 Depletion of such organic molecules by the early reproducing life forms could have continued until a point was reached where the supply of one of these molecules limited further multiplication.By a process of mutation and natural selection, these life forms then evolved a catalytic activity for utilizing another related available molecule by catalyzing its conversion to the previously limiting molecule.When this precursor molecule was in turn eventually depleted, further mutations allowed the conversion of another available substrate (a pre- precursor) into the precursor.