Free energy conversion in the LUCA: Quo vadis?

Living entities are unimaginable without means to harvest free energy from the environment, that is, without bioenergetics. The quest to understand the bioenergetic ways of early life therefore is one of the crucial elements to understand the emergence of life on our planet. Over the last few years, several mutually exclusive scenarios for primordial bioenergetics have been put forward, all of which are based on some sort of empirical observation, a remarkable step forward from the previous, essentially untestable, ab initio models. We here try to present and compare these scenarios while at the same time discuss their respective empirical weaknesses. The goal of this article is to harness crucial new expertise from the entire field by stimulating a larger part of the bioenergetics community to become involved in "origin-of-energy-metabolism" research. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.