What aspect of his evolutionary past was creeping up?
Arguably none, there is no reason why a mutation has to be an atavism (reversion to an ancestral state). Mutations occur producing embryos with 2 heads, that doesn't mean 2 headed organisms were part of those embryos' evolutionary lineage.
Did our ape ancestors have extra digits too?
The answer depends on what question exactly you are asking. If you mean did ancestral species of great apes commonly posess extra digits the answer is no. If you mean did our ape ancestors also suffer from the sporadic appearance of supernumerary digits, the answer is almost certainly yes.
If you go further back in the evolutionary history of tetrapods though you find many examples of primitive limbs with many more digits than modern tetrapods, principal ones being
Acanthostega and
Icthyostega with 8 and 7 digits respectively.
Considering the additional ossification and the webbed fingers the most likely developmental pathway to have been affected is probably BMP (Bone morphogenetic protein) which is associated with bone growth, obviously, and also with the regulation of apoptosis in interdigital regions which is responsible for removing the tissue that is present there in the early embryo. As for frog ancestors, certainly an amphibian ancestor is on the cards, again something along the lines of
Acanthostega.
TTFN,
WK
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