Figure 3: Systematic exploration of prebiotic scenarios reveals a core organo-sulfur network.

(a) A thermodynamically constrained network expansion algorithm was used to simulate the early expansion of proto-metabolism under various scenarios, including the availability of reductants in the environment, pH, temperature, and the availability of fixed carbon sources and thiols. The proportion of molecules selected KEGG modules involved carbon metabolism are plotted as a heatmap to the right of the parameters. (b) A representation of the core network producible from a prebiotically plausible seed set without nitrogen or phosphate (bottom left box). Acetyl-thioesters are first produced, potentially from a primitive Wood-Ljungdahl pathway^8,19 from acetate and thiols provided as seed molecules (green). Acetyl-thioesters enable the production of all intermediates in the reductive tricarboxylic acid (rTCA) cycle, with the exception of phosphoenolpyruvate. ATP-dependent reactions in the rTCA cycle may have been substituted with a primitive malate synthase and transthioesterification of succinate as well as the recently discovered reversible citrate synthase ^66,67. The keto acid precursors for 8 common amino acids (A,D,E,G,I,L,S,V) are highlighted in purple, while routes to thioester-mediated polymerization of fatty acids and polyketides are highlighted in pink.