Defender's Guide to Science and Creationism

Mark Vuletic

Assertion

Ultraviolet 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 a natural origin of life altogether, since the early Earth did not have an ozone layer to screen out ultraviolet light. Other creationists are more restrained, arguing that it rules out only a contribution to the origin of life by prebiotic synthesis in certain specific contexts.

Analysis

I. Deep water

If the origin of life occurred near deep-sea hydrothermal vents, as one hypothesis proposes, it would have occurred at depths where ultraviolet light does not penetrate.

II. Medium-depth to shallow water

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

  • An oceanic ice layer, expected in the absence of greenhouse gases, would have scattered ultraviolet light.
  • Oil slicks that would have absorbed or scattered ultraviolet light may have been formed by the photopolymerization of methane or by the accumulation of hydrocarbons carried by meteorites.
  • A layer of foam that would have scattered ultraviolet light may have been generated by that very same light acting on compounds like pyrene and hexadecene.
  • Sea water itself is opaque to ultraviolet light 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, would have protected against ultraviolet light.
  • An adequate concentration of Fe2+ or SH-, one of which would have been in excess in the early oceans, would have protected against ultraviolet light.

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. Space

Ultraviolet radiation does not even rule out a role for 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.

Last update: 15 Jan 2015

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