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I am working out a future scenario in which humans begin to discover alien life; I want their discoveries to be plausible.

For example, I've frequently heard that alien life will most likely be carbon-based; because carbon can form so many more molecules than any other element.

The question is: What parts of molecular biology are commonly believed to be the most likely to occur if life evolved from scratch elsewhere in the Universe? I put it this way because I am interested in both what we probably have in common, but also what could be different but plausible.

As an aside (and not a separate question) I am interested in any likely ramifications of these molecular-biology similarities and differences on subsequent evolution: Such as if they would lead to oxygen-breathing aliens, or aliens adapted to much hotter or colder conditions, terrestrial vs. aquatic aliens, and so on.

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    $\begingroup$ we are almost completely certain they will obey the laws of chemistry, after that well... maybe water based? $\endgroup$ – John Aug 16 '17 at 2:55
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If we postulate that it's carbon-based, then water is also the most likely biosolvent. It just has a lot of really convenient properties that aren't found in any other single molecule- strong polarity, high heat capacity, large liquid range over a wide range of pressures, and a really good solvent for lots and lots of things. Non-polar solvents, like liquid methane or supercritical carbon dioxide, might work, but we don't know. It might be water plus something else, like a mixture of water and ammonia, or water and hydrogen peroxide, and pure water may be toxic to organisms using those mixtures (much like fresh water is harmful to saltwater fish) but the involvement of water is pretty likely. At lower temperatures, pure ammonia might work, too, but water just seems better.

Since most alien life is probably primitive bacteria-like organisms, breathing oxygen doesn't seem super likely. But if we're talking just about complex, animal-like life, whether or not they breathe oxygen will depend on other details of the environment. If it is possible to build up large quantities of oxygen in the atmosphere, then yeah, they'll probably breathe oxygen. Chlorine isn't quite as good an oxidizer, and there's no real good reason for autotrophs to produce it as a waste product, and while one can come up with somewhat contrived scenarios in which fluorine might be produced biologically, it's way less abundant than oxygen. If oxygen cannot be accumulated in the environment (e.g., because the oceans contain a high fraction of ammonia, or the atmosphere is mostly hydrogen, or something like that), they might breathe hydrogen instead (with redox metabolism being exactly reversed from ours), or they might not breathe at all, instead relying on eating a suitable balance of more-reducing vs. more-oxidizing foods (like the nitro-life from Hal Clement's The Nitrogen Fix).

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For example, I've frequently heard that alien life will most likely be carbon-based; because carbon can form so many more molecules than any other element.

This assessment is true only if you consider life in conditions similar to the ones we know.

referring wikipedia:

Why carbon?

  • Silicon biochemistry is a topic engaging lot of studies. This seam to be irrelevant for earth condition chemistry but is quite well adapted to hot and/or chemically aggressive environment
  • Bore/Nitrogen are not so far from carbon. Actually nitroborane crystals and nano-sheet show interesting properties close to the full carbons ones.

What is life? why would it be molecular based?

You focus on molecular biology but as your question is based on exotic life form mechanism i want to highlight this can be a restrictive vision of life.

Mainly you can consider life as an organized, entropy fighting, auto-reproductive system. This give you some place for other medias than biochemistry for life.

  • Plasma in a solar atmosphere. Which is a highly fluid medium, containing complex and fast physical process so why not some kind of organized system.
  • Metal based life, either metallic complexes or pure liquid metal in a contained area such as planet core
  • Highly advanced robotic can in some way be considered as life.

As a bonus, and for the sake of fun, alien could be made of antimatter which allow them to have the exact same physiology than us but would annihilate at contact ;-)

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TL;DR -- the basics would be the same, but more complex structures would diverge wildly.

Hmm. If you assume unguided evolution, it stands to reason that the kinds of chemical compounds we see on Earth showed up because it was ... easy ... for them to show up. It wouldn't be crazy to assume that they'd show up on another planet as well. So I'd expect on an Earthlike planet to see amino acids, lipids, and the like.

It wouldn't be identical. Some thoughts...

If they have a DNA-analogue, the encoding of bases to amino acids would likely be different. I doubt alien viruses could infect you.

Some key difficult-to-evolve chemicals might be just plain missing; after all, it took hundreds of millions of years of advanced plant life before they got around to evolving lignin. Some snake venoms contain compounds which are otherwise rare in nature (ie other creatures).

There are some other things which might be rare. The "invention" of mitochondria seems like a low-probability event, likely a botched eating attempt. You might see a lot of low-metabolism life on Planet X.

Not sure about bilateral vs other symmetries.

Overall, on an Earthlike world (in terms of temperature, gravity, sunlight, etc) I speculate that life would look a lot like it does here. You could eat it, but you can't mate with it.

Heck, imagine a world where oxygen respiration never evolved, so there's a CO2 atmosphere and oxygen is a deadly poison. If the inhabitants get frisky, the Terran Space Navy could bombard them with blue-green algae bombs!

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If we make a blanket assumption that life must be based on DNA/RNA and amino acid based proteins then we see how far alien life could diverge from us even with that restriction. Our DNA/RNA is based on three specific purines and two pyrimidines out of hundreds of different ones. The Proteins that the DNA/RNA makes are restricted to 22 amino acids out of hundreds. On that basis alone something could evolve that could breathe oxygen and drink water but would be utterly different from us. Some proteins made by life on our own world are highly toxic to us and some are serious allergens. Also billions of years of co-evolution on our planet had developed a sort of balance between an organism and it's biological environment. Just breathing alien air could bring in biochemicals that your body has no evolved defense mechanism to deal with. Our cells have evolved systems to detect and destroy DNA in the food we eat to prevent it from interacting with our own DNA. We know when we eat something it will not be trying to eat us. That may not be true on an alien world.

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