Source of water
- “Recently, enstatites have been in the news because a recent study has shown that they contain sufficient Hydrogen with a specific isotopic signature to have contributed to the emergence of a major fraction of the water on the Earth. This strengthens a big role for the enstatite bodies in the origin of the Earth and its water. … The authors of the said recent study admit this fact and propose that carbonaceous chondrites of the Ivuna-type could have supplied additional water after the initial formation of the Earth to result in its current isotopic signature.” MT
Source of organics
- “This came to be known as the famous Murchison meteorite. An analysis of its composition revealed that it had >10% water and \approx 2.2% carbon by weight, consistent with its smoky disintegration. Since then the organics of the Murchison meteorite have been intensely studied and the following have been detected: 1) Over 70 distinct amino acids; 2) fatty acids; 3) purines; 4) pyrimidines; 5) A complex mixture of sugars; 6) alcohols; 7) aldehydes; 8) ketones; 9) amines; 10) amides; 11) aliphatic and aromatic hydrocarbons; 12) some heterocyclic aromatics; 13) ethers; 14) organo-sulfur and organo-phosphorus compounds. Some of these are at very low concentrations like amines while fatty acids are quite abundant. The fact that these compounds show a mixture of chiralities and a distinct Carbon 13 isotopic signature showed that they had a celestial rather than an earthly biological origin. " MT
- “Given that the Murchison meteorite represents material as old or considerably older than the Earth, it established beyond doubt that interstellar organic matter was a component of the material that formed the original rocky bodies of the solar system. It also suggests that upon accretion into asteroidal bodies, the originally simple interstellar organic molecules reacted abiotically by the action or water and heat to form the entire range of more complex molecules that could serve as building blocks of life. However, given that life is characterized by homochirality of its building blocks, it also suggested at least a subset of such bodies was not transmitting life itself but only its building blocks.” MT
Emergence of homochirality
- “Murchison meteorite that cleverly avoided the effects of contamination and racemization indicated that there was a
- “earlier studies in organic chemistry which have shown that asymmetric photolysis by circularly polarized light could produce notable enantiomeric excess (e.g. the photolysis of camphor).”
- “Thus, early on, it had been proposed that pulsars emitting circularly polarized synchrotron radiation could have caused the initial enantiomeric anisotropy. But it has been pointed out that such radiation could break up amino acids. Others have hence suggested circularly polarized light scattered from dusty regions in the Milky Way could provide the appropriate light for such reactions. Such light has been directly observed in the reflection nebulae of the OMC-1 star-forming cloud in the Orion region, which is rich in organic compounds.”
Panspermia
MT’s summary
- Source MT
- “In our youth, when we first read of the Murchison meteorite, we believed the majority view that meteors such as this had delivered the building blocks of life to the early Earth and it was followed by local pre-biotic evolution resulting in life. However, as our understanding of biology improved we increasingly started moving towards panspermia though cellular life on Earth had a single common ancestor.”
- “First, the archaeo-bacterial split implied a certain barrier to lateral gene transfer that had since then broken down on Earth. This could be most easily explained by two seeding events, one which brought the bacteria and the other which brought the archaea.”
- “Second, consistent with the above, there are many protein divergences among housekeeping functions that imply divergence time that likely greater than the age of the Earth, even assuming early acceleration in virus-like primitive replicators.”
- “Third, genomic analysis strongly favors a hetero-trophic(=ingesting food rather than manufacturing) ancestral organism. Abundant food sources that would have allowed such organisms to get started before autotrophy evolved are not likely to have existed on the early Earth. Being in the hot inner Solar System it is less likely to have had materials like tholins that would have fed the early heterotrophs. Hence, we again see hints that life emerged elsewhere in a tholin-rich region and secondarily reached Earth. "
- Homochirality is observed in life.
- “In any case, the photochemical enantiomer selection remains the most likely possibility, and those conditions are not found on Earth.”
- “Now one could still argue that after this excess was established such an enantiomerically biased mixture was delivered to Earth by meteorites and that provided the building blocks for life on Earth. However, this does not take into account the racemization and re-equilibration of enantiomers on Earth post-landing.”