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The amino acids that can be found everywhere include ten of the simpler amino-acids that are used in proteins (glycine, the 2 acids, the 3 branched, alanine, pr

The amino acids that can be found everywhere include ten of the simpler amino-acids that are used in proteins (glycine, the 2 acids, the 3 branched, alanine, proline and the 2 alcohols). The other 11 amino-acids from proteins have never been found where life does not exist. They are more complex and they seem to have been developed by living beings long after the appearance of life and the appearance of the genetic code (they seem to have substituted later the simpler amino-acids in certain locations of the map of the original genetic code, which encoded fewer amino-acids). Moreover, while the simple amino-acids, including the ten that are used in proteins, can be found pretty much everywhere, wherever they were not produced by living beings they have been found in racemic mixtures, i.e. in equal amounts of left-handed and right-handed isomers, while in proteins only the left-handed isomers are used, so the living beings normally produce almost only left-handed isomers. Very small quantities of right-handed isomers are produced by some living beings, for other purposes than making proteins. So it is relatively easy to distinguish amino-acids that have been produced by living beings from amino-acids that have been produced in abiotic conditions (i.e. the amino-acids produced in abiotic conditions are recognized by the absence of complex amino-acids and by the presence of great quantities of right-handed isomers).

asdff 5mo ago
I don't see the left handed aspect as necessary for life. To me this just suggests our common ancestor for life on earth made use of this chirality. Another group of organisms somewhere else could have evolved from an ancestor that makes use of right handed chirality. Or one that is hand-blind.
- adrian_b 5mo ago
  I agree. It seems that the choice between left-handed and right-handed amino-acids was random. However, it is unlikely that other kinds of life forms could use both kinds indiscriminately, because mixing them creates difficulties in the assembling of polymers. So it is likely that amino-acids produced by some extra-terrestrial life form would also be predominantly of only one orientation, but it could happen to be the right-handed variant. Moreover, extra-terrestrial life forms could use very different complex amino-acids, because there are much more of those than the 11 that have been added to the simple amino-acids in the terrestrial proteins.
  - asdff 5mo ago
    That makes sense that the chirality can affect downstream polymer assembly or even folding in the higher order structures. Likely we are all left handed on earth because our left handed ancestor outcompeted the right handed organisms in the primordial soup. Or the right handed organisms just didn't evolve in the first place here on earth and there was nothing to outcompete. There might still be some higher order advantages to shifting chirality one way or another. Certain molecules, such as methamphetamine, have differing bioactivity based on chirality. Maybe this can be regulated in some way such as to control the rate of some other downstream process. In an abstracted sense, chemists here on earth are already this organism as they refine reactions to produce desired chirality and reduce expenditure on undesired chirality. ET could be using different amino acids, or more or fewer. I would hazard to guess there is immense selection to reduce the amino acid set to its most necessary components. This pressure has gotten to the point here on earth where even these necessary components might not all be produced endogenously by the organism who needs them, but consumed from the environment saving energy spent on synthesis. But this requires your neighbor to be producing these AAs, such that you consume them, and you having sufficient feedback mechanisms to not immediately consume all of your neighbor's species and put your own insufficient lineage to extinction.
    - cyberax 5mo ago
      Life can even use something other than amino acids. They are really inconvenient when you think about it. Fixed nitrogen is extremely rare, and there are no nitrogen-containing minerals other than some exotic exceptions. Amino acids are useful because they can be easily joined together and split apart (via the C-N bond). But there are other types of "molecular glues" that are viable, like sulfur or phosphorus.
      - adrian_b 5mo ago
        Amino acids are much more likely to be involved in the appearance of life anywhere than other molecules. For instance it would be much less surprising if an alien life form used another kind of polymer to store information, instead of nucleic acids, than if it would not use amino acids. The fact that on Earth the living beings eventually used ATP and RNA appears to have been determined in great part by chance, while the use of amino acids seems to have been much more deterministic. Some of the simple amino acids are very easy to be synthesized in abiotic conditions, which is why they are ubiquitous in many celestial bodies. The advantage of amino acids is that they do not contain only one end that can be attached to other molecules, but that they contain two such ends. A molecule with only one connector would attach to another, forming a dimer, after which no further reaction is possible. A molecule with two connectors, like an amino acid that has both a carboxyl end and an amine end, can be daisy chained into a polymer of arbitrary length. This allows building complex structures. There are other molecules with two connectors, but they are much more unlikely to appear in abiotic conditions. Thioesters, i.e. a kind of organic molecules that are bound by a sulfur bridge, like you mention, appear to have been much more important when life has appeared on Earth than today, but such molecules were important as intermediates in metabolic reactions, not as structural blocks, like amino acids, and there are no known naturally-produced molecules with sulfur that could be used as easily as amino acids to make molecules with arbitrary complex shapes.

asdff 5mo ago

I don't see the left handed aspect as necessary for life. To me this just suggests our common ancestor for life on earth made use of this chirality. Another group of organisms somewhere else could have evolved from an ancestor that makes use of right handed chirality. Or one that is hand-blind.

adrian_b 5mo ago
I agree. It seems that the choice between left-handed and right-handed amino-acids was random. However, it is unlikely that other kinds of life forms could use both kinds indiscriminately, because mixing them creates difficulties in the assembling of polymers. So it is likely that amino-acids produced by some extra-terrestrial life form would also be predominantly of only one orientation, but it could happen to be the right-handed variant. Moreover, extra-terrestrial life forms could use very different complex amino-acids, because there are much more of those than the 11 that have been added to the simple amino-acids in the terrestrial proteins.
- asdff 5mo ago
  That makes sense that the chirality can affect downstream polymer assembly or even folding in the higher order structures. Likely we are all left handed on earth because our left handed ancestor outcompeted the right handed organisms in the primordial soup. Or the right handed organisms just didn't evolve in the first place here on earth and there was nothing to outcompete. There might still be some higher order advantages to shifting chirality one way or another. Certain molecules, such as methamphetamine, have differing bioactivity based on chirality. Maybe this can be regulated in some way such as to control the rate of some other downstream process. In an abstracted sense, chemists here on earth are already this organism as they refine reactions to produce desired chirality and reduce expenditure on undesired chirality. ET could be using different amino acids, or more or fewer. I would hazard to guess there is immense selection to reduce the amino acid set to its most necessary components. This pressure has gotten to the point here on earth where even these necessary components might not all be produced endogenously by the organism who needs them, but consumed from the environment saving energy spent on synthesis. But this requires your neighbor to be producing these AAs, such that you consume them, and you having sufficient feedback mechanisms to not immediately consume all of your neighbor's species and put your own insufficient lineage to extinction.
  - cyberax 5mo ago
    Life can even use something other than amino acids. They are really inconvenient when you think about it. Fixed nitrogen is extremely rare, and there are no nitrogen-containing minerals other than some exotic exceptions. Amino acids are useful because they can be easily joined together and split apart (via the C-N bond). But there are other types of "molecular glues" that are viable, like sulfur or phosphorus.
    - adrian_b 5mo ago
      Amino acids are much more likely to be involved in the appearance of life anywhere than other molecules. For instance it would be much less surprising if an alien life form used another kind of polymer to store information, instead of nucleic acids, than if it would not use amino acids. The fact that on Earth the living beings eventually used ATP and RNA appears to have been determined in great part by chance, while the use of amino acids seems to have been much more deterministic. Some of the simple amino acids are very easy to be synthesized in abiotic conditions, which is why they are ubiquitous in many celestial bodies. The advantage of amino acids is that they do not contain only one end that can be attached to other molecules, but that they contain two such ends. A molecule with only one connector would attach to another, forming a dimer, after which no further reaction is possible. A molecule with two connectors, like an amino acid that has both a carboxyl end and an amine end, can be daisy chained into a polymer of arbitrary length. This allows building complex structures. There are other molecules with two connectors, but they are much more unlikely to appear in abiotic conditions. Thioesters, i.e. a kind of organic molecules that are bound by a sulfur bridge, like you mention, appear to have been much more important when life has appeared on Earth than today, but such molecules were important as intermediates in metabolic reactions, not as structural blocks, like amino acids, and there are no known naturally-produced molecules with sulfur that could be used as easily as amino acids to make molecules with arbitrary complex shapes.