapting to their environment through phenotypic plasticity. However this happens far too slow for humans to notice without a time lapse.
  - gremlinsinc 7y ago
    This is why it takes so long to hold a meeting of the Ents.

schoen 7y ago

Probably, although a not-widely-accepted but interesting theory is that there might be microorganisms that we can't detect because we're not looking for the right things (because the things we look for are based on macroscopic life and things that interact biologically with it). https://en.wikipedia.org/wiki/Shadow_life https://en.wikipedia.org/wiki/Shadow_life xkcd just ran a strip with a joke about this theory last week https://xkcd.com/2171/ https://xkcd.com/2171/ I think it's unlikely because it seems hard to imagine no interaction between the kinds of life ever after billions of years of coevolution. On the other hand, there are tons of microorganisms that do share our biochemistry that we haven't noticed or identified yet, so maybe it's not impossible to imagine some that don't that we also haven't noticed or identified (since it would be more difficult for us to recognize them).

orwin 7y ago
It is interesting, but following Popper's epistemology rules, this hypothesis is not scientific. So saying it is a not-widely-accepted theory is right, but with reasons: this is not a theory. Not that just because a hypothesis is not scientific, it doesn't mean that people can not research or fine-tune it until it become a scientific theory, but in this state, spending public research credit on it is not a very good choice (especially in the current state of funding for biology). (it can be read wrong, my command of english is not great: i'm just adding thing to your comment, this is not a critisism of you or what you said)
- mannykannot 7y ago
  Popper's view is neither the last word on what science is, nor the final arbiter as to whether something is scientific: what he did was to add some powerful insights to a description of how science works. Having said that, it is neither unscientific nor against Popper to postulate something for which there is currently no evidence (the search for exoplanets was started before there was evidence of them) - you would have to postulate something that cannot, in principle, be empirically determined (such as p-zombies) to violate the falsifiability principle (if that's what your specific issue is here.) Such hypotheses are not outside of science; they are an essential part of it, and some unspecified part of scientific spending is effectively spent on generating them. On the other hand, as Hossenfelder has pointed out [1], falsifiability is a low bar. In the case of shadow life, it is reasonable to ask both how it could remain isolated and why have we not noticed it yet. [1] https://backreaction.blogspot.com/2019/01/just-because-its-falsifiable-doesnt.html https://backreaction.blogspot.com/2019/01/just-because-its-f...

dalbasal 7y ago

An alternative explanation (s) is that "there's only one way to do life" or that the origin of life is extremely rare/unlikely. But panspermia too.

teekert 7y ago

Interesting but your thought experiment only "proves" that the likelihood for something that can replicate itself to emerge is far smaller than the likelihood that that something eventually evolves into something extremely complex. Nothing more imho.

ozy 7y ago

> The fact that all known life shares a common genetic > structure implies a common ancestor. Or a (local) optimum.

tandr 7y ago
Prions have a more optimal optimum, but they are not exactly "life" from what I have read.

crimsonalucard 7y ago

You can't quantify the complexity of something like an eye. The eye could actually be very very simple to form, due to circumstances and variables outside of our knowledge. Scientists have discovered that the animal with the most genes--about 31,000--is the near-microscopic freshwater crustacean Daphnia pulex, or water flea. By comparison, humans have about 23,000 genes. One of the most astonishing features of the D. pulex genome is its compactness: despite being around 200 Mb in size (around 16-fold smaller than the human genome which is 3,200 Mb in size); its 12 chromosomes contain a minimum set of 30,907 predicted protein-coding genes, more than the 20,000–25,000 contained in the human counterpart. The unexpected complexity of the water flea genome compared with a human genome illustrates unknown dimensions of complexity that we cannot fully comprehend. What this means is this. You do not know how complicated the original template of life is... the complexity for the first genome to produce instructions that allow for self replication to kick off evolution may be so complex that it could only happen once on our planet, once in this galaxy or once in the universe. We simply do not have the knowledge to know the probability space of life on earth or life as we know it.

joycian 7y ago

Why are there no other human species (anymore)? Because there was no room for them anymore and they could not compete. In the same way, it it very well possible that life utilizing other genetic code got stamped out (early on).

ivanhoe 7y ago

well, if you look on viruses and bacteria situation stops looking that much uniform, there're some odd bugs there https://upload.wikimedia.org/wikipedia/commons/9/98/A_Novel_Representation_Of_The_Tree_Of_Life.png https://upload.wikimedia.org/wikipedia/commons/9/98/A_Novel_...

kitd 7y ago

OK, I'm not any kind of expert in this area, but is RNA not considered an early form of genetic coding? And DNA evolved from it? Also, don't viruses replicate by "borrowing" the mechanism of their hosts? My understanding is that life visible to the naked eye is a minority of life forms on the planet. Maybe other forms of genetic coding than DNA are more prevalent than we anthropocentrists think.

orwin 7y ago
Yes. DNA probably "evolved" from RNA. Keep im mind that the time scale is huge and that at the time, the earth was really hot and pressurized, so we can only have speculations. A posituve thing is that in Miller experiment, he found out that those conditions create aminate acids. That also indicate that panspermia theory is unlikely (not impossible, but not Occam's razor choice at least).
gomijacogeo 7y ago
My own tin-hat theory on that is DNA probably co-evolved with RNA pretty quickly. The main difference between RNA and DNA is that a single RNA strand can fold up and bond with itself to make interesting() 3d shapes while DNA can't self-fold. I think single-stranded DNA initially served as a durable, robust structure for RNA to assemble against before it went off and folded up and did whatever it did chemically. This might help explain why RNA polymerase only runs 3'->5'; in fact one of the early functions of one of the RNA polymerase components was maybe to keep the single DNA strand "tidy" and free of random base-pairings. Also, it might help explain why the 3'->5' DNA strand has to jump through so many extra hoops to get replicated - double-stranded DNA came later. It might also explain the reason reverse transcriptase exists at all. Again, I am probably provably a crank on this. () This is the whole appeal of the "RNA world" hypothesis, you have one molecule that can serve both as "code" and catalyst for other chemical reactions.

nabla9 7y ago

On the contrary. If life started multiple times on earth competitive exclusion probably took care of all other primitive forms rapidly. Even small differences, energy efficiency and reproduction lead to situation where only one form of primitive life survives. Once the life really started going, it changed the chemistry of the earth rapidly. Similar chemical evolution starting from scratch was not possible once the life started. Eventually when photosynthesis started, biologically induced molecular oxygen caused so called Oxygen Catastrophe where Earth's chemistry turned from reducing into oxidizing. Oxygen also changed the chemistry in the seas and rocks. Molecular oxygen is great disinfectant.

amelius 7y ago
How about different chirality? Different chiral forms would have the same advantages.
- gomijacogeo 7y ago
  Most precursor molecules (e.g. CH4, NH2, etc) aren't chiral, so differently-chiral'd proto-life would still compete for the basic building blocks. Each time one set of complexes managed to get more efficient at something, it probably drove more than half of its competitors to extinction. Repeat a few times and you wind up with essentially a single global consensus and the arms-race moves on to higher levels of abstraction.
  - amelius 7y ago
    > Most precursor molecules (e.g. CH4, NH2, etc) aren't chiral, so differently-chiral'd proto-life would still compete for the basic building blocks. Yes, but horses and cows also compete for the same building blocks. And yet, we have horses and cows. Your argument could be right if there wasn't an abundance of these building blocks.
    - gomijacogeo 7y ago
      And yet we have no more mastodons, dinosaurs, trilobites, ... Stuff still goes extinct, it's just a lot slower with cows than with molecules. I think you're just failing to grasp the sheer numbers involved here: there are probably around 10^9 cow-like critters and they interact with other, competing critters every, let's say, 10^4 seconds, with the odds of any one of these interactions being fatal being, again, let's say 1 in 10^4. By comparison, there were probably something like 10^37 (if I counted my orders of magnitude right) biomolecules interacting every 10^-6 seconds where every time some new chemical "A-ha!" moment occurred, that reaction now became 1000x more favorable than any other competing reaction. That's like, what, 41 orders of magnitude; even if I'm off by 10+ orders of magnitude, there was just a whole lot more selective pressure going on during the pre-biotic era. Less favorable chemical pathways were outcompeted on scales so vast you'd need trillions of cow-years to approximate how de-de-de-deselected-for they were.

TeMPOraL 7y ago

> The fact that all known life shares a common genetic structure implies a common ancestor. The lack of competing genetic codes could be interpreted to imply that life only started on earth once, or that it only made it to earth once. The more I read about DNA and RNA, the more it seems to me they're somewhat fundamentally grounded in chemistry - you can't just make codons mean arbitrary things by messing with the replication mechanisms. There's less path dependence, and more of "one optimal way of doing things". That said, I think it's reasonable to assume we all have common lineage[0] - it's arguably a winner-takes-all scenario, as there's a huge first-mover advantage for the first self-replicating family of things that got loose on the world. -- [0] - causally, not genetically; the history of life is not a tree structure due to constant lateral gene transfer that bacteria and archaea engage in.

Angostura 7y ago

The genetic code isn't quite as universal as you might think. Here's an interesting article on the peculiarities of encoding in the mitochrondial genome, and what it tells us about evolution. https://whyevolutionistrue.wordpress.com/2014/09/28/guest-post-why-the-genetic-code-is-not-universal/ https://whyevolutionistrue.wordpress.com/2014/09/28/guest-po...

gomijacogeo 7y ago
Also, https://en.wikipedia.org/wiki/Genetic_code#Alternative_genetic_codes https://en.wikipedia.org/wiki/Genetic_code#Alternative_genet... Although most (all?) of the variability is in the one place you can make it without almost-certainly killing your host organism - in repurposing the STOP codons. There are usually three (and likely UGG was a fourth before it got hijacked by Tryptophan in the before time) and all you need is one.

doctorstupid 7y ago

Perhaps genetic replication did emerge multiple times on Earth, but always to the same solution which had the highest probability?

ifrancium 7y ago

Drew Endy gives a pretty fun talk along the same lines: https://www.youtube.com/watch?v=GY28A0tBNPU https://www.youtube.com/watch?v=GY28A0tBNPU

2a96eb7d685a49c 7y ago

Perhaps the exact conditions that allowed for abiogensis were only present for a relatively short period of time. That would explain why it only happened once. The fact that we haven't been able to replicate it leads one to think that the conditions must be very unlikely. Also, your argument suggest the reason there are not different genetic structures on Earth is because the one we have came from outer space. If one genetic pattern can come from outer space why haven't others?

Balgair 7y ago

The panspermania hypothesis has one very large hurdle to get over: the half-life of DNA is ~500 years [0]. If life came from outside our planet via an asteroid or something, then it had to be a LOT of DNA that came over. More exotic theories can be concieved, but the jist is that it is really really hard for DNA to travel the stars and between planets intact. The simplier explination is that it started here. [0] https://royalsocietypublishing.org/doi/full/10.1098/rspb.2012.1745 https://royalsocietypublishing.org/doi/full/10.1098/rspb.201...

asfarley 7y ago
I get the impression that this article is mostly about dating bones. When people talk about DNA-based-panspermia, I've always assumed that they meant either a living or dormant organism (with the potential for DNA replication/error-correction), rather than a completely inert blob of DNA.
- Balgair 7y ago
  I always figured that the extremes of interstellar space automatically precluded anything that was even remotely alive. The temperature swings, vacuum, radiation, and ejection and re-entry violence always seemed to me to be so crazy that I assumed that nothing could survive it; just bits of genetric material, a dead ameoboid carcass, etc. I'll have to ponder on the idea that anything could get past all those 'gates'. If it did, woah, what a hardy little thing!

liberte82 7y ago

There's one possibility that explains it: that the probability of a genetic code evolving naturally is really, really, really low. Like, less likely to happen than once per universe. Then, once this barrier is attained, other forms of evolution occur more readily. As we don't know the mechanisms for how it happens, it's a possibility. The anthropic principle allows for it, but it likely requires multiverse theory and/or infinite universes coming and going over time. There's just no consciousness in all the infinite universes where it never occurred, so there's no one there wondering why there's no life.

jhedwards 7y ago

> The notion of abiogenesis occurring only once on earth seems improbable considering complex organs like eyes have evolved independently several different times. Not quite. Simple life forms probably existed for far longer than DNA. The fact that we see convergent evolution in complex life forms, which emerged very late in the history of life, only suggests that there are recurring patterns in the system that enabled the formation of higher life. It doesn't suggest anything about the origin of cellular life itself, which existed for hundreds of millions of years before eyes or lungs.

dragonwriter 7y ago

> The notion of abiogenesis occurring only once on earth seems improbable considering complex organs like eyes have evolved independently several different times. Evolution of eyes and abiogenesis aren't obviously similar enough for this analogy to be compelling without some narrative as to why what occurred with one should shape expectations of the other. The conditions in which life first emerged in either the Hadean or Eoarchean eons weren't around for much more of the history of the Earth; if the average expected frequency of abiogenesis under the conditions it actually emerged was once every quarter billion years or so, then—no matter which point in the possible range of time it first occurred—conditions wouldn't have been right for more than one to a very few instances, and if there were more than one, still one would likely be much more established by time available to spread alone, and then there were be massive environmental changes likely producing extinctions. Only one lineage surviving that bottleneck isn't surprising even if there were more before, and while we have evidence of early life it's far from complete either in coverage or ability to determine biochemistry and lineage. It's really not necessary to invoke extraterrestrial origin to explain only one lineage on Earth (and it actually may make the problem worse: because then you have to explain why we haven't gotten alternate lineages from the same or other extraplanetary sources.)

la_barba 7y ago

>Natural selection is a constant force, presumably new genetic codes could offer an advantage, e.g., robustness against damaging mutation, resulting in multiple competing genetic codes. But what would the advantage look like? Isn't successfully surviving and propagating for billions of years, the ultimate advantage? It would seem like our current state is the best one then..