It's not that crappy given that newborns can not only recognize faces with significant accuracy, but also recognize facial expressions.

Do you have a link to that? 'Newborn' can mean many things - the visual system starts learning from the second the eyes open, and perhaps even before that through pattern generators projected onto the retina which help to 'pretrain' the viscortex.

I know that infants have initial face detectors from the second they open their eyes, but from what I remember reading - they are pretty crappy indeed, and initially can't tell a human face apart from a simple cartoon with 3 blobs for eyes and mouth.

It seems more likely that there is a single face recognition module which is genetically specified and then it becomes fine tuned by learning.

Except that it isn't that simple, because - amongst other evidence - congenitally blind people still learn a model and recognizer for attractive people, and can discern someone's relative beauty by scanning faces with their fingertips.

Even without a concept of "even number", wouldn't this neolithic human be able to figure out an algorithm to compute the right answer?

Not sure - we are getting into hypothetical scenarios here. Your visual version, with black and white pebbles laid out in a line, implicitly helps simplify the problem and may guide the priors in the right way. I am reasonably sure that this setup would also help any brain-like AGI.