2020 Volume 129 Issue 6 Pages 779-804
Previously proposed hypotheses on the origin of life are reviewed and it is demonstrated that none of them can provide the energy flux of ionizing radiation (UV/X/γ photons, and high-energy charged particles and neutrons) required to synthesize organic materials as demonstrated by the experiments by Miller and Urey in 1953. In order to overcome this difficulty, Ebisuzaki and Maruyama, in 2017, proposed a new hypothesis called the “Nuclear Geyser Model” of the origin of life, in which high-energy flux from a natural nuclear reactor drives chemical reactions to produce major biological molecules, such as amino acids, nucleotides, sugars, and fatty acids from raw molecules (H2O, N2, and CO2). Natural nuclear reactors were common on the surface of Hadean Earth, because the 235U/238U ratio was as high as 20%, which is much higher than the present value (0.7%), due to the shorter half-life of 235U than 238U. Ebisuzaki and Maruyama further posited that aqueous electrons and glyceraldehyde play key roles in the networks of chemical reactions in a nuclear geyser and suggested that primordial life depended on glyceraldehyde phosphate (GAP) from the nuclear geyser system as energy, carbon, and phosphate sources, pointing to a possible parallelism with the anaerobic glycolysis pathway; in particular, the lower stem path starting from GAP through Acetyl Coenzyme A to produce ATP and reduction power. It is shown that microbes (members of candidate division OD1) inhabiting high alkali hot springs, a modern analogue of the Hadean Earth environment, do not possess genes associated with conventional metabolisms, such as those of the TCA cycle, but only have genes in the lower stem path of the glycolysis. This is named the “Hadean Primordial Pathway”, because it is believed that this striking result points to a plausible origin of metabolic pathways of extant organisms. Also proposed is a step-by-step scenario of the evolution of the metabolism: 1) Chemical degradation of GAP supplied from the nuclear geyser to lactate; 2) Catalytic reactions to produce reductive power and acetyl coenzyme A (or its primitive form) and self-reproductive reactions by ribozymes on the surface of minerals (pyrite and struvite), which precipitate in a nuclear geyser (RNA world); 3) Enzymatic reactions by proteins with pyrites and the struvite in their reaction centers (RNP world); and, 4) Metabolism of extant organisms with the full assembly of enzymes produced by translating molecular machines with information stored in DNA sequences (DNA world). It is further inferred that relics of primordial metabolic evolution in the Hadean nuclear geyser can be seen at the reaction centers of enzymes of both pyrite and struvite types, nucleotide-like molecules as a cofactor of the enzymes, Calvin Cycle of photosynthesis, and chemical abundance of cytoplasm.