Life uses only left-handed amino acids

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Claim

The 20 amino acids which are used by life are all the left-handed variety. This is very unlikely to have occurred by chance.

Source

Responses

  1. Firstly, this claim is based on false premises. Not only are more than 20 types of amino acids used by life, but right-handed (or 'D') ones do occur naturally, although usually less frequently than left-handed (or 'L') amino acids. First discovered in the cells walls of bacteria in the 60s, D-amino acids have since been found to be synthesized and utilized in all animals as well.
  2. This can be easily explained by the theory of evolution. If you start out with a mixture of life forms based on left-handed and life forms based on right-handed amino-acids, genetic drift will after some time lead to one of the two outliving the other. The more "left-handed life" there is, the easier it becomes for "left-handed life" to survive and the more difficult for "right-handed life". That is, equal amounts of both is an unstable situation.
  3. Since abiogenesis requires self-assembly of the first peptidoid structures and pre-proteins, the fact that left-handed amino acids are used, may well lie in the way the stack in self-organizing structures. It might be the first Darwinian selection that left handed stacks stack better to form auto-assembled peptides, whereas the right handed variety doesn't aggregate conveniently. Experiments with self-assembling supramolecular structures by DSM suggests that left-handed helical structures form more readily and stably than right handed helices.
  4. Although it could have arisen by chance quite easily (the claim ignores the actual laws of probability and how they pertain to events on Earth), it is believed today that it is not due to chance, but rather is a consequence of the laws of physics. This is borne out by the fact that meteoric amino acids also tend to be left-handed more often than right. The so-called Bonner-Rubenstein hypothesis suggests that stellar processes can lead to the separation of racemic mixtures into their enantiomers. This would make it a straight coin-flip whether any particular planet had an excess of one or the other.
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Fallacies contained in this claim

External Links

References

  1. Corrigan, J.J., 1969 "D-amino acids in animals" Science 164 (876), 11 April 1969, p142-149
  2. Sabine A. Fuchs, Ruud Berger, Leo W.J. Klomp and Tom J. de Koning, "d-Amino acids in the central nervous system in health and disease", Molecular Genetics and Metabolism, 85, (3), July 2005, p168-180.
  3. Cronin, J. R. & Pizzarello, S., 1999. "Amino acid enantomer excesses in meteorites: Origin and significance". Advances in Space Research 23(2), p293-299.
  4. Saghatelian A, Yokobayashi Y, Soltani K, Ghadiri MR. 2001. "A chiroselective peptide replicator". Nature 409 (6822), p797-801.
  5. Service, RF, 1999. "Does life's handedness come from within?" Science 286 (5443), p1282-1283.
  6. TSRI, 2001 (15 Feb.). New Study by Scientists at The Scripps Research Institute Suggests an Answer for One of the Oldest Questions in Biology. [2]

Further Reading

  • Clark S., 1999 (Jul/Aug). Polarized starlight and the handedness of Life. American Scientist 87(4): 336-343; [3] (abstract and sample illustrations only).

Related claims

See Also

Why is Creationism not a Scientific Theory?

Acknowledgments

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