it looks as if kids don't learn language in the way predicted by a universal grammar; rather, they start with small pockets of reliable patterns in the language they hear, such as Where's the X?, I wanna X, More X, It's a X, I'm X-ing it, Put X here, Mommy's X-ing it, Let's X it, Throw X, X gone, I X-ed it, Sit on the X, Open X, X here, There's a X, X broken … and gradually build their grammar on these patterns, from the "bottom up".
...Importantly, these same basic processes of intention-reading are necessary not only for language, but also for discerning what someone is communicating when they simply poke their index finger out in a particular direction for the purpose of communication. To understand why someone is pointing to, for example, a bicycle leaning against a tree, one must share some background experience and knowledge with that person to determine why on earth they would be directing one's attention to this particular situation at this particular moment.
The idea is that something (we don't precisely know what) in our evolutionary history placed pressure on us (but not chimpanzees) to evolve the kind of mental machinery that allows us to read communicative intentions. One of the consequences of this was that it provided a key mental capacity for language. But it also put in place the potential for us to take part in ever more complex and large-scale cooperative ventures that form the fabric of our different cultures.