In astrobiology, a proposed way of testing whether life found on another world is alien or originates from Earth is to determine the chirality of its molecules. This is of course only useful when the biochemistry is similar enough that one may suspect contamination.

But this test has a weakness. While different chirality can confirm alien origin, similar chirality does not exclude alien origin.

To mend this, a plausible idea is to cross check the chirality of a wide range of compounds. A false positive suggesting Earth origin would then have an exponentially smaller chance to match every single compound tested.
...or so I hope.

The issue is that I fear various chiral compounds do not have statistically independent default chirality. For instance, common biochemical pathways may produce one compound from another, leading the chirality of the first molecule to directly determine the chirality of the second.

I'm looking for a high level perspective: Does the chirality of a single compound more or less determine the chirality of all other compounds in an entire biochemistry, or are there a great number of compounds that have an arbitrary default chirality compared to others?

  • $\begingroup$ If the chirality is not conserved along the biochemical pathways, then the test outlined in the first paragraph is not valid. Although, concievably one could design other tests, comparing various compounds and chains. $\endgroup$
    – Roger V.
    Dec 6, 2020 at 8:34

2 Answers 2


Hypothetically if an alien species were found , and if they contained similar biochemical polymers (DNA with A,T,C,G , Protein with 20 Amino acids etc) then we MAY be able to conclude that they share an origin with us. (Just similar chirality does not necessitates the theory of same origin)

Does the chirality of a single compound more or less determine the chirality of all other compounds in an entire biochemistry?

Yes. As Chiral molecules are almost always produced/purified from chiral medium (containing other chiral molecules and/or chiral physical attributes e.g. polarized light).

Therefore (Assuming that the reaction is enantioselective and you want pure chemical with a specific stereochemistry) the configuration of the chiral center of a new molecule does depend on the reagent chirality. Still this relation is not constant and differs from one reaction to another. For instance textbook SN2 reaction leads to inversion of configuration while Williamson ether synthesis reaction maintains the chirality.

A non-racemic mixture of a chiral compound is resulted from previous non-racemic mixture and so on. therefore the stereochemistry of the first non-racemic mixture determined the S/R , L/R ,(-/+) chirality of the entire life per se whose identity may never be fully understood.


Biologic reactions preserve chirality whereas non-biologic reactions generally do not (although there are examples of rxns where chirality is preserved ) I think this strategy would work for comparing biomolecules with multiple chiral centers. For example, from an IUPAC perspective, D-glucose is (2R,3S,4R,5R)-2,3,4,5,6-Pentahydroxyhexanal and simply among aldoses & ketals , finding a single biomolecule (not just carbohydrates,) with multiple chiral centers where a single carbon involves a different chiralty than one used by life on earth, would highly suggest that non-earth originated life was responsible.


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