How primordial cells assembled biosynthetic pathways

The emergence and evolution of metabolic pathways represented a crucial step in molecular and cellular evolution allowing primitive organisms to become less-dependent on exogenous sources of organic compounds. Comparative analyses of genes and genomes from organisms belonging to Archaea, Bacteria and Eukarya revealed that, during evolution, different forces and molecular mechanisms might have driven the shaping of genomes and the arisal of new metabolic abilities. Among these gene elongations, gene and operon duplications undoubtedly played a major role since they can lead to the (immediate) appearance of new genetic material that, in turn, might undergo evolutionary divergence giving rise to new genes coding for new metabolic abilities. This work will review the main theories and mechanisms proposed to account for the appearance and refinement of extant metabolic pathways, focusing on the paradigmatic study-case represented by histidine biosynthetic route.