Ofcourse. But I wonder wonder if the sixth finger arose in humans or if it's an expression of an previously common structure that has been suppressed for the last couple of million years. I would wager on the latter.

It's an anomaly. Look at the analogous structures in other vertebrates -- in species where digits are discernable (i.e, discounting mammals with hooves or without front limbs at all), there are practically always five. Even a structure as unusual as a bat's wing is made up of five "fingers".

Losing digits seems relatively easy - that's where hooves come from (horses have one toe left, goats two). It's gaining digits that seems very hard. It's possible that some configuration of six fingers could be better, but we probably can't get from our local optimum to that without going through major disadvantages. Apparently five has been the maximum number for most creatures for 420 million years: https://www.reddit.com/r/askscience/comments/1org2e/why_do_m...