When I was doing my Knight fellowship at MIT, I met a fascinating man called Shigeru Miyagawa. I was sitting in on his class on Japanese culture, but I learned later that his day job is actually as a professor of linguistics. MIT has some of the world's leading linguists (most famously, of course, Noam Chomsky) and at the time we met, Miyagawa happened to be working on a new idea that promised to shake up the field: that human language evolved from two different layers that pre-exist in nature. One layer comprises simple, grunt-type utterances, or words, which have meaning (like we see in our fellow primates) and the other layer is more melodic and expressive but not really meaningful in itself (like birdsong). He calls this radical idea the integration hypothesis.
My BBC Radio 4 documentary, produced by Rami Tzabar, about his work and the more general idea that birds might help explain the origins of human language aired this morning (incidentally, the baby babbling at the start of the documentary is my son, Aneurin. I dare you not to be utterly charmed). You can catch it online on BBC iPlayer if you missed it, or you can hear it on the BBC World Service in a couple of weeks. If radio is not your bag (what?) then I've written a BBC Earth feature to accompany the story.
Since I met Miyagawa, he's published a few papers on the topic and produced a very informative MOOC for students, which covers the basics of human language as well as the integration hypothesis in more detail. They're all hugely interesting, as is he.
Zebra finch photo above by Maurice van Bruggen.