Sign Language

NextBigFuture speculated that no one will learn second languages soon because of machine translation. This misses quite a lot of the way we use language that machine translation can never do (such as using esoteric or group-specific features of a language as a way of establishing membership of a group). However my posted comment concentrated on one of my pet theories:

Human brains are designed to know multiple languages and some bits of the brain don't develop properly if not (which we know because people with one language have more trouble learning a new language than people who learn more than one when very young). And there is a second language that everyone should learn early: sign language. Reasons: (a) Talk in noisy environments; (b) Easier to have a private conversation because light waves are easier to block than sound waves; (c) Babies can learn sign language earlier (and parents are very frustrated till they can communicate with their kids). Sign language was our ancestors first language: we needed a language first to justify all the voice box changes needed for language.