My understanding is that hand paintings like this fall out fairly naturally- after collecting and grinding the iron oxide for paint (there's a great but now unavailable Google+ post by Yonatan Zunger that explains why barns are red for the same reason: supernovas), you want to use some sort of mask or template, and a hand is pretty much the most available thing for early societies.
After he wrote it I pointed out most historical barns in many parts of the US are white, using whitewash (https://en.wikipedia.org/wiki/Whitewash) which doesn't really change the conclusion of the article, which itself is a masterclass in explaining the root cause of metallicity in the universe.
