In a survey of more than 3,700 languages, Everett and his collaborators found that those with complex tones do indeed occur less frequently in dry areas than they do in humid ones, even after accounting for the clustering of related languages. For instance, more than half of the hundreds of languages spoken in tropical sub-Saharan locations feature complex tones, whereas none of the two dozen languages in the Sahara do. Overall, only one in 30 complex tonal languages flourished in dry areas; one in three nontonal languages cropped up in those same regions. The results appeared in February in the Proceedings of the National Academy of Sciences USA.

Those conclusions run counter to a traditional linguistic view that the structure of language is independent of its environment, says Robert Ladd, a linguistics researcher at the University of Edinburgh. To bolster the Miami group’s findings, researchers in the field will need to prove that tonal languages require a precise control of pitch.

Along those lines, Everett and his team will next measure experimentally how well people voice complex tones in arid air. Although the evolution of tonal languages over the course of centuries cannot be observed, witnessing the physiological effect under controlled conditions could really make the hypothesis sing.