Chiral biomarkers and microfossils in carbonaceous meteorites

Abstract

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.

Filaments in carbonaceous meteorites: mineral crystals, modern bio-contaminants or indigenous microfossils of trichomic prokaryotes?

“…7 During the past 14 years, many large, complex filaments, consortia and mat assemblages have been found with undeniable biological characteristics that were interpreted as indigenous microfossils in carbonaceous meteorites during both independent and collaborative SEM, ESEM and FESEM investigations carried out in the United States and Russia. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Many of the filamentous and coccoidal microstructures found during this research were consistent with forms found half a century ago by earlier researchers (e.g., Nagy, Claus, Fitch, Palik, Timofeyev and van Landingham) and interpreted as indigenous microfossils of blue-green algae (cyanophytes), bacteria, and acritarchs or dismissed as recent bacterial contaminants or pollen grains. [23][24][25][26][27][28][29][30][31][32] The filaments detected at NASA/MSFC were imaged with secondary and backscattered electron detectors and analyzed by EDS to obtain spot data and 2D x-ray maps at voltages ranging from 5 keV to 15 keV.…”

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“…Therefore the detection of racemic mixtures of amino acids or other biologically produced molecules could simply indicate they are ancient rather than produced by abiotic mechanisms. Several of the chiral biomolecules found in Orgueil, Murchison and other carbonaceous meteorites exhibit an excess of the L-enantiomer that is much greater that can be produced by any known abiotic production/amplification mechanism, but consistent with ancient life processes [26].…”

Epithermal Neutron Activation Analysis of Carbonaceous Chondrites and the Polonnaruwa/Aralaganwila Stones

“…Therefore the detection of racemic mixtures of amino acids or other biologically produced molecules could simply indicate they are ancient rather than produced by abiotic mechanisms. Several of the chiral biomolecules found in Orgueil, Murchison and other carbonaceous meteorites exhibit an excess of the L-enantiomer that is much greater that can be produced by any known abiotic production/amplification mechanism, but consistent with ancient life processes [26].…”