It is well known that the evolution of life is affected by environmental factors, and this should be a fundamental perspective when investigating the origin of life; however, this perspective has not been fully addressed in biology. The Hadean Earth had a completely different surface environment from that of today, with no free oxygen, but instead a local environment rich in H2 which was generated by serpentinization, while energy–material circulation was driven by nuclear geysers. It is proposed that an anoxic hot-spring environment, with abundant hydrogen produced by serpentinization, was the birthplace of life. It is also proposed that the Hakuba hot spring in Nagano, Japan, is a Hadean-Earth-like environment with an H2-rich environment. A microbe found there, designated Hakuba OD1, is a member of the Candidate Phyla Radiation (CPR) bacteria group. In this review, CPR bacteria are described and their importance for the origin of life is discussed. The CPR is a bacterial supergroup consisting of dozens of phylum-level lineages of very small bacteria. This group was recently discovered with a metagenomics analysis that allowed unculturable environmental samples to be detected. Biochemical approaches to the CPR bacteria have not yet been successful because almost all the bacteria are unculturable or have not been isolated. However, with the development of massive parallel sequencing technology (next-generation sequencing), the phylogenetic characteristics of the CPR bacteria are becoming clear, and genomic analyses of these bacteria have led to unique discoveries. The sizes of the CPR bacterial genomes range from 400 to 1,500 kilobases (kb), and they contain approximately 400-1,500 genes. Thus, their genomes are remarkably small compared to other well-known and ordinary bacteria, represented by Escherichia coli, which have over 4,000 genes, but are similar to those of symbiotic or parasitic bacteria. The CPR bacterial genomes also lack many of the genes involved in essential metabolic pathways, such as the tricarboxylic acid (TCA) cycle and amino acid biosynthesis, so they seem to obtain their essential metabolites from their environments. It is proposed that this knowledge is important when considering the chemical changes that occurred on primitive Earth, which gave rise to the first forms of life through the processes of chemical evolution. Therefore, it is essential to understand the kinds of protein that are encoded in CPR bacterial genomes when studying the origin of life.
