Common sense dictates that as human groups go on to occupy disparate geographical areas, both their genes and languages should diverge.
Although intuitive, this theory has been notoriously difficult to prove.
A new study, recently published in Proceedings of the National Academy of Sciences, finally succeeded at quantifying the complex relationship between geographic, linguistic and genetic data.
The research group compared how alleles (trait-defining stretches of DNA) relate to phonemes (distinct units of sounds that make up a language) in terms of geographic dispersal.
“Our study directly compares the signatures of human demographic history in microsatellite polymorphisms [short DNA sequences that vary from person to person] from 264 worldwide populations and complete sets of phonemes for 2,082 languages,” wrote the authors.
Since languages and genes experience descent with modification, and both are affected by evolutionary processes, such as migration, population divergence and drift, “combining linguistic and genetic analyses is a natural approach to studying human evolution.”
The map below shows how – in most parts of the world – alleles and phonemes occupy the same areas and even appear to have travelled along similar trajectories.
Even though these datasets were available for quite a while now, it has never been examined in the same place. “The thing we’ve done that no one else has is match worldwide genetic populations to their languages, so that you’re looking at a comparable set,” explained study co-author Nicole Creanza.
Only one language – Assamese – fully broke from the trend in its geographic group, while all the others proved to be closely linked to their genetic counterparts.
This data, coupled with advanced statistical techniques, helped the researchers demonstrate once and for all that genes and languages are closely tied and tend to travel together.
Another interesting finding is that geographically proximate languages tend to share properties even if they are not linguistically related.
“When two languages were geographically near, they tended to share more phonemes even if they were not closely related, suggesting a relationship between phonemes and geography within and between language families.”
The study also found that language isolation leads to more diversity in phonemes – an opposite result of genetic separation. This suggests that even though both languages and genes are related geographically, they evolve at highly variable speeds.