Life on earth can be defined as the metabolic things possessing DNA as genetic information and the system to transcribe and translate DNA to protein using own ribosomes. Three domains of life, the Bacteria, Archaea (monophyly or paraphyly of Archaea is unresolved) and Eukarya, are generally accepted. Although the interrelationships among these domains are not completely resolved, the Archaea and Eukarya are considered to be related at least in the transcribe and translate system. Because all eukaryotes (probably) possess mitochondrion or its homologous organelle, the symbiotic event between common ancestor of the Eukarya and α-proteobacterium is assumed.