The Panspermia Hypothesis - A Discussion Of Molecular Evidence

Analysis of the chirality of purines and amino acids found in carbonaceous chondrites correlates with that seen in those in nature. The large asymmetry in isovaline and other ?-dialkyl amino acids found in altered CI and CM meteorites, may account for the ubiquitous left-handedness of the amino acids found in organisms today, as the amino acids delivered by asteroids, comets, and their fragments may have biased the Earth's prebiotic organic inventory with left-handed molecules before the origin of life. If this can account for the left-handed homochirality of biotic amino acids, it is possible the right-handed homochirality of our genetic material was influenced similarly. The mechanism by which the initial enantiomeric excesses were imparted upon the organic molecules has still yet to be ascertained. Bailey et al, 1998 propose UV circularly polarized light, similar to the IR circularly polarized light formed by dust scattering in reflection nebulae in the Orion OMC-1 star-formation region, may have imparted the Lee excesses to the amino acids found on the Murchison Meteorite. The results of Callahan et al, 2011 however were inconsistent with this theory and instead indicate that amplification of a small initial isovaline asymmetry in Murchison.

The panspermia hypotheses which support an exogenous supply of life to the early Earth currently suffer from a lack of primary evidence and are somewhat speculatory, thus perhaps should be disregarded for the time being. In contrast there is extensive evidence for a great wealth of prebiotic, organic material found in cometary dust and on meteorites found on Earth. This supports the idea of a 'prebiotic panspermia' which catalysed the evolution of life on the 'Goldilocks planet' Earth and may even account for the homochirality of our amino acids and nucleobases. However, like all the endogenous synthesis hypotheses, the panspermia hypotheses do not address the evolutionary 'gap' between prebiotic organic molecules and biotic, self-replicating material.