A startling theory (description from the FAQ):
Everett, Blasi & Roberts (2015) review literature on how inhaling dry air affects phonation, suggesting that lexical tone is harder to produce and perceive in dry environments. This leads to a prediction that languages should adapt to this pressure, so that lexical tone should not be found in dry climates, and the paper presents statistical evidence in favour of this prediction.
It’s reminiscent of this bit of japery, but this is serious, and the FAQ answers some obvious questions, e.g., re exceptions:
There are certainly exceptions to the prediction. But we’re not expecting a hard and fast rule, just a statistical tendency. We were aware of such exceptions before even embarking on the analysis of the database. Our account is not simply deterministic, but suggestive of gradual pressures operating at the same time as other pressures known to impact the evolution of sound systems. Occasionally such influences may even be at odds. We note in the SI, however, that even in a language family like Tibeto-Burman, in which there are exceptions, the overall pattern holds in the predicted direction. Tibeto-Burman languages spoken in more desiccated regions are less likely to employ complex tone.