We live in a more complex world, this produce someting more complex, culture, ideas, knowledge, tasks, technology, craps.. but basic intelligence i think is the same (if not better in the past, less "pollution")
When I first read it, I remember thinking that this was the most interesting book I've read that is probably wrong.
Probably in the past minds can contemplate things we are not more aware today, who know? is not all about rational and scientific thinking, is not more or less advanced, is simply different, and in the past humans needed a quite different set of ideas and tools than today.
Just travel to Papua-NewGuinea and check with some tribe in a lost valley, straigh from Neolythic, some does not have a concept of "3", but children can learn arithmetic as everyone else, they can contemplate every concept we have, given the right path.