This figure is the phylogenetic tree based on 16S and 18S rRNA sequences proposed by Woese. The position of last universal common ancestor is proposed by Gogarten and Iwabe. The last universal common ancestor gCommonoteh diverged into two domains approximately 350-400 million years ago. One branch is domain Bacteria; which included Gram-positive bacteria (Bacillus), Gram-negative bacteria (Escherichia coli), cyanobacteria and so on. Another branch is further branched into two Domains: Domain Archaea and domain Eukarya: Archaea contain methanogens, halophiles, sulfur-dependent thermophiles and so on. Eukarya contain animals, plants, fungi, microsporidia and so on. Mitochondria and chloroplast, organelles in Eukarya cells, are thought to originate from Gram-negative bacteria and cyanobacteria, respectively. Optimum growth temperature of each organism are shown in the figure. In this phylogenetic tree, thermophilic organisms exist in the deepest branches of Archaea and Bacteria. Therefore, several authors proposed a thermophilic common ancestor of the extant life on the earth. We have been testing this hypothesis by experimental method.