Chirality

Life is left-handed: all but one amino acids could be a right-handed or a left-handed molecule, yet in living organisms, all amino acids are left-handed. Many other biochemically vital molecules are "handed" too: DNA is right-handed, and so are all the sugars we can use. Why? and when did this "chirality" begin?

Well, obviously, the origin of chirality is linked with the origin of life as we know it, so that same sorts of problems arise. There are multiple suggestions, set in multiple environments, yet everyone who worries about chirality has to admit that it might be a cosmic accident. To work properly, all life had to be right-handed or left-handed, and perhaps it didn't matter which to begin with. Once life had evolved as left-handed, then it was vital that the chirality be maintained. Biochemistry would have been lethally crippled in any right-handed mutants.

There are several theories for the origin of chirality, none of them obviously superior to the others. The simplest story is that the origin was random, but self-perpetuating and irreversible.

My only thought on the topic would be that since the important consequences of chirality for living things are biochemical, the origin of chirality occurred with the origin of life, for biochemical reasons. I prefer the simplest suggestion, that it was a cosmic accident. Next comes the mineral-surface version, which also calls for a cosmic accident, but which has the added requirement that life first began on a "handed" mineral surface.

Here is a review of the arguments from Science News, August 2007

Last updated August 18, 2007.

Links checked August 18, 2007.

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