**1. Introduction**

The origin of life has long been interested to human since old times. Indeed, Aristotle proposed "spontaneous generation" more than 2000 years ago, although this idea was disproved by Louis Pasteur in experiments using "swan neck flasks." Our great interest in the origin of life might be expressed by the following philosophical words: *Where do we come from? What are we? Where are we going?* These words were written by French artist Paul Gauguin on his canvas in Tahiti in 1897.

The development of nucleotide sequencing technology [1, 2] has contributed to progress in molecular biology, including the analysis of a complete bacterial genome first carried out in 1995 [3], and, subsequently, the draft human genome, which was reported in 2001 [4, 5]. At present (June 19, 2019), 498 eukaryote, 5159 bacterial, and 296 archaeal complete genomes were determined. However, the origin of life is still unclear. Assuming that the replacement rates of nucleotides or amino acids in genes are constant [6], phylogenetic trees were drawn [6–11]. However, we know that their exact replacement rates differ between genes and between species. Studies based on nucleotide or amino acid sequences are applicable to genes

whose nucleotide or amino acid numbers are much smaller than those of complete genomes, but not to genomes consisting of huge numbers of nucleotides and many genes. Of course, simple comparison of sequence differences between genes in the same species and the same genes in different species is useful.
