Although living organisms have a symmetrical appearance at the macroscopic level, biological systems are composed of typical asymmetrical molecules: nucleic acids (RNA and DNA) have ribose with D-configuration while proteins have alpha-carbons with L-configuration. However, the origin of biomolecular homochirality is still unknown. Proteins are synthesized on the ribosome by the elongation of L-amino acids that are attached to tRNAs. Therefore, aminoacylation of tRNA could be the key step in the origin of amino acid homochirality. With this in mind, we attempted non-enzymatic aminoacylation of an RNA minihelix (primordial tRNA) with an aminoacyl-phosphate-D-oligonucleotide, which revealed chiral-selective aminoacylation of the RNA minihelix with a clear preference for L-amino acids. A mirror-image RNA system with L-ribose exhibited aminoacylation with the preference for D-amino acids. These results suggest that the stereochemistry of RNA could be the determinant of chiral-selectivity of amino acids. The D-ribose-based “RNA world” was probably established by chiral-selective ligation of oligonucleotides, which would have generated a “winner” sequence with an important chemical ability for evolution of life.
One of the leading hypotheses proposes asymmetric photolysis induced by circularly polarized light in space was triggered the origin of homochirality. Asymmetric photolysis induced by circularly polarized ultraviolet has been well-studied. On the other hand, contribution of circularly polarized soft X-ray has not been examined well. Herein, I briefly review an estimation of enantiomeric excess induced by circularly polarized soft X-ray (photon energy = 532.7 eV) calculated by using absorption spectrum, circular dichroism spectrum and Kagan’s equation.
Anciet submarine hydrothermal environments have been considered as important locations where the first life flourished. On the other hand, many experimental results postulate if submarine hydrothermal environments were suitable for the chemical evolution for origin of life. Geological records of Archean submarine hydrothermal ore deposits are present allover the world. Those ore deposits often accompany with carbonaceous sediments, suggesting high microbial activities around the discharging hydrothermal fluids. Such carbonaceous sediments may provide an opportunity to examine evolution of early life in submarine hydrothermal environments. Here I present the evidence of high microbial activities at ca. 3.2-billion-years-old submarine hydrothermal fields of Sulfur Springs in Australia. It is found that examine samples associated with the ore deposit were rich in organic carbon. Relatively high Mo concentrations were found in those organic matter. Such Mo behavior implies direct microbial mediation of bio-essential elements from submarine hydrothermal fluids. Microscopic occurrence of organic carbon is often closely associated with sulfide minerals forming onion- or stromatolite-like structures. Such structures imply biologically induced sulfide mineralization in submarine hydrothermal environments. Stable isotope analyses indicate high activities of methanogens and methanotrophs with sulfate reducers only around the hydrothermal discharging zone. Those results indicate that ancient submarine hydrothermal fields were important for early ecosystem not only for metabolic energy usage but also uptake of bio-essential elements. However, the total energy flux from interior of the early Earth is known to be small. Such small energy flux constraints the extents of submarine hydrothermal activities on the early ocean floor and further suggests that biomass around the ancient submarine hydrothermal environments was also small. Therefore it is still an open question as to if ancient submarine hydrothermal environments were major geological fields for evolution of early life.
We have been focusing on the chemical and physical environments in the vicinity of hydrothermal vents in the primitive ocean with regard to the chemical evolutions of life. We used a flow reactor that was constructed for simulating the pressure and temperature conditions of the hydrothermal vents. In the flow reactor, a high-temperature high-pressure fluid at 200 ºC, 24 MPa was injected into a low temperature (0 ºC, 24 MPa). Temperature gradient should exist at the interface between high- and low-temperature fluids in the low-temperature chamber. Identification of the oligomeric products was made with the aid of an HPLC analysis.
The yield of diglycine was adopted as an index for the capacity of oligomerization. The amount of oligomerization was found to depend on the quenching rate of the temperature. Furthermore, the rate was enhanced by the presence of proteinoid microspheres made from five kinds of amino acids. These results suggest that both chemical and physical environments at none-equilibrium states should have a powerful effects on the prebiotic oligomerizations of amino acids during chemical evolutions of life on the primitive Earth.
The hydrothermal origin-of-life hypothesis has been experimentally verified using the hydrothermal flow reactor systems in our group. In this review, the reason why we have examined RNA and proteins from the viewpoint of the hydrothermal origin-of-life and the process for development of the hydrothermal reactors is briefly described. Based on these investigations, the importance of the stability and prebiotic formation of biopolymers under hydrothermal environments on primitive earth, and the interactions and solubility of prebiotic biomolecules will be addressed. Furthermore, the studies led to another requisites for a primitive life-like system as a living system as well as the stability, formation, interaction, and solubility of prebiotic molecules. The stability of the primitive life-like system, which is one of the requisites, would have been the base for continuous chemical evolutions towards a higher level of the system.
Marine hydrothermal fields are thought to be an environment where microorganisms thrived in early Earth. However, the physiological characteristics of these microorganisms are unknown. The study of microorganisms in the present hydrothermal fields will provide clues to elucidate the ancient ecosystem. For a better understanding of the ancient ecosystem, it is important to know the relationship between geological, geochemical and microbiological diversities of the present marine hydrothermal systems. Microbiological breakthroughs for both culture-dependent and -independent methods are needed to reveal the physiology of the microorganisms living in the present marine hydrothermal fields.