Figure 1.

General approach for studies on the origin of life (bottom-up approach), which are carried out to elucidate what happened on the primitive Earth, where the first life emerged. Molecular evolution from simple inorganic compounds would occur in order of physical evolution (dotted arrow), chemical and biochemical evolutions (broken arrow), and biological evolution (solid arrow). The difficulty in understanding the evolutionary processes with the bottom-up approach is shown through the order of the thick arrows, from white to gray to black. The thin broken arrow represents the direction of the study with bottom-up approach.