People are quick to discount the value of education (or upbringing) from the early age. Humans have started developing quickly with the development of language and especially writing. Take that away, and we are looking at another dozens of thousands of years of getting back to it, if at all.

The best evidence to me is how those "lost in woods, found years later" kids have developed: mostly mimicking other animals (https://en.wikipedia.org/wiki/Feral_child). It's even hard to change that later on.