Chirality – What is the Murchinson meteorite?
In 1969, fragments of a meteorite landed 2 kilometers south of the small village of Murchison, Australia. Research was undertaken in view of determining the presence of organic molecules. One of the first discoveries was amino acids, the components of proteins. Later, there were dubious claims that some of the amino acids had a slight excess of the correct homochirality required for life.
A team led by Zita Martins of Imperial College, London and Leiden Institute of Chemistry, Netherlands, reported that they had found two nucleobases in this meteorite. If true, this would be of fundamental importance because these are the letters of the genetic code. Specifically, the bases found were uracil and xanthine. Uracil is found in RNA and pairs with adenine. Xanthine does not form part of the genetic code but is found in living organisms from the breakdown of guanine, which is a base in both DNA and RNA.
Many scientists, however, are rightly skeptical of the claim that these nucleobases formed beyond earth, and contend rather that contamination is a real possibility.
Supposing that the nucleobases found on the Murchinson meteorite were indeed from extraterrestrial sources, would this solve the problem of the origin of life? It seems unlikely. To form DNA and RNA requires far more than a random collection of nucleobases. The nucleobases must be purified from other closely related compounds. They must combine with ribose to form a nucleoside (this reaction cannot occur in water, and even in a dry environment, pyrmidine nucleosides are extremely difficult to form). They must then combine with phosphate to form a nucleotide. These nucleotides then have to be activated, purified and concentrated so that they will combine to form polymers.
As Jonathan Sarfati rightly concludes regarding the Murchinson meteorite: “Rather than proof of chemical evolution, the observations are evidence against it. This is because we never see them progress any further -- they are chemical dead ends.”
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