Defender's Guide to Science and Creationism

Assertion: UV light would have destroyed important prebiotic compounds.

Background

Direct exposure to unattenuated ultraviolet light destroys amino acids and nucleic acid bases. Some creationists argue that this rules out abiogenesis altogether, since the early Earth did not have an ozone layer to screen out UV rays. Other creationists are more restrained, arguing that it rules out a contribution to abiogenesis by prebiotic synthesis in certain specific contexts, such as in the early Earth's atmosphere or in outer space.

Analysis

(i) If abiogenesis occurred near deep-sea hydrothermal vents, as one line of origin of life research proposes, it would have occurred at depths where UV light does not penetrate.

(ii) According to Cleaves and Miller (1998) there are a number of things that could have protected prebiotic compounds in the early oceans at medium or shallow depths:

• A UV-scattering oceanic ice layer, expected in the absence of greenhouse gases.
• UV-absorbing and -scattering oil slicks that may have been formed by the photopolymerization of methane or by the accumulation of hydrocarbons carried by meteorites.
• A UV-scattering layer of foam that may have been generated by UV light acting on compounds like pyrene and hexadecene.
• The UV-opacity of sea water itself in the 200-220 nanometer range, which would have protected aliphatic amino acids (but not aromatic amino acids or nucleic acid bases, which need protection in the 260 nanometer range).
• An adequate concentration of some of the organic compounds produced by prebiotic synthesis, including (but not limited to) the junk polymers generated in spark discharge experiments.
• An adequate concentration of Fe²⁺ or SH^-, one of which would have been in excess in the early oceans.

Cleaves and Miller (1998) conclude:

This analysis shows that there would have been a wide variety of both organic and inorganic UV absorbers in the primitive ocean that would have protected the UV-sensitive organic compounds such as the purines and pyrimidines, with amino acids being protected effectively by aqueous salts. Our models suggest that it is entirely possible that incident UV flux could have been attenuated to minimal intensities after passage through as little as 2 mm of ocean water (<1% T at 218 nm and 260 nm).

We recognize that any one of these concentration estimates may be in considerable error. However, entirely removing any one of the absorbers still leaves a variety of alternative absorbers that still would offer adequate protection.

(iii) UV radiation does not rule out a contribution to abiogenesis even from prebiotic synthesis in space; we know this because important prebiotic compounds have been detected in space. For instance, one team has detected the amino acid glycine in interstellar clouds (Kuan 2003).

References

Cleaves HJ and Miller SL. 1998. Oceanic protection of prebiotic organic compounds from UV radiation. Proc. Natl. Acad. Sci. USA 95:7260-7263.

Kuan Y-J et al. 2003. Interstellar glycine. The Astrophysical Journal 593:848-867.

Defender's Guide to Science and Creationism
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