Evolution of modern respiration

The crystal structure of the catalytic core of a membrane hydrogenase reveals how the quinone reduction site of the ubiquitous Complex I (NADH quinone oxidoreductase) evolved from a hydrogen gas-evolving ancestor.  The use of high potential quinone, rather than low potential hydrogen-gas forming protons, as the electron acceptor of reductant provided by ferredoxin is proposed as the evolutionary driving force.  This major accomplishment would have propelled the diversification of life whereby the quinone functions as the electron donor to a subsequent respiratory step, accomplished through evolution of other respiratory membrane oxidoreductases that oxidize nitrate, polysulfide or arsenate and eventually oxygen.