"As the metabolic complexity of protobiological systems increased, so
did the functional diversity of macromolecules carrying out metabolic
reactions. Novel catalysts might have emerged from random sequences, as
discussed above, but other, more efficient mechanisms might have been
at play. In particular, evolutionary flexibility may have allowed those
early proteins to improve their catalytic efficiency, or to alter their
substrate specificity in response to a small number of mutations. This
mechanism has been demonstrated for a number of highly evolved enzymes
from contemporary cells (Aharoni et al., 2005, Yoshikuni et al., 2006,
Khersonsky et al., 2006). We propose that the same mechanism applied to
primitive functional proteins, thus providing a powerful fitness criterion.
Proteins lacking evolutionary flexibility would not have been very useful
during natural selection."

— Andrew Pohorille et al., "Origins of Functional Proteins and the Early
Evolution of Metabolism."