Before revision:  We find that the core of both backbones are mainly composed of reactions in ancient pathways, meaning that those reactions still remain at present significant for biomass production.
After revision:  We find that the core of both backbones are mainly composed of reactions in ancient pathways, meaning that those reactions still retain at present the central role in the evolved metabolism. In E. coli, the analysis of the core reveals a dominant direction with the synthesis of purines and pyrimidines and the metabolism of lipids ensuing after energy metabolism.