Why would life on a different planet use DNA?

Life on Earth pretty early on settled on deoxyribonucleic acid organized in chains of base pairs as the means to code for the construction of proteins which make up a lifeform. It also has the benefit that it can be biologically copied relatively simply and accurately.

But why would life that has evolved separately from Earth life end up with the same solution to the problem of coding for construction of proteins and inheritance of such coding?

The specific base pairs in DNA are cytosine ("C", C4H5N3O), guanine ("G", C5H5N5O), adenine ("A", C5H5N5) and thymine ("T", C5H6N2O2). These happen to be able to form the fairly well-known "double helix" DNA structure. Here already we can see a strong dependence on an environment rich in carbon, nitrogen and oxygen (as well as hydrogen), which works well on Earth and with Earth life.

Assuming that life develops independently (no common origin) on different planets, possibly in different solar systems, each able to support some kind of life which may be either similar to or dissimilar from Earth life, is there any plausible reason, or plausible set of criteria, why life would happen onto specifically DNA (as used by Earth life) on different planets? Or is it simply a random chance thing and there is no reason whatsoever why alien life wouldn't just as well happen onto something utterly and completely different that solves the same problem?

Please note that I am not asking about the specific proteins being coded for, or the mechanism by which DNA is used to actually drive protein production, or the need to find a solution to the problems that DNA solves in Earth life, but simply about the use of specifically DNA itself. If you want to address those issues as well, then feel free, but they are not the focus of this question.

I'm not going to tag this as hard-science, but the harder the science in the answers, the better.

• I don't think anybody can answer this question properly. Honestly humanities knowledge on the limits of organic chemistry is woefully lacking. We've only just started to understand abiogenisis here on Earth. We truly don't know if there are better molecules for encoding information. Of the two major types of information encoding molecules that we know of (DNA and RNA), DNA is indeed better suited to this function but that doesn't tell us if there couldn't be something even better than it. We don't know how life/earths specific organic chemistry arose, was it inevitable, or chance? ¯_(ツ)_/¯ – AngelPray Dec 1 '17 at 21:48
• @AngelPray Evolution doesn't need better, or best, it just needs something that gets the job done and doesn't get in the way. :-) – a CVn Dec 1 '17 at 21:51
• DNA is easier than DNA with identical nucleotides to earth life which do you mean? – John Dec 1 '17 at 21:57
• @AngelPray: Not only do we not know if there's anything better than DNA/RNA, we don't (AFAIK, anyway) even know whether there's anything else that would actually work. – jamesqf Dec 2 '17 at 2:31
• I don't think anybody can answer this question properly. - nah. There are plenty of useful and "proper" answers to questions like this. A biologist/chemist (which I am not) may know of good reasons why these specific molecules would form. A physicist may have found them in asteroids (created naturally). A philosopher may argue the "earth is nothing special" argument and come from a purely statistical viewpoint. And so on. It is obvious that we cannot know until we do interstellar travel, but up to then, fine answers are not unlikely to arrive at. – AnoE Dec 3 '17 at 12:53

Because those nucleotides occur naturally.

...probably. We've been finding the building blocks of DNA on meteorites for a while now. According to NASA there's a good chance they occur naturally.

The team found adenine and guanine, which are components of DNA called nucleobases, as well as hypoxanthine and xanthine. ...Hypoxanthine and xanthine are not found in DNA, but are used in other biological processes.

Also, in two of the meteorites, the team discovered for the first time trace amounts of three molecules related to nucleobases: purine, 2,6-diaminopurine, and 6,8-diaminopurine; the latter two almost never used in biology. These compounds have the same core molecule as nucleobases but with a structure added or removed. ... "However, if asteroids are behaving like chemical 'factories' cranking out prebiotic material, you would expect them to produce many variants of nucleobases, not just the biological ones, due to the wide variety of ingredients and conditions in each asteroid."

Thirdly, the team found these nucleobases -- both the biological and non-biological ones -- were produced in a completely non-biological reaction. "In the lab, an identical suite of nucleobases and nucleobase analogs were generated in non-biological chemical reactions containing hydrogen cyanide, ammonia, and water. This provides a plausible mechanism for their synthesis in the asteroid parent bodies, and supports the notion that they are extraterrestrial," says Callahan.

NASA scientists studying the origin of life have reproduced uracil, cytosine, and thymine, three key components of our hereditary material, in the laboratory. They discovered that an ice sample containing pyrimidine exposed to ultraviolet radiation under space-like conditions produces these essential ingredients of life.

Rather than being a fluke that happened once on Earth, DNA, or at least its building blocks, appear to occur naturally. So another planet would have the same chemical base pairs available for proto-life to produce DNA.

DNA and RNA are very good at what they do: encode the blueprints for an organism very efficiently, accurately enough to ensure stability, but allowing sufficient inaccuracies for evolutionary variations to respond to changes in the environment. It's so good at what it does that despite billions of years of evolution and endless variety no life on Earth does anything else. This implies that even if several competing forms of life arise on another planet, DNA/RNA based life will win.

RNA might be more likely.

Just because the base pairs are there doesn't mean you get DNA. You might get RNA first, known as the RNA World Hypothesis. RNA has many of the properties of DNA which are important to life: it's self-replicating, it can act as a catalyst, and it can make proteins. RNA is more fragile, making it suitable for only simple life, but that's exactly where life starts out.

• A/T/G/C are nucleotides, not amino acids. Well, adenosine monophosphate is a nucleotide. Adenosine is a nucleoside. Adenine (like hypoxanthine and xanthine) is a nucleic acid base. -- In any case, they're not amino acids. – R.M. Dec 2 '17 at 13:58
• Note in theory, the mirror images of the nucleotides, which are not identical to the originals, ought to be equally likely to form at random. But that's only a factor of two applied to a probability we have no good estimates of. – aschepler Dec 2 '17 at 17:41

Having compatible DNA is very unlikely, just about the only way it could happen is if you had some form of cross contamination, like a panspermia event. Just having DNA may not be hard but have a compatible sequence (same nucleotides and/or codons) is just too unlikely to be believable without a common ancestor.

• Note that the OP specifically does not ask about anything regarding the compatibility, i.e., a "delivering mechanism" or the actual "code", but specifically about those 4 molecules. – AnoE Dec 3 '17 at 12:49

RNA is basic, DNA is sophisticated

It took a long time for life on earth to switch from a purely RNA based genome to the more stable DNA. RNA is still essential for life processes.

RNA is chemically simple and its ingredients are sythesised in Urey-style experiments. So it is pretty likely that an alien lifeform will use RNA as an information encoding molecule. It may later develop DNA or something different.

While RNA is basic, the code is arbitrary

An exobiological life form will for sure have a different genetic code being incompatible with the Earth's life forms. Even if it uses the same bases (they look quite optimal for their purpose, but so-called exotic bases exist in nature and can be synthesised by chemists), the mapping to aminic acids is completely arbitrary and will be different for sure (for statistical reasons). The set of aminic acids will be also different.

There may be mirror life

Sugars, aminic acids, and bases are asymetrical molecules, they have mirror images with the same physical features (melting point, acidity, etc.) but that are still different. Biological life has selected for a long time one of the mirror forms to build their more complex molecules from and cannot use the mirror images in general. On another planet, the mirror images of the Earth's molecules may be chosen by life.

• Mirror Milk! I've lost track of it now, but there was a drug that caused cancer/birth defects because a good portion of the active molecule was right-handed due to the manufacturing process (all life on Earth uses left-handed--chiral--proteins). That said, mirror milk is not always dangerous. There's at least one common sugar substitute that is a mirror molecule: it activates the same receptors in our tongue, but can't be digested. – Draco18s Dec 2 '17 at 22:34
• @Draco18s That's aspartame, commonly sold as Equal. I remember when it first came out. The mirror molecule was a big selling point as they were bringing it to market. It was certainly an improvement on saccharin, which was at the time the only available artificial sweetener. – BobRodes Dec 3 '17 at 4:15
• @BobRodes That's the sugar one, yes. Still can't remember the drug that caused issues (was back in like the 50s or 60s before the handedness of proteins was well understood). I see a topic about it every couple of years and can never remember what it was (and today...I was unable to Google Fu either one; kept finding Tagatose being listed as a mirror molecule). – Draco18s Dec 3 '17 at 5:09
• @Draco18s I recall that a study was done back in the 70s where saccharin caused cancer in rats. The general understanding among us teenagers (so perhaps an inaccurate understanding, because, well, teenagers) was that people would have to drink a bathtub full of the stuff every day for quite a while to get as much saccharin as they used in the rats. – BobRodes Dec 3 '17 at 5:46
• @BobRodes Not the one I was thinking of, unfortunately. Have run across it though. – Draco18s Dec 3 '17 at 6:03

EDIT: After some edits to the question, I think the bulk of my answer is not directly relevant as the question seems to be asking about the physical molecule of DNA and not an abstract idea of molecular organization. Given this, my response would be the "short answer" below.

I think a point of clarity here is what you mean by DNA. Do you mean literal, biological DNA the molecule, or the higher level concept of DNA as a molecular scale data storage mechanism?

If you mean literal DNA, then I would say that the chance is pretty small, approaching impossible. Convergent evolution can only explain so much, and the odds of two completely independent ecosystems evolving exactly the same solution to a problem is incredibly small.

However, if you mean DNA in a more abstract sense, then I would say that it is all but guaranteed.

Most definitions of life require a few key properties:

• Input/output of energy
• Self replication/perpetuation

The most relevant of these properties to this discussion is self replication. This requires some form of data storage at some level. Here on earth, organic organisms use DNA to store information from generation to generation and to differentiate between different types of organism.

DNA has a few key qualities that make it ideal for this task, again speaking in high-level, abstract terms:

• Recursive (it contains the instructions to read, build, and modify itself)
• Malleable (can be modified during replication, either intentionally or by chance)
• Simple structure (very basic components, four base pairs, are used to form very complex output)

While we obviously do not know for certainty, it seems reasonable to assume that any sort of complex life would require some similar mechanism that accomplishes these goals as well.

• The title has a "why" question and the body has an "either/or" question. How can the answer to either be "no"? – Spencer Dec 2 '17 at 5:31

We don’t know for sure, but probably not

There are so many possible different chemical variations that it would seem unlikely in the extreme that a rerun of evolution would arrive at exactly the same “design”. Whilst a lot would be down to the exact conditions present, a lot would also be down to chance.

Assuming that the life on this alien world was carbon and water based (reasonable assumptions but by no means a given). And assuming that some form of sugar based acid is going to be used (hard to be certain, but I would have thought there are plenty of other arrangements of molecules that could do the same job) and assuming that it happens to be pentose sugars that are use (rather than hexose sugars) then there is still a fair amount of choice as you can see here.

Even assuming all this there are lots of potential bases other than pyrimidines and purines and a good variety of pyrimidines and purines as well. Although not as famous as those of biological importance, there are plenty of variants available as can be seen by looking through this index

I would say the best way to ensure that DNA was found as the building block of life on another planet would be to make sure that the conditions on the alien world exactly matched those on Earth and additionally to assume that DNA is the only possible solution to the problem. Although this is a very big ask, it might just be that there is only one way of solving the problem using the chemistry available to life on an Earth like world (we don’t know for sure).

If this were the case then I think that it would be a suspiciously god like coincidence. And we might even imagine that intelligent humanoid life forms are the best solution for any land based animal and if we are not careful we can persuade ourselves into a star trek like universe where all the aliens are basically just like us ;o)

• One observation that may be relevant to the likelihood of similar organization of lifeforms in the universe is that water is fairly likely to exist universe-wide. – BobRodes Dec 3 '17 at 4:19

There are other methods to store genetic information similar to DNA that we have discovered. They are called XNA for short. I would assume, there will be different way to code genetic information in the universe, however, DNA would be one of them. So far, we know 9 other methods. If you incorporate other genetic methods into your universe, but explain that your story is passing on the worlds with DNA due to compatibility reasons, no one will question the science behind it. After all, this evolutionary path has been taken at least once, there is nothing that stops it to be taken more than once.

You can also consider using RNA as it is simpler to form and statistically speaking should be more abundant. RNA is compatible with our biology thus RNA based life forms can (and does) infect humans or can be used as food source.

• "XNA" - Xeno Nucleic Acid - just because of the "xeno" part it must be used for a story! :) More seriously, though - consider including the link to nucleic acid analogue - which are also alternative building blocks. – G0BLiN Dec 3 '17 at 15:04

DNA isn't necessary for life, but the instructions on how to function, how to reproduce, and the imperative for improvement, are necessary for life. You could implement those instructions in a variety of ways - in the book/film The Andromeda Strain, the foreign life was of a crystalline structure that thrived and grew under radiation. How that worked, the story didn't explain.

So to consider your hypothesis, you need to consider exactly how DNA is used in a living cell.

It is not DNA itself, but the instruction sets contained in DNA sequences, plus the supporting chemical structures, that enable life to function, just as a microprocessor by itself can't do anything. It's the instructions executed by the microprocessor that makes the system functional.

The living cell exists by executing instructions found in DNA, or in the case of very crude bacteria, the simpler and less long lived RNA. This miniature 'computer' runs on chemicals and not electricity, and it has no 'clock' (though it can multitask like crazy), but like a microprocessor, the cell executes an instruction set in response to stimuli (both internal and external), to achieve a desired goal. On top of the 30k or so instructions in DNA, plus a backup copy, DNA also contains templates for the multicellular, multiorgan creatures. This chemical software has been running for a few billion years on earth, fixing itself, reproducing itself and improving itself.

Consider also that as far back as we can find identifiable complete DNA strands, around 150-200 million years in bugs trapped in amber, the basic components and representation of instructions (4 chemicals, always referenced in groups of four... a 256 bit medium), have not changed. The templates for the creatures have evolved considerably, but not the basic instructions. The basic functioning of DNA does not appear to be evolving, although it does facilitate evolution of the templates.

Some of the instructions in DNA may well predate life on earth. Evolution in a living creature, as in variations in the DNA, only happens during reproduction when the DNA replicates, or a new strand is combined from two donors in the case of sexual reproduction. So how did living creatures evolve to reproduce, if reproduction is necessary for evolution? The living cell had to come into existence with the ability to reproduce, which also happens to be its most complex function, or it would not survive, let alone evolve and thrive.

It is interesting to note that, as far as we can determine, life on earth began not too long after the ecosphere changed to conditions that would support life, as in temperature, presence of water, etc... almost as if life was waiting for those conditions. Perhaps the basic instructions in DNA for functioning and reproducing/evolving aren't the result of random chance, but a deliberate effort. By whom and for what purpose might make for an interesting story.

You can accomplish the same goal with mechanisms other than DNA/RNA/amino acids/proteins. As it is, DNA and the supporting chemical structures are an elegant solution that has been proven on earth for a few billion years... repairing itself, replicating itself, and improving itself. Life, as far as we can observe, is the only entity in the physical universe that appears to be following a coded instruction set to improve itself. Science has never observed a rock or a body of water consulting an internal instruction set on what to do next.

One of the currently most supported theories on where life came from, states that we're descendants of bacteria that came to Earth with a meteorite. Given how far comets can travel and how often they presumably crash into other celestial objects (if you consider the impact of gravitation on their route), it's not too unlikely that extraterrestrial life would be descendants of the very same bacteria - the evolutionary line would, of course, split up long before the bacteria would have arrived on Earth, but probably not yet before DNA or at least RNA developed.

So, if we and the aliens happen to both have evolved from the same bacteria species that already had DNA when evoluton split up (which is, as explained above, not quite far-fetched), it's extremely likely that the genetic structure is DNA-based for the aliens as well - it hasn't changed much, as it seems, during the long travel of the bacteria across space to Earth, so it appears to be quite stable. It might be well-imaginable, though, that the aliens developed some comparably minor changes in the structure, thus having, say, 5 pairs of nucleobases rather than 2 like humans. Since earth life is the only known example, there's no telling how likely that is.

I think this information would be pretty much solid enough to build your entire concept upon, and the idea to be relatives of the aliens via a "life before life" of sorts might have some interesting impacts on the story.

• "Most supported" based on what? I've heard the hypothesis previously, so I'm certainly not claiming that it is unique to you. However, deep sea vents is what I would think of as "most supported" to be the first life. That said, the criterion is plausibility rather than "most supported" -- so this doesn't invalidate the rest of your answer. – Brythan Dec 3 '17 at 18:04
• You might want to change Panspermia to Pseudo-panspermia and include some links or just say that the hypothesis is there (just not the most supported one). And Statistics say that it should be only a 2 pair system with Panspermia, migt be even true for Pseudo-panspermia, but we only have one sample there. Some changes in proteine encoding will happen in the next billion years though. – Henning M. Dec 3 '17 at 18:14
• @HenningM. Thanks for pointing me towards the difference, but I'll probably stick to Panspermia as Pseudo-panspermia is basically what another answer states. As of the links, though, that's probably a good idea. – Egor Hans Dec 3 '17 at 18:21

There are people who speculate that DNA predates life

So the short answer is that with only one data point we don't know if DNA is super-special (I personally don't think it is)

In https://www.newscientist.com/article/mg21528795-500-dna-could-have-existed-long-before-life-itself/ it is speculated that DNA existed before life started to use it which would indicate to me that early life might well have just been using what was handy and another planet might well have something else handy.

• Read that New Scientist article and found it completely disappointing. The authors did not get DNA fragments from their Urey-style experiments, they only almost succeeded and claim not to be far from the goal. ON the other hand, RNA fragment can be synthesised under suitable conditions. – jknappen - Reinstate Monica Dec 3 '17 at 20:49

I can't say for sure, but the DNA/RNA system works wonderfully well. RNA might be better for simplicity. Think about it: Each three-base pair codon codes for a specific amino acid (although some different codons are for the same amino acid, e.g., Alanine is coded for both by mRNA codons GCU and GCC (DNA codons CGA and CGG)) The essential amino acids (for us) are the twenty covered by these codons, as well as start (Methionine, mRNA codon AUG) and stop codons (mRNA UAA UAG and UGA). You usually only have 2 types of DNA: DNA and mtDNA (mitochondrial). But RNA comes in many types: mRNA (messenger) tRNA (transfer; it brings the amino acids to the ribosome coded for by mRNA) lncRNA (long-noncoding) rRNA (ribosomal structural) crRNA (CRISPER RNA, used as a guide for the restriction enzyme Cas-9) snRNA (small nuclear.) and viral RNA (Which is unusual in that it can be double-stranded. Then agian, viruses also can have single-stranded DNA, so not so unusual.)