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I read once that we have two sources for proteins:

  1. Protein that is made in cells
  2. Protein that is made in labs

However, based on Miller–Urey experiment, it's argued that amino acids can be made from initial Earth's condition, and from this link, it seems that protein-first world is a hypothesis.

Yet if we accept that proteins could be made before cells, then the process that made proteins in the first place is still out there. This means that we should be able to find proteins outside cells or labs in nature.

Do we find protein not of cellular origin in nature?

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    $\begingroup$ I would suggest rewording your question to refer to proteins/peptides/amino acids not of cellular origin instead of "in nature". $\endgroup$ – MattDMo Jan 17 at 20:07
  • $\begingroup$ Viruses are composed of proteins. They exist outside of cells freely in nature. $\endgroup$ – Roni Saiba Jan 18 at 4:45
  • $\begingroup$ Vituses reproduce in living cells and use host cell's machinery to replicate. Thus they don't count. $\endgroup$ – Saeed Neamati Jan 18 at 12:03
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On our planet, there is such an incredibly massive amount of protein creation by biological organisms (as well as smaller peptide constructs) that it would be essentially impossible to identify any that was created by a natural non-biological process.

Moreover, now that there is life, any location with the right conditions for non-biological creation of protein would likely also be a good place for modern microbes to take advantage of those same resources to grow themselves, diverting the necessary inputs or quickly eating the outputs.

It is for this reason that we search for precursors of life in space, and have long ago found the basic building blocks. This is also why probes to other planets are carefully sterilized, because otherwise we will never know if life found on Mars is just hardy colonist organisms transplanted from Earth.

To the best of my knowledge, however, no non-Earth molecule has yet been found that has the full complexity of a protein, as opposed to just isolated amino acids.

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  • $\begingroup$ @user1136 Sure. But if you found material with L-amino acids, you wouldn't know. $\endgroup$ – jakebeal Jan 18 at 13:55
  • $\begingroup$ We're in agreement: finding proteins based on D-amino acids would be interesting. I'm saying that if we didn't sterilize spacecraft and we found proteins based on L-amino acids, we might have a hard time determining if it came from the planet or the spacecraft. $\endgroup$ – jakebeal Jan 18 at 14:12

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