Mark I. Vuletic

Last updated 20 July 2008

Background

Some molecules have their constituent atoms arranged so that their mirror images can be superimposed back upon the originals. Others are not: such molecules come in two different forms (enantiomers), called left-handed (L) and right-handed (D). When such molecules are produced by a random process, the L-enantiomers and D-enantiomers are produced in roughly equal proportions, resulting in what is known as a racemic mixture.

Of the twenty amino acids produced by living creatures, nineteen have a left-handed and right-handed form. However, only the L-enantiomer of these nineteen are utilized by living creatures. Creationists argue that if life arose without the direct intervention of a god, then it must have arisen from a random process, which they in turn interpret to mean that the important molecules in life should be approximately 50% left-handed and 50% right-handed. The discrepancy between this prediction and observation is known as the problem of chirality.

Analysis

That some creationists (e.g. McCombs 2004) go so far as to claim that scientists do not recognize or want to discuss the issue of chirality ("chirality" is the technical word for "handedness"), must make one wonder whether they keep track of scientific literature at all, since the issue has been under active research for considerable time.

(i) According to chemist Andre Brack, there are two classes of proposals for how the left-handedness of amino acids in organisms is to be explained: "those which call for a chance mechanism and those which call for a determinate mechanism resulting from an asymmetrical environment originating from the universe or from the Earth." (Brack 1998:5).

Chance proposals call for a random fluctuation in small samples of molecules, which is amplified until the basic chemical processes come to favor one enantiomer over the other. Brack describes one example:

In a rather simple kinetic model proposed by Franck, an open flow reactor, run in far-from equilibrium conditions, is fed by achiral compounds and form two enantiomers reversibly and autocatalytically. If the two enantiomers can react to form an irreversible combination flowing out of the reactor, by precipitation for instance, and if certain conditions of the fluxes and concentrations are reached, the racemic production may become metastable and the system may switch permanently toward the production of either one or the other enantiomer, depending on a small excess in one enantiomer (Brack 1998:5-6)

Proposals for a determinate mechanism include parity nonconservation (though Brack thinks the effect is too weak), asymmetry in the weak force (also thought by many to have too small an effect; but see Service 2000), circularly polarized light acting on the Earth's surface (which Brack discounts), and circularly polarized synchrotron radiation from neutron stars acting on interstellar clouds (which Brack thinks is plausible).

The proposals involving determinate mechanisms typically do not propose that these mechanisms are themselves sufficient to establish one enantiomer over the other; rather, they propose that determinate mechanisms create small excesses of one enantiomer, which are then (just as in the random fluctuation models) amplified into dominance by normal chemical processes.

(ii) The weak force proposal not only predicts left-handed amino acids, but right-handed sugars, so it is suggestive that the two sugar components of nucleotides are in fact right-handed.

(iii) The molecules on the Murchison meteorite generally occur more often in right-handed form (Cronin 1998:135); while this does not, of course, explain why life on Earth uses left-handed amino acids (while earlier studies revealed an excess of left-handed amino acids on the Murchison meteorite, these findings appear to be in dispute, so the meteorite does not yet indicate an unequivocal solution to the problem), it also demonstrates that nature does not prefer equal proportions of both enantiomers for all molecules in all inorganic situations.

References

Brack A (ed.). 1998. The Molecular Origins of Life: Assembling Pieces of the Puzzle. Cambridge: Cambridge University Press.

Cronin JR. 1988. Clues from the origin of the solar system: meteorites. pp. 119-146 in Brack 1998.

McCombs C. 2004. Evolution hopes you don't know chemistry: the problem with chirality. Impact 371.