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The issues we attempt to tackle here are what the first peptides did looklike when they emerged on the primitive earth, and what simple catalyticactivities they fulfilled. We conjecture that the early functional peptideswere short (3 to 8 amino acids long), were made of those amino acids, Gly, Ala,Val and Asp, that are abundantly produced in many prebiotic synthesisexperiments and observed in meteorites, and that the neutralization of Asp'snegative charge is achieved by metal ions. We further assume that some tracesof these prebiotic peptides still exist, in the form of active sites inpresent-day proteins. Searching these proteins for prebiotic peptide candidatesled us to identify three main classes of motifs, bound mainly to Mg^{2+} ions:D(F/Y)DGD corresponding to the active site in RNA polymerases, DGD(G/A)Dpresent in some kinds of mutases, and DAKVGDGD in dihydroxyacetone kinase. Allthree motifs contain a DGD submotif, which is suggested to be the commonancestor of all active peptides. Moreover, all three manipulate phosphategroups, which was probably a very important biological function in the veryfirst stages of life. The statistical significance of our results is supportedby the frequency of these motifs in today's proteins, which is three timeshigher than expected by chance, with a P-value of 3 10^{-2}. The implicationsof our findings in the context of the appearance of life and the possibility ofan experimental validation are discussed.

The first peptides: the evolutionary transition between prebiotic amino acids and early proteins