Tracing human genetic history
Human tooth shape varies greatly among individuals and populations. Examples of common dental features include the groove patterns in crowns, the relative size of cusps, the number of roots, and the presence or absence of wisdom teeth. These dental traits are heritable, with certain traits commonly observed within families. Some of them occur at different frequencies across populations. In a way, that is similar to the inheritance and variation of DNA.
“Dental traits can be used in population genetic studies when DNA is not available,” says Dr. Hannes Rathmann, a researcher at the University of Tübingen. Teeth are the hardest tissue in the human body and individuals’ dental remains are often well preserved, even when their skeletal and DNA preservation is poor.
Neutral traits yield valuable information
“Most human dental traits probably arose by chance as a result of genetic drift,” Rathmann says. “That is an evolutionary process that is considered to be neutral, having no particular advantages or disadvantages for individuals or the population.” On the other hand, it is believed that some traits evolved in a non-neutral manner. This is a result of natural selection and adaptation, perhaps in response to chewing behavior or environmental factors. “Teeth that evolve neutrally are useful for inferring genetic relationships and can be highly informative for reconstructing the human past,” adds Dr. Hugo Reyes-Centeno.
In order to separate the neutral evolutionary mechanisms from the non-neutral ones, the researchers compared the variation in dental traits to the variation in neutrally evolving DNA across various populations around the world. They performed extensive calculations and looked at more than 130 million possible combinations of dental traits. This enabled them to identify a set of highly informative trait combinations that preserve neutral genetic signals best.
The wide-ranging implications
The findings could be applied in many different contexts, including the reconstruction of the origin and evolution of our species using human fossils. “In such investigations, it may be that DNA cannot be retrieved due to poor preservation,” says Reyes-Centeno. That is when dental traits become a very useful proxy for DNA. “We propose that future studies should prioritize the most effective dental traits and trait combinations found in our study, as they allow more accurate conclusions to be drawn about genetic relationships.”