What preceded ATP synthase? ATP Synthase is ubiquitous throughout life on earth and so most probably evolved within the last universal common ancestor (LUCA) before that lineage diversified into the various kingdoms of life. It is suggested that the functional domains comprising the ATP Synthase complex were cobbled together from pre-existing sub-units, each with with a related activity (e.g. the union of a proton motor with a DNA Helicase with ATPase activity), which seems reasonable, although a bit vague in the details. The fact that a primitive ATP Synthase arose in the first place suggests that ATP already had a central role in cellular metabolism and so begs the question, what preceded ATP Synthase in the role of generating ATP in the LUCA cell?

There are metabolic processes in which ATP is synthesised without the involvement of ATP synthase. The best examples are, in fact, two steps in the glycolytic pathway, catalysed by phosphoglycerate kinase and pyruvate kinase. This is why, in the absence of any aerobic metabolism, many organisms (like yeast for example) can grow quite happily, producing two molecules of ATP for each molecule of glucose that is metabolised.

Presumably this process is what enabled ATP to gain its central role in energy metabolism without any need for an ATP synthase.