Until the 1950s, American Sign Language was widely seen as pantomime or broken English. In 1960, William Stokoe, who taught at Gallaudet College in Washington, published 'Sign Language Structure', showing that every sign breaks down into units such as handshape, location and movement, much like the sounds of a spoken language. His 1965 dictionary drove the point home, though it took years to win people over: ASL has its own grammar.
In Kuşköy, a mountain village in northern Turkey, people talk across valleys by whistling Turkish. A 2015 study showed that whistled Turkish draws on both hemispheres fairly evenly, whereas spoken Turkish relies mostly on the left. The physical form of the signal, it turns out, matters to the brain.
Speech has no gaps between words. In 1996, researchers played 8-month-old babies a continuous stream of made-up syllables for just two minutes. The infants picked out the 'words' by tracking which syllables tend to follow one another. That's probability calculation, done without a calculator or any visible effort.