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. 2013 Mar 13;8(3):e59300. doi: 10.1371/journal.pone.0059300

Figure 4. Early evolution of the purine metabolic network.

Figure 4

A. Origin of nucleotide metabolism ∼3.8 Gy ago; ndFF  = 0). B. Emergence of the nucleotide interconversion (INT), catabolism and salvage (CAT) and biosynthetic (BIO) pathways ∼3.5 Gy ago (ndFF  = 0.061–0.073). C. Fully connected INT, BIO and CAT pathways ∼3 Gy ago (ndFF  = 0.187). Pathways mediated by prebiotic chemistries that are plausible and most parsimonious are depicted in red and enable the growth of the emergent protein enzyme-mediated pathways of purine metabolism by structural and functional innovation and piecemeal recruitment (recruited FFs are indicated with numbers). Unknown candidate or withering prebiotic pathways are indicated with dashed lines. We note that primordial reactions of the BIO pathway (top of metabolic diagram) in B could have been non-operational in the absence of suitable prebiotic chemistries until later in evolution. FF structures associated with individual enzymatic activities (described in EC nomenclature) are painted according to their age, in a scale of node distance (ndFF) that ranges from 0 (the oldest enzymes; ∼3.8 Gy ago) to 0.2 (∼3.0 Gy ago).