"The language of sin was universal, the original Esperanto" ~ Joe Hill

Psychologist, linguist, and recipient of an unfair number of IQ points at birth, Stephen Pinker, wrote this great book I'm very slowly reading right now called The Language Instinct. The book reveals that the one unique instinct that humans have is the instinct to use language. Language isn't a skill, it isn't an invention, it's not a discovery that humans found and passed down to their children like the wheel or fire. Language is the one thing that every human being is hard-wired to learn, and if a language doesn't already exist, children will make one. 
And it is children, mostly. Kids start learning language from the instant they're born, and start using it the moment they're physically capable. Have you ever noticed that the words for mother in most languages is really similar? From the English mama to the Arabic ahm to the Punjabi mai, the sound made by the letter m features prominently in most languages' word for mother. This isn't an accident. Ma, along with da and ba are among the first sounds a muchkin can make (oddly, babies make the g and k sounds during the "babbling" phase of vocal development, before they have any real control of their vocal tract, but they lose the ability to make those sounds when they're a few months old, and don't get it back until many months after that). Of course, you could say that while babies might make the sounds, it is the adults who assign meaning to them, and that's true to some degree. But even without adults around to communicate word meaning, kids do a pretty amazing job of word creation on their own.
One case that Pinker uses to support his point is that of Nicaraguan sign language. Before the Sandinista government took over there in 1979, the country had no schools for the deaf - so no universal sign language existed. Deaf folks communicated with their hearing friends and family with their own attempts at sign language, but it wasn't until deaf kids got together that a fully functioning sign-language was born.

And anatomically correct

And born more quickly than you can imagine. While schools tried with no success to teach them how to speak and read lips, the kids were creating their own sign language, a language younger than me but a rich one, one with its own unique grammar, vocabulary, and idioms; one in which speakers can discuss concepts as abstract as surrealism and philosophy. A language developed entirely by children in grade school who had never known any other language.
Totally mind-blowing.