An interesting question, one that linguists have long debated, is whether climate and geography affect language. The challenge has been to untangle the factors that cause sounds to change.

To find a relationship between the climate and the evolution of language, one needs to discover an association between the environment and vocal sounds that is consistent throughout the world and present in different languages. And that is precisely what a group of researchers has done.

Many languages of the world use tone or pitch to give meaning to their words. University of Miami (UM) linguist Caleb Everett and his collaborators have uncovered that languages with complex tones --those that use three or more tones for sound contrast -- are much more likely to occur in humid regions of the world, while languages with simple tone occur more frequently in desiccated regions, whether frigid areas or dry deserts.

"In my estimation, it changes a bit our understanding of how languages evolve," said Everett, associate professor in the Department of Anthropology at the UM College of Arts and Sciences, and lead investigator of this project. "It does not imply that languages are completely determined by climate, but that climate can, over the long haul, be one of the factors that helps shape languages."

"More broadly, this suggests another non-conscious way in which humans have adapted to their very different and harsh environments," Everett said. "Also, there may be some health benefits to certain sound patterns in certain climates, but more research is needed to establish that in a satisfactory way."

One explanation, supported by extensive experimental data discussed in the study, is that inhaling dry air causes laryngeal dehydration and decreases vocal fold elasticity. It's probably more difficult to achieve complex tones in arid climates--particularly very cold ones--when contrasted to warmer and more humid climates. The result is that deviations of sounds, including increased jitter and shimmer, are associated with very cold or desiccated climates, the study says.

The findings are published in the Proceedings of the National Academy of Sciences. They provide extensive evidence that sound systems of human languages are adaptive and can be influenced by climate. The findings are supported by data relating to over half of the world's languages and to previous extensive experimental research on the properties of the human larynx that affect tonality.

The team examined more than 3,700 languages and found 629 languages with complex tones. Most were found in tropical regions, throughout Africa and Southeast Asia, but also in some humid regions of North America, Amazonia and New Guinea.