It is one of life's great unexplained chemical quirks: the 20 amino acids found in all living things almost always have the same geometric configuration. The structure can best be demonstrated by crystallizing the amino acids and passing polarized light through them; the light waves are always rotated to the left. Yet after an organism dies, its amino acids undergo a radical change. The lefthanded molecules gradually become righthanded. Although it has long been known that the mirror-like reversal progresses steadily as time passes, a practical use for the phenomenon has now been suggested: it can be used as a geological...
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