Do humans have an innate universal grammar, i.e., are we all born with certain foundational rules of language "hard-wired" in our brain—and which we don't need to learn? The dominant theory in linguistics, long associated with Noam Chomsky, says yes. However, this is not entirely accepted in the field, and challengers have only increased. Many now lean away from innate universal rules, and towards innate capacities or instincts that are shaped by culture into rules. Here is an excellent article on the debate and the work of a leading challenger, Dan Everett.
A Christian missionary sets out to convert a remote Amazonian tribe. He lives with them for years in primitive conditions, learns their extremely difficult language, risks his life battling malaria, giant anacondas, and sometimes the tribe itself. In a plot twist, instead of converting them he loses his faith, morphing from an evangelist trying to translate the Bible into an academic determined to understand the people he's come to respect and love.
Along the way, the former missionary discovers that the language these people speak doesn't follow one of the fundamental tenets of linguistics, a finding that would seem to turn the field on its head, undermine basic assumptions about how children learn to communicate, and dethrone the discipline's long-reigning king, who also happens to be among the most well-known and influential intellectuals of the 20th century.
More here. Some discussion on this article, including by Everett, took place here. This builds upon a really good New Yorker essay from 2007, The Interpreter (which carried the photo above). Also check out The Myth of Language Universals, which argues that the "claims of Universal Grammar ... are either empirically false, unfalsifiable, or misleading in that they refer to tendencies rather than strict universals."