You're familiar with the polydactyl petroglyphs at Pueblo Bonito and other Ancestral Pueblo Great Houses, yes? They aren't that old, but technically are prehistoric, dating back to 600 to 1280 CE.

Here's two polydactyl feet petroglyphs embedded in an interior wall of Pueblo Bonita. Here's some extremely polydactyl feet petroglyphs from Ivins, Utah.

As a physician, I can only comment on the trait part of your question. Polydactyly isn't considered in medicine as a specific 'trait' but of multifactorial origin (involving mutations in genetic clusters, particularly homeobox and sonic hedgehog genes). It also occurs in other mammals, so it probably predates humans.