Beating the acetyl coenzyme A-pathway to the origin of life.

Attempts to draft plausible scenarios for the origin of life have in the past mainly built upon palaeogeochemical boundary conditions while, as detailed in a companion article in this issue, frequently neglecting to comply with fundamental thermodynamic laws. Even if demands from both palaeogeochemistry and thermodynamics are respected, then a plethora of strongly differing models are still conceivable. Although we have no guarantee that life at its origin necessarily resembled biology in extant organisms, we consider that the only empirical way to deduce how life may have emerged is by taking the stance of assuming continuity of biology from its inception to the present day. Building upon this conviction, we have assessed extant types of energy and carbon metabolism for their appropriateness to conditions probably pertaining in those settings of the Hadean planet that fulfil the thermodynamic requirements for life to come into being. Wood-Ljungdahl (WL) pathways leading to acetyl CoA formation are excellent candidates for such primordial metabolism. Based on a review of our present understanding of the biochemistry and biophysics of acetogenic, methanogenic and methanotrophic pathways and on a phylogenetic analysis of involved enzymes, we propose that a variant of modern methanotrophy is more likely than traditional WL systems to date back to the origin of life. The proposed model furthermore better fits basic thermodynamic demands and palaeogeochemical conditions suggested by recent results from extant alkaline hydrothermal seeps.