HANDEDNESS
Organic molecules are commonly built around a carbon atom attached to four other atoms or groups of atoms. When at least two of these atoms (or groups of atoms) are the same, the structure is mirror-symmetric; its mirror image is itself, only rotated. When all four are different, however, the molecule lacks this symmetry; it comes in two distinct varieties, like your hands. In general these varieties behave the same chemically, yet life on our planet uses only one of them. That, in itself, is not mysterious: shapes are crucial in biochemical reactions and providing for both mirror images might have been too complicated.

Yet why did life choose the variety it did? Some think it was mere chance, but others think the choice is a clue to the origin of life. Perhaps crystals, which can separate the two varieties, provided a scaffolding for protolife, or perhaps the building blocks of life came from outer space, where polarized starlight had sifted out the varieties. —George Musser

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Excerpted from: HANDEDNESS. August 21, 2009