Although nickel (Ni) is a minor element of the Earth's crust, it has played a major role in the evolution of life. This metal is a component of the active sites of several archaeal and bacterial anaerobic enzymes essential for bioenergy processes such as H2 and CO oxidation and CO2 fixation. Furthermore, Ni of meteoritic origin was probably involved in primordial organic phosphorylations. However, depending on its concentration, Ni can also be extremely toxic to most species. Through Earth's history this paradoxical situation has provoked complex interactions between microorganisms, such as sulfate-reducing bacteria and the highly Ni-dependent methanogens. Ni-rich volcanic emissions have resulted in alterations of the biological carbon cycle caused by high archaeal production of greenhouse CH4 gas and the ensuing global temperature elevation. These emissions are also thought to have directly helped producing the most serious of the five major extinctions at the end of the Permian period.