Life builds its molecules from carbon dioxide (CO2) and breaks them back down again through the intermediacy of just five metabolites, which are the universal hubs of biochemistry(1). However, it is unclear how core biological metabolism began and why it uses the intermediates, reactions and pathways that it does. Here we describe a purely chemical reaction network promoted by ferrous iron, in which aqueous pyruvate and glyoxylate-two products of abiotic CO2 reduction(2-4)-build up 9 of the 11 intermediates of the biological Krebs (or tricarboxylic acid) cycle, including all 5 universal metabolic precursors. The intermediates simultaneously break down to CO2 in a life-like regime that resembles biological anabolism and catabolism(5). Adding hydroxylamine(6-8) and metallic iron into the system produces four biological amino acids in a manner that parallels biosynthesis. The observed network overlaps substantially with the Krebs and glyoxylate cycles(9,10), and may represent a prebiotic precursor to these core metabolic pathways.