|
Homochirality of the biomolecules (D-sugars of DNA and RNA and L-amino acids of proteins) is a fundamental
property of all life on Earth. Abiotic mechanisms yield racemic mixtures (D/L=1) of chiral molecules and after the death
of an organism, the enantiopure chiral biomolecules slowly racemize. Several independent investigators have now
established that the amino acids present in CI1 and CM2 carbonaceous meteorites have a moderate to strong excess of
the L-enantiomer. Stable isotope data have established that these amino acids are both indigenous and extraterrestrial.
Carbonaceous meteorites also contain many other strong chemical biomarkers including purines and pyrimidines
(nitrogen heterocycles of nucleic acids); pristine and phytane (components of the chlorophyll pigment) and
morphological biomarkers (microfossils of filamentous cyanobacteria). Energy dispersive X-ray Spectroscopy (EDS)
analysis reveals that nitrogen is below the detectability level in most of the meteorite filaments as well as in Cambrian
Trilobites and filaments of 2.7 Gya Archaean cyanobacteria from Karelia. The deficiency of nitrogen in the filaments
and the total absence of sugars, of twelve of the life-critical protein amino acids, and two of the nucleobases of DNA and
RNA provide clear and convincing evidence that these filaments are not modern biological contaminants. This paper
reviews the chiral, chemical biomarkers morphological biomarkers and microfossils in carbonaceous meteorites. This
paper reviews chiral and morphological biomarkers and discusses the missing nitrogen, sugars, protein amino acids, and
nucleobases as "bio-discriminators" that exclude modern biological contaminants as a possible explanation for the
permineralized cyanobacterial filaments found in the meteorites.
|
