They assert that the source of prior support for language acquisition must originate within the brain, on the unstated assumption that there is no other possible source. Chomsky argues that much of the child's knowledge of grammar and syntax is not learned in the way that words are. It is discovered, though not by introspection of rules already available in the brain. I think this appearance of lucky coincidence accurately captures what happens, though it is not luck that is responsible. Languages evolve to make it easy for children to learn them, i.e., so that childrens guesses are likely to be good guesses.
Languages evolve very fast compared to biological evolution.
Work with Bonobo chimp named Kanzi is cited. Kanzi understands lexigrams and English, has a mastery of simple grammar. He was not trained directly. His mother was trained in language experiments which she did poorly in. Neural net experiments in learning grammatical structure are cited.