The living organisms are divided into two groups, eukaryotes and prokaryotes. Eukaryotes contain animals, plants, molds and protozoans. Prokaryotes are separated into Bacteria and Archaea. Bacteria include Escherichia, Bacilli and Lactobacilli. Prokaryotic cells are small in size (1 um) and do not have intracellular membranous structure. However, eukaryotic cells are big in size (10~100 um) and have organelles (mitochondria and chloroplasts) and intracellular membranes (endoplasmic reticula, golgi body, etc.). Cell symbiosis theory presented by Margulis suggests that aerobic bacteria became mitochondria by intracellular symbiosis, and cyanobacteria became chloroplasts. The host cell of symbiosis may share the ancestor with archaea. However, the ancestral archaeon and its characteristics are not clear yet. Furthermore, the origins of eukaryote specific components, such as cytoskeletons and steroid synthetic pathway, are not clear yet.