I've been ranting too much about European linguistics. I've been shamelessly unfair to the other continents who have patiently been reading my rants and feeling unrepresented. This is a big world so let's get cracking on... [Glen swiftly spins a globe and then stops it suddenly with his finger]... ah, yes, Sino-Tibetan.
Don't know what Sino-Tibetan is? Then check out this explanation from Berkeley University in California. Learn it because this is where the most popular language in the world comes from. No, no, not English, silly. Mandarin!
When I was a wee lad, there was this nagging question in the back of my quirky mind: Why does Chinese have tones? No matter how educated one happens to be, most people honestly haven't the foggiest clue about how to answer that question. Afterall they don't teach language origins in high school for some crazy reason. I guess if they started that programme, people would feel more united and have one less reason to start wars with other countries. Surely global comradery would cause a collapse of civilization. (Okay, that was a terribly bitter thing to say, but I just finished watching George Orwell's 1984 on YouTube and it just makes ya think, y'know?)
Anyways, it was only in my twenties when I finally discovered the juicy answer. The keyword here is called tonogenesis. Tonogenesis is the development of tonal contrasts in a language that previously didn't have any. Apparently, this happens all the time. It probably happened in languages before the Ice Age. It's certainly happened into the modern day, and if we live long enough and haven't evolved into waddling fish-people, there will be new languages with tones, perhaps derived from English, in the future.
Asian languages aren't the only tongues with tones. Did you know that Swedish has two tones? See The Tone-bearing Unit in Swedish and Norwegian [pdf]. And have you ever heard of a Nilo-Saharan language called Nobiin? Where have you been? Read this article entitled A sketch of Nobiin tone [pdf] from the University of Leiden. And we can't forget our Native American brothers and sisters, some of whom speak Navajo.
Tonogenesis in Sino-Tibetan is pretty interesting. Surprisingly linguists have reconstructed Proto-Sino-Tibetan with tongue-twisting consonant clusters and no tones. Frankly, it sounds more like Russian than anything we'd recognize in China today. All the tonal contrasts that Chinese languages now use (four tones in Mandarin; six or more in Cantonese) came after Proto-Sino-Tibetan. I have to chuckle every time I think about that crazy thought. It's a mind-bender. The word "eight", for example, is pronounced ba with high tone in Mandarin and as baat in Cantonese with middle tone. The two words, including their tone are related, of course. Sometimes you can even guess what the tone will be in Cantonese if you know the Mandarin word but, trust me, be careful with that because I personally have miscalculated a few times and had egg on my face. Both Mandarin ba and Cantonese baat go back to Proto-Sino-Tibetan *bʀyat but this word is without any reconstructable tone. Yep, just like Russian. So where did these tones come from then? Thin air? How does that work?
It turns out that voiceless and voiced sounds that we make unconsciously when we speak a language have an inherent acoustic frequency or "tone". In the word *bʀyat the initial consonant cluster is voiced and therefore gives off a low frequency but the letter at the end, *t, is voiceless and has high frequency. In other words, the surrounding sounds of the vowel were the seeds for later tone and they eventually determined the tonal shape of the word, whether the tone was low or high, whether it rose to a high pitch or fell sharply, etc. I suppose we could say that the vowel assimilated the unconscious frequencies of consonants and this evolved into more conscious tones, now intrinsic to the meaning of that word. Different Chinese languages have different kinds and numbers of "tonemes", just as all spoken languages have different kinds and numbers of phonemes.
Similar tonogenesis happened in Navajo, a language which derives from Proto-Athabaskan. Proto-Athabaskan also doesn't appear to have had tonal contrasts. This article Vietnamese and Tonogenesis: revising the model is also interesting for those who want to really get their hands dirty.