Talking is something that is unique to humans, yet it still remains a mystery.
Why Do We Talk? meets the scientists beginning to unlock the secrets of speech - including a father who is filming every second of his son's first three years in order to discover how we learn to talk, the autistic savant who can speak more than 20 languages, and the first scientist to identify a gene that makes speech possible.
Horizon also hears from the godfather of linguistics, Noam Chomsky, the first to suggest that our ability to talk is innate.
A unique experiment shows how a new alien language can emerge in just one afternoon, in a bid to understand where language comes from and why it is the way it is.