For the most part, the thousands of languages in the world today fall into one of two categories (notable exceptions being Japanese, some Scandinavian dialects and northern Spain's Basque tongue): tonal or nontonal.

Two linguists believe they know the genetic underpinnings for these differences. During a study of linguistic and genetic data from 49 distinct populations, the authors discovered a striking correlation between two genes involved in brain development and language tonality. Populations that speak nontonal languages (where the pitch of a spoken word does not affect its meaning) have newer versions of the genes, with mutations that began to appear roughly 37 thousand years ago.

"You can consider this as the first of the many possible studies that we could do to try to find a genetic basis for language and language typology and the different populations that speak a language," says Patrick Wong, an assistant professor of communication sciences and disorders at Northwestern University, who was not involved in this study.

In English, the pitch at which a word is spoken conveys emotion but usually does not affect its meaning. But in many sub-Saharan Africa, Southeast Asian and Latin American languages tone changes the meaning of words. For instance, the Chinese word huar said in a high pitch means flower, but in a dipping pitch means picture.

The new research, published this week inProceedings of the National Academy of Sciences USA ties this difference to two genes, ASPM and Microcephalin. The exact functions of both genes are still open to debate, but they are known to affect brain size during embryonic development. "They presumably have something to do with brain structure, because there are deleterious mutations of the genes that lead to microcephaly" (a condition in which a person's brain is much smaller than the average size for his or her age), says senior study author, Robert Ladd, a professor of linguistics at the University of Edinburgh in Scotland.

Ladd and colleague Dan Dediu, a fellow linguist at the university, focused on one particular variation of each of these two genes. "They're versions of these genes that are not only newer, but also show signatures of strong natural selection in modern humans," Ladd says. In their report, the authors note that previous studies indicate that these popular new mutations do not appear to affect intelligence, brain size or social ability. But based on their strong correlation with language tone, they surmise that they may contribute to slight differences in the cerebral cortex, the outermost layer of the brain, which, among many other functions, plays a role in our ability to understand language.

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.

Northwestern's Wong says that in a field in which researchers struggle to determine whether differences arise from experience or genetics, the new study "gives us an idea that there is a genetic side to things." He says the research indicates that small differences in brain organization determined by genetic makeup may be amplified by cultural factors and contact with other languages through war or migration, creating today's dichotomy in language tonality.

"Even remarkable correlations can arise by coincidence—or, in this case, possibly by prehistoric migration factors that are currently unknown to anthropology and archaeology—so we can't rule that out," Ladd says. "The next step is to attempt to correlate individual genotypes with measurably different behaviors on experimental tasks that are plausibly related to language and speech."