5 ms·

Hypothetical question: Let's assume for a while that amino acids are everywhere in the solar system or galaxy (if not universe). Let's also assume that there

Hypothetical question: Let's assume for a while that amino acids are everywhere in the solar system or galaxy (if not universe). Let's also assume that there are many earth-like planets. If we found alien life on another earth-like planet, could it be possible that it too would have independently-evolved DNA? Not just DNA-like substances, but chemically identical DNA? I guess I am wondering was DNA such an incredible fluke here on earth that we'll never see it again, or is it "the logical conclusion" for a bunch of similar amino acids in very similar environments? IIRC DNA is a chemically stable structure etc

rococode 4y ago
Convergent evolution has been observed many times on Earth. I think it's plausible that a similar process of evolution may be possible for DNA. https://en.wikipedia.org/wiki/Convergent_evolution https://en.wikipedia.org/wiki/Convergent_evolution
yababa_y 4y ago
In https://chemrxiv.org/engage/chemrxiv/article-details/60c75885ee301ce974c7b74e https://chemrxiv.org/engage/chemrxiv/article-details/60c7588..., they find the common nucleobases are found earlier in enumeration, so you might expect most life use it
cdumler 4y ago
It's a good question. It seems there are meany sources, including production on earth[1] and in space. It seems a lot of commonly available chemicals in space if hit with light or electricity will generate them. The open question has been what does it take to generate RNA/DNA. There has been suggestion that RNA would be unlikely to form in open, active water like found on early Earth. However, Mars would have had shallow oceans with large amount of shore area for chemicals to combine in quiet. The Panspermia theory is that RNA/DNA formed there and was ejected by asteroid hits to Earth. Support for that theory is that life appears to have formed within a few hundred million years of it being possible on Earth, which seems far too short of a time for RNA/DNA to form spontaneously in an ocean Earth. If this is all true, life may be a fluke and requiring a rather stable set of environments to work. All of this is hard to know with a sample size of one, though. [1] https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment
- stubish 4y ago
  Just recently on HN was RNA forms on basalt lava glass in the presence of nucleoside triphosphates, https://news.ycombinator.com/item?id=31628832 https://news.ycombinator.com/item?id=31628832 . Not a complete answer though, as I don't think we know if there where nucleoside triphosphates around (here, or on Mars for that matter).
mywittyname 4y ago
I'm by no means an expert on this subject. There are many different forms that RNA takes, and some of those forms are double-stranded, but differ from DNA in certain key ways. There are even "non-canonical" forms of DNA which exist whose nucleobases differ from the traditional four bases found in most DNA. Thus, you're premise that all DNA on Earth is "chemically identical" is flawed. But, if you expand the scope of your a bit, then the answer is a little more satisfying.
arrosenberg 4y ago
Probably not chemically identical, but similar. Different nucleotides and protein sequences, but you'd certainly see alot of the same structures (alpha helix, beta pleated sheet, etc.) and things like methylation. I would expect this based on the fact that DNA is incredibly information dense, so it would be surprising if the universe had completely novel ways to optimize for that problem.
echelon 4y ago
"DNA" itself is so complicated that there are different conformations [1] of the winding, sense, rise, pucker, etc. There are an unlimited number of different bases that could be used to encode information, too! [1] https://en.wikipedia.org/wiki/A-DNA https://en.wikipedia.org/wiki/A-DNA
sebosp 4y ago
If this was the case, then here on earth would we see more and more DNA appearing that doesn't follow an expected evolution tree? but then also our current DNA creatures would/could absorb/eat them before they are allowed to adapt to pressure in the surrounding environment?
gilleain 4y ago
Difficult question, but there's an analogous question for 'carbon-based' life - are there non-carbon based life? The answer is likely "no", simply because the number of molecules based on carbon is many orders of magnitude larger than for the next largest (sulfur). Similarly for DNA: there are other polymers that have some of the same properties, but not to the same extent. Specifically, the ability to store information is so great in DNA. Most polymers are either regular like (-CH2-)n or random (eg lignin) while DNA (and RNA) are 'structured random', for lack of a better term. Add to this the ability to 'unzip' double-stranded DNA into two copyable halves, its great stability, and ability to be coiled up. Like water - which is another example of something with an abundance of useful features - it's very hard to see any rival molecule that could do as many things. Of course, you could just argue that we have not discovered or invented such a molecule yet. People have tried to make alternate compounds, and there are - as one of the other repliers pointed out - many variations in biology such as alternate nucleobases, or things like double-stranded RNA viruses, etc. More generally, you could of course imagine distributing the properties of DNA across multiple molecules or molecular systems. That suffers from Occam's razor a little, but cannot be ruled out, I suppose.
lend000 4y ago
There are so many examples of convergent evolution [0] here on Earth that it seems reasonable to speculate that many of the most useful compounds for life would evolve similarly many times over a large number of trials/planets. Since we don't have access to such data, some metrics that seem useful for speculation about a property are: - How often it has evolved independently on Earth - How fast it evolved and remained dominant That being said, there is a potential positive feedback loop going on, where most life depends on or benefits from compounds it gets from other life. Because there is an abundance of certain types of amino acids, we could hypothetically be stuck in a less optimal overall biosphere state, whereas a more fit biosphere might exist with a different composition of amino acids or other building blocks that played the same role. I'm defining fitness of the biosphere as the amount of energy from the sun that life harnesses for work. [0] https://en.wikipedia.org/wiki/Convergent_evolution https://en.wikipedia.org/wiki/Convergent_evolution
- hypertele-Xii 4y ago
  > There are so many examples of convergent evolution [0] here on Earth that it seems reasonable to speculate that many of the most useful compounds for life would evolve similarly many times over a large number of trials/planets. Examples of convergent evolution of the most useful compunds for life on Earth. It makes sense that in a unified environment (of Earth) there is a tendency to evolve similar mechanisms to survive - on Earth. But we just don't know what kind of other chemical environments might be out there, in the virtually infinite cosmos. As beings of Earth-based life we fail to imagine any other possibility. This is a failing of our own understanding, not of chemistry to produce life. We just don't know, so we can't really even begin to guess. All we know is, our own backyard looks pretty dead except for us.
  - lend000 4y ago
    We can actually get a pretty good idea from looking at the spectral emissions, at least which environments are likely to be more common. On ptable.com [0] you can compare the abundance of elements in a human to the abundance on Earth, meteors, the solar system, and the universe. It seems like despite an abundance of aluminum, silicon, iron, and titanium, relatively trace amounts are found in humans (a typical animal), and most heavier elements are not needed at all and are actively toxic. The more versatile, modular, lightweight elements are very highly represented in humans. There's a reason organic chemistry is its own field -- those "more elementary" elements are simply more unique than heavier ones. At the top of the periodic table where the nuclei have small masses and closer orbitals, the relative impact of unfilled orbitals is much more significant. The standard reduction potential of Lithium (element no. 3) is a prime example of why it is so uniquely dominant for battery chemistry. I agree with you that our universe is a big place and a lot could happen, but my point is that attractor states exist and I would expect orders of magnitude more life to be based on organic chemistry, and probably even similar amino acids / a DNA double helix than not. Those lighter elements also tend to be more common elements throughout the universe, so the amount of planets where life with some inferior chemistry based on germanium (everything would be much heavier for the same reactivity patterns as an organic compound) would be limited to planets where there is a ton of germanium and not much carbon or silicon, which would be very rare (although it would be awesome to see what would happen!). [0] https://ptable.com/?lang=en#Properties/Abundance/Human https://ptable.com/?lang=en#Properties/Abundance/Human
gus_massa 4y ago
Nobody is sure, but if you allow me to guess ... Perhaps DNA and RNA are inevitable, or at least they are the easiest solution for they problem they solve. I guess even the same sugars and some nucleotides. The only variation on Earth is that some parts of the DNA are methylated, but it can be added and removed. There are some weird bases in the RNA for very special task, but my guess is that they will use the same main bases. I'd not be surprised if alien has DNA and RNA that is chemically identical to us (but in other order). Even proteins are probably inevitable. Here there is more wiggling room. We use 20 amino acids, but there are like 5 more weird amino acids that are mostly modifications of usual amino acids and are made after the protein is formed. For example collagen uses a lot of proline that is a normal amino acid, and also hidroxyproline that is a modification of proline made during the creation of collagen https://en.wikipedia.org/wiki/Collagen#Synthesis https://en.wikipedia.org/wiki/Collagen#Synthesis I guess that the list of amino acids used by aliens will be slightly different. Perhaps between 15 and 25 instead of the 20 we use. Perhaps also some replacements. But perhaps the list has some special property we have not discovered yet. I don't expect a smoking gun here to determine if aliens are unrelated to us. The most interesting part is the translation from DNA/mRNA to proteins https://en.wikipedia.org/wiki/Genetic_code https://en.wikipedia.org/wiki/Genetic_code It's a translation from the 64 combinations of 3 bases of DNA/RNA to amino acids. It's almost blocked, so there are 16 blocks with 4 options in each one. But some blocks have small changes. We know like 20 versions of the small changes. The 20 versions are very similar, they are not very different. So it's possible to make changes in this translation table. The most interesting part are the 16 blocks. As far as we know the elections it arbitrary, like a frozen random initial election that was inherited by all of living things on Earth. So the first thing to check in an alien life is if they translate in the 16 blocks the same amino acid that we translate. I'd be very surprised if this is the case. My guess is that it's almost random, and any election is fine, and each independent alien life will have a very different initial random election.
zackees 4y ago
[dead]
nomel 4y ago
There’s a great Lex Fridman podcast related to this, about “Assembly Theory”, by Lee Cronin. I thought it was fascinating. Youtube: https://youtu.be/ZecQ64l-gKM https://youtu.be/ZecQ64l-gKM Spotify: https://open.spotify.com/episode/4U3teONvRzNh32qqtX6X2w?si=50R638K1TAu0C0FK9LLBzw https://open.spotify.com/episode/4U3teONvRzNh32qqtX6X2w?si=5...

rococode 4y ago

Convergent evolution has been observed many times on Earth. I think it's plausible that a similar process of evolution may be possible for DNA. https://en.wikipedia.org/wiki/Convergent_evolution https://en.wikipedia.org/wiki/Convergent_evolution

yababa_y 4y ago

In https://chemrxiv.org/engage/chemrxiv/article-details/60c75885ee301ce974c7b74e https://chemrxiv.org/engage/chemrxiv/article-details/60c7588..., they find the common nucleobases are found earlier in enumeration, so you might expect most life use it

cdumler 4y ago

It's a good question. It seems there are meany sources, including production on earth[1] and in space. It seems a lot of commonly available chemicals in space if hit with light or electricity will generate them. The open question has been what does it take to generate RNA/DNA. There has been suggestion that RNA would be unlikely to form in open, active water like found on early Earth. However, Mars would have had shallow oceans with large amount of shore area for chemicals to combine in quiet. The Panspermia theory is that RNA/DNA formed there and was ejected by asteroid hits to Earth. Support for that theory is that life appears to have formed within a few hundred million years of it being possible on Earth, which seems far too short of a time for RNA/DNA to form spontaneously in an ocean Earth. If this is all true, life may be a fluke and requiring a rather stable set of environments to work. All of this is hard to know with a sample size of one, though. [1] https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment

stubish 4y ago
Just recently on HN was RNA forms on basalt lava glass in the presence of nucleoside triphosphates, https://news.ycombinator.com/item?id=31628832 https://news.ycombinator.com/item?id=31628832 . Not a complete answer though, as I don't think we know if there where nucleoside triphosphates around (here, or on Mars for that matter).

mywittyname 4y ago

I'm by no means an expert on this subject. There are many different forms that RNA takes, and some of those forms are double-stranded, but differ from DNA in certain key ways. There are even "non-canonical" forms of DNA which exist whose nucleobases differ from the traditional four bases found in most DNA. Thus, you're premise that all DNA on Earth is "chemically identical" is flawed. But, if you expand the scope of your a bit, then the answer is a little more satisfying.

arrosenberg 4y ago

Probably not chemically identical, but similar. Different nucleotides and protein sequences, but you'd certainly see alot of the same structures (alpha helix, beta pleated sheet, etc.) and things like methylation. I would expect this based on the fact that DNA is incredibly information dense, so it would be surprising if the universe had completely novel ways to optimize for that problem.

echelon 4y ago

"DNA" itself is so complicated that there are different conformations [1] of the winding, sense, rise, pucker, etc. There are an unlimited number of different bases that could be used to encode information, too! [1] https://en.wikipedia.org/wiki/A-DNA https://en.wikipedia.org/wiki/A-DNA

sebosp 4y ago

If this was the case, then here on earth would we see more and more DNA appearing that doesn't follow an expected evolution tree? but then also our current DNA creatures would/could absorb/eat them before they are allowed to adapt to pressure in the surrounding environment?

gilleain 4y ago

Difficult question, but there's an analogous question for 'carbon-based' life - are there non-carbon based life? The answer is likely "no", simply because the number of molecules based on carbon is many orders of magnitude larger than for the next largest (sulfur). Similarly for DNA: there are other polymers that have some of the same properties, but not to the same extent. Specifically, the ability to store information is so great in DNA. Most polymers are either regular like (-CH2-)n or random (eg lignin) while DNA (and RNA) are 'structured random', for lack of a better term. Add to this the ability to 'unzip' double-stranded DNA into two copyable halves, its great stability, and ability to be coiled up. Like water - which is another example of something with an abundance of useful features - it's very hard to see any rival molecule that could do as many things. Of course, you could just argue that we have not discovered or invented such a molecule yet. People have tried to make alternate compounds, and there are - as one of the other repliers pointed out - many variations in biology such as alternate nucleobases, or things like double-stranded RNA viruses, etc. More generally, you could of course imagine distributing the properties of DNA across multiple molecules or molecular systems. That suffers from Occam's razor a little, but cannot be ruled out, I suppose.

lend000 4y ago

There are so many examples of convergent evolution [0] here on Earth that it seems reasonable to speculate that many of the most useful compounds for life would evolve similarly many times over a large number of trials/planets. Since we don't have access to such data, some metrics that seem useful for speculation about a property are: - How often it has evolved independently on Earth - How fast it evolved and remained dominant That being said, there is a potential positive feedback loop going on, where most life depends on or benefits from compounds it gets from other life. Because there is an abundance of certain types of amino acids, we could hypothetically be stuck in a less optimal overall biosphere state, whereas a more fit biosphere might exist with a different composition of amino acids or other building blocks that played the same role. I'm defining fitness of the biosphere as the amount of energy from the sun that life harnesses for work. [0] https://en.wikipedia.org/wiki/Convergent_evolution https://en.wikipedia.org/wiki/Convergent_evolution

hypertele-Xii 4y ago
> There are so many examples of convergent evolution [0] here on Earth that it seems reasonable to speculate that many of the most useful compounds for life would evolve similarly many times over a large number of trials/planets. Examples of convergent evolution of the most useful compunds for life on Earth. It makes sense that in a unified environment (of Earth) there is a tendency to evolve similar mechanisms to survive - on Earth. But we just don't know what kind of other chemical environments might be out there, in the virtually infinite cosmos. As beings of Earth-based life we fail to imagine any other possibility. This is a failing of our own understanding, not of chemistry to produce life. We just don't know, so we can't really even begin to guess. All we know is, our own backyard looks pretty dead except for us.
- lend000 4y ago
  We can actually get a pretty good idea from looking at the spectral emissions, at least which environments are likely to be more common. On ptable.com [0] you can compare the abundance of elements in a human to the abundance on Earth, meteors, the solar system, and the universe. It seems like despite an abundance of aluminum, silicon, iron, and titanium, relatively trace amounts are found in humans (a typical animal), and most heavier elements are not needed at all and are actively toxic. The more versatile, modular, lightweight elements are very highly represented in humans. There's a reason organic chemistry is its own field -- those "more elementary" elements are simply more unique than heavier ones. At the top of the periodic table where the nuclei have small masses and closer orbitals, the relative impact of unfilled orbitals is much more significant. The standard reduction potential of Lithium (element no. 3) is a prime example of why it is so uniquely dominant for battery chemistry. I agree with you that our universe is a big place and a lot could happen, but my point is that attractor states exist and I would expect orders of magnitude more life to be based on organic chemistry, and probably even similar amino acids / a DNA double helix than not. Those lighter elements also tend to be more common elements throughout the universe, so the amount of planets where life with some inferior chemistry based on germanium (everything would be much heavier for the same reactivity patterns as an organic compound) would be limited to planets where there is a ton of germanium and not much carbon or silicon, which would be very rare (although it would be awesome to see what would happen!). [0] https://ptable.com/?lang=en#Properties/Abundance/Human https://ptable.com/?lang=en#Properties/Abundance/Human

gus_massa 4y ago

Nobody is sure, but if you allow me to guess ... Perhaps DNA and RNA are inevitable, or at least they are the easiest solution for they problem they solve. I guess even the same sugars and some nucleotides. The only variation on Earth is that some parts of the DNA are methylated, but it can be added and removed. There are some weird bases in the RNA for very special task, but my guess is that they will use the same main bases. I'd not be surprised if alien has DNA and RNA that is chemically identical to us (but in other order). Even proteins are probably inevitable. Here there is more wiggling room. We use 20 amino acids, but there are like 5 more weird amino acids that are mostly modifications of usual amino acids and are made after the protein is formed. For example collagen uses a lot of proline that is a normal amino acid, and also hidroxyproline that is a modification of proline made during the creation of collagen https://en.wikipedia.org/wiki/Collagen#Synthesis https://en.wikipedia.org/wiki/Collagen#Synthesis I guess that the list of amino acids used by aliens will be slightly different. Perhaps between 15 and 25 instead of the 20 we use. Perhaps also some replacements. But perhaps the list has some special property we have not discovered yet. I don't expect a smoking gun here to determine if aliens are unrelated to us. The most interesting part is the translation from DNA/mRNA to proteins https://en.wikipedia.org/wiki/Genetic_code https://en.wikipedia.org/wiki/Genetic_code It's a translation from the 64 combinations of 3 bases of DNA/RNA to amino acids. It's almost blocked, so there are 16 blocks with 4 options in each one. But some blocks have small changes. We know like 20 versions of the small changes. The 20 versions are very similar, they are not very different. So it's possible to make changes in this translation table. The most interesting part are the 16 blocks. As far as we know the elections it arbitrary, like a frozen random initial election that was inherited by all of living things on Earth. So the first thing to check in an alien life is if they translate in the 16 blocks the same amino acid that we translate. I'd be very surprised if this is the case. My guess is that it's almost random, and any election is fine, and each independent alien life will have a very different initial random election.

zackees 4y ago

[dead]

nomel 4y ago

There’s a great Lex Fridman podcast related to this, about “Assembly Theory”, by Lee Cronin. I thought it was fascinating. Youtube: https://youtu.be/ZecQ64l-gKM https://youtu.be/ZecQ64l-gKM Spotify: https://open.spotify.com/episode/4U3teONvRzNh32qqtX6X2w?si=50R638K1TAu0C0FK9LLBzw https://open.spotify.com/episode/4U3teONvRzNh32qqtX6X2w?si=5...