Puckering in response to sour taste is one of three “gusto-facial” reflexes (the others are smiling at sweet tastes and frowning at bitter taste) that have been shown to exist not only in newborn humans, but to have correlates in most primate species. The mechanism for this reaction is hard-wired in at the brain stem level; we have it even before we ever experience a sour taste.
Pure speculation on my part, but I wonder if this might have served to signal to other members of a group of primates whether a food being sampled by one of the members was good to eat. See a pucker or a frown, and take a pass.