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 I had some of these same thoughts. It seems to me that there might exist two distinct categories (among others) that might have some influence on "learned" versus "not-learned" when it comes to the central pattern generator; those categories are instinctual and not. For instance, it seems that walking (humans) and flying (birds) are instinctual behaviors and so require no learning in order to occur. But what makes something instinctual? What about, as you suggest, talking? In an environment with no talking, would a human instinctually try to speak (can we ever know?)? Since biking is not an instinct (in response to the bike-riding comment), isn't that why it must be learned from observation and practice? What makes something instinctual and another thing not?