As the number of atoms in a molecule increases, so does the number of ways of arranging the atoms in space. Of those arrangements, called structural isomers, by far the most numerous are chiral. Such isomers have a definite handedness and can’t be superposed on their mirror images.

Given the preponderance of chiral isomers, it’s not surprising that all the amino acids in our bodies except the simplest, glycine, are chiral. What’s remarkable, however, is that naturally occurring amino acids share the same chirality. Although the homochirality of biomolecules astonished Louis Pasteur, who discovered the trait in 1848, its existence does make sense. To function, a protein must reliably adopt a unique complex and distinct three-dimensional shape. Building homochiral proteins from homochiral amino acids serves that goal.

It’s harder to understand how homochirality ever arose in the first place before life began. Energetically, left-handed and right-handed isomers are identical...

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