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They are formed when bacteria break down amino acids containing sulfur.

Liu observed that these two amino acids were appearing at different concentrations.

These include 14 of the 20 amino acids that life on Earth uses to build proteins and all four of the ring-shaped molecules that make up DNA - adenine, guanine, cytosine and thymine.

So if you were to engineer synthetic mirror image versions of these, the amino acids would have right-handed chirality and the DNA would have left-handed chirality.

But raw milk advocates say that drinking unpasteurized milk is a “natural” food that contains more amino acids, vitamins, minerals and fatty acids.