Which raises a number of questions:
ANCESTRAL PROTEINS can only be inferred for modern proteins similar enough to be grouped into families, of which S100s are one example. This leaves out deeper protein relationships between families. As a rule of thumb, a pair of proteins matching at fewer than ~30% of their positions cannot be confidently aligned. This is because such levels of similarity are likely due to chance alone.26 As a result, it is only possible to scratch the surface of evolutionary history—only those proteins which diverged relatively recently remain similar enough to compare with confidence. The deepest questions about the origins of novel gene families remain shrouded in mystery.
This includes protein specificity. The ancestor of S100A5 and S100A6 may not have been more general than its descendants—but is the same true of other proteins and protein families? If so, how far back can this trend, or lack thereof, be extrapolated? Which targets were actually present in their environments? How did the primordial archetypes which gave rise to the modern protein families evolve, and had their own ancestors been more general? And, are more general proteins easier to chance upon in sequence space, that is, could they be reasonably expected to have arisen as evolutionary starting points?
To answer these questions, a lot more work just like that of Wheeler and Harms will need to be done.Chase Nelson, “Reconstructing ancestral proteins” at Inference Review
Wheeler and Harms: Lucas Wheeler and Michael Harms, “Were Ancestral Proteins Less Specific?” Molecular Biology and Evolution 38, no. 6 (2021): 2,227–39