ABSTRACT
One of the rst scientists to ponder this question, at least in print, was the biochemist Norman Horowitz. In 1945, he proposed a general scenario in which a primitive metabolic system on the early Earth initially used only those abiotically produced organic compounds that were available in its environment. As abiotic production diminished and each of those compounds became depleted, the system began to accumulate their immediate chemical precursors and then to evolve catalytic capabilities for converting those precursors into the needed compounds. In this way, Horowitz imagined that metabolic pathways evolved in reverse, from end products back to the initial precursors used in modern metabolism.
