Claim CB040:
The twenty amino acids used by life are all the left-handed variety. This
is very unlikely to have occurred by chance.
Source:
Watchtower Bible and Tract Society. 1985. Life--How Did It Get Here?
Brooklyn, NY, pg. 43
Response:
- The amino acids that are used in life, like most other aspects of
living things, are very likely not the product of chance. Instead, they
likely resulted from a selection process. A simple peptide replicator
can amplify the proportion of a single handedness in an initially
random mixture of left- and right-handed fragments (Saghatelian et al.
2001; TSRI 2001). Self-assemblies on two-dimensional surfaces can also
amplify a single handedness (Zepik et al. 2002). Serine forms stable
clusters of a single handedness which can select other amino acids of
like handedness by subtituting them for serine; these clusters also
incorporate other biologically important molecules such as
glyceraldehyde, glucose, and phosphoric acid (Takats et al. 2003). An
excess of handedness in one kind of amino acid catalyzes the handedness
of other organic products, such as threose, which may have figured
prominently in proto-life (Pizzarello and Weber 2004).
- Amino acids found in meteorites from space, which must have formed
abiotically, also show significantly more of the left-handed variety,
perhaps from circularly polarized UV light in the early solar system
(Engel and Macko 1997; Cronin and Pizzarello 1999). The weak nuclear
force, responsible for beta decay, produces only electrons with
left-handed spin, and chemicals exposed to these electrons are far more
likely to form left-handed crystals (Service 1999). Such mechanisms
might also have been responsible for the prevalence of left-handed
amino acids on earth.
- The first self-replicator may have had eight or fewer types of amino
acids (Cavalier-Smith 2001). It is not all that unlikely that the same
handedness might occur so few times by chance, especially if one of the
amino acids was glycine, which has no handedness.
- Some bacteria use right-handed amino acids, too (McCarthy et al. 1998).
Links:
Jacoby, Mitch. 2003. Serine flavors the primordial soup. Chemical and
Engineering News 81(32): 5.
http://pubs.acs.org/cen/topstory/8132/8132notw1.html
References:
- Cavalier-Smith T. 2001. Obcells as proto-organisms: membrane
heredity, lithophosphorylation, and the origins of the genetic code,
the first cells, and photosynthesis. Journal of Molecular Evolution
53: 555-595.
- Cronin, J. R. and S. Pizzarello. 1999. Amino acid enantiomer excesses
in meteorites: Origin and significance. Advances in Space Research
23(2): 293-299.
- Engel, M. H. and S. A. Macko. 1997. Isotopic evidence for
extraterrestrial non-racemic amino acids in the Murchison meteorite.
Nature 389: 265-268. See also: Chyba, C. R., 1997. A left-handed
Solar System? Nature 389: 234-235.
- McCarthy, Matthew D., John I. Hedges and Ronald Benner. 1998. Major
bacterial contribution to marine dissolved organic nitrogen.
Science
281: 231-234.
- Pizzarello, S. and A. L. Weber. 2004. Prebiotic amino acids as
asymmetric catalysts. Science 303: 1151.
- Saghatelian, A., Y. Yokobayashi, K. Soltani and M. R. Ghadiri. 2001. A
chiroselective peptide replicator. Nature 409: 797-801.
- Service, R. F. 1999. Does life's handedness come from within?
Science 286: 1282-1283.
- Takats, Zoltan, Sergio C. Nanita and R. Graham Cooks. 2003. Serine
octamer reactions: indicators of prebiotic relevance. Angewandte
Chemie International Edition 42: 3521-3523.
- TSRI. 2001 (15 Feb.). New study by scientists at the Scripps Research
Institute suggests an answer for one of the oldest questions in
biology. http://www.scripps.edu/news/press/021401.html
- Zepik, H. et al. 2002. Chiral amplification of oligopeptides in
two-dimensional crystalline self-assemblies on water. Science 295:
1266-1269.
Further Reading:
Clark, Stuart. 1999. Polarized starlight and the handedness of life.
American Scientist 87(4) (Jul/Aug): 336-343.
Guterman, Lila. 1998. Why life on Earth leans to the left. New
Scientist, 160(2164): 16.
created 2001-3-31, modified 2004-11-16