Wired for Life Syntrophic bacteria live on the metabolic by-products of a partner species. The exchange of the by-products accompanies a flow of electrons in the opposite direction that helps some species grow in conditions that would otherwise be unfavorable. In mixed anaerobic cultures of two related Geobacter species, Summers et al. (p. 1413 ) observed that one species evolved to promote the transfer of electrons directly to the other, in large aggregated cell clusters, without coupling to common anaerobic by-products such as hydrogen or formate. Selection pressures in nine parallel populations all resulted in a point mutation that truncated a protein involved in the production of small hairlike projections involved in intercellular communication—pili—and indirectly increased the expression of a c -type multiheme cytochrome responsible for extracellular electron transfer. The evolved aggregates were conductive, suggesting that the direct exchange of electrons between partner species is a possible alternative route to anaerobic syntrophy rather than interspecies hydrogen transfer; indeed, deleting a gene that encodes a hydrogenase involved in hydrogen transfer conferred a growth advantage in the cocultures.