Re: Ch. 4 - How to make us human
Jared Diamond has theorized that two evolutionary developments allowed the modern human to outcompete other species of early hominids: longevity (so we could pass down more information to our young) and the ability to pronounce complex phonetics that would lead to the cultural development of language. Our very concept of reality is based on language. We use categories to define the parameters of our existence and categories are expressed linguistically as words. In this sense, language is reality. Without language we could not comprehend abstract ideas. We would be as primitive as our simian cousins.
There's the very interesting story of the Wild Boy of Aveyron, although this is obviously not evidence.http://en.wikipedia.org/wiki/Victor_of_Aveyron