There are many many possible amino acids, it is as universal as it is arbitrary that all life on earth uses just 20 standard amino acids. All of these from Alainine to Valine are said to be left-handed, that is they all twist to the left.

This feature translates to our DNA, which is built upon and with those same left-handed amino acids. It’s what allows genetic engineers to mix and match genes from different organisms, they all fit together because they’re all shaped the same. This is totally arbitrary, a world with right-handed amino acids would look exactly the same as this one.

Sometime in the distant past, nacient life must have—for one reason or another preferred left-handed amino acids over right-handed ones. A new study by Daniel P. Glavin and Jason P. Dworkin of NASA’s Goddard Space Flight Center, published in the Proceedings of the National Academy of Sciences examines amino acids found in carbonacious meteroites, and found a preference of 15%-18% of the left-handed version of the amino acid Isovaline in meterorites containg water. This could mean that formation of amino acids in space favors a left-handed orientation. This slight bias would be picked up by developing life competing for resources, allowing life using the slightly more common left-handed orientation to win out over life using the right-handed amino acids.

Glavin, D., & Dworkin, J. (2009). Enrichment of the amino acid L-isovaline by aqueous alteration on CI and CM meteorite parent bodies Proceedings of the National Academy of Sciences, 106 (14), 5487-5492 DOI: 10.1073/pnas.0811618106