Did respiration or photosynthesis come first?

The similarity of the mechanisms in photosynthetic and in oxidative phosphorylation suggests a common origin (conversion hypothesis). It is proposed that an early form of electron flow with oxidative phosphorylation (“prerespiration”), to therminal electron acceptors available in a reducing biosphere, was supplemented by a photocatalyst capable of a redox reaction. In this way, cyclic photophosphorylation arose. Further stages in evolution were reverse electron flow, powered by ATP, to make NADH as a reductant for CO₂, and subsequently noncyclic electron flow. These processes concomitantly provided the oxidants indispensable for full development of oxidative phosphorylation, i.e. for normal respiration: sulphate, O₂, and, with participation of the nitrificants, nitrite and nitrate. Thus prerespiration preceded photosynthesis, and this preceded respiration. It is also suggested that nonredox photoprocesses of the Halobacterium type are not part of the mainstream of bioenergetic evolution. They do not lead to photoprocesses with electron flow.