Tones and genes
According to an article in the New Scientist, researchers at the University of Edinburgh have demonstrated using statistical analysis that two genes, ASPM and Microcephalin, that govern aspects of brain development tend to differ between regions where tonal languages are spoken, and regions where non-tonal languages are spoken.
The article also mentions that there are some differences in brain structure between English speakers with facility for learning tonal languages and those who find such languages difficult. So if you are struggling with the tones of a language like Mandarin or Thai, maybe it’s because your brain has evolved to cope best with non-tonal languages.
Another article on this subject in Scientific American gives more details of the research:
Ladd and Dediu compared 24 linguistic features — such as subject-verb word order, passive tense, and rounded vowels — with 981 versions of the two genes found in the 49 populations studied. Most of the language contrasts could be explained by geographic or historical differences. But tone seemed to be inextricably tied to the variations of ASPM and Microcephalin observed by the authors. The mutations were absent in populations that speak tonal languages, but abundant in nontonal speakers.
Further details are available on this blog, which is written by the son of one of the researchers.