Unless you're forced to have some unpleasant dental work done, it's likely you will never know if you have a three-rooted lower second molar. If you do, however, it probably reveals your connection to the Denisovans – that mysterious species of human known only from a few fragments of bone and teeth.
Most of the people of the world have two roots anchoring their second molars to the jawbone, even though the first molar usually has three. Some people get by with a single root to their second molar. In China, however, up to 40 percent of the population have three-rooted lower second molars, and something similar is true of people whose ancestry is primarily indigenous to the Americas. In other parts of the world, its less than 3.5 percent.
This was just a curiosity for dentists until the discovery of part of a 160,000-year-old Denisovan jaw in Tibet earlier this year. Analysis has revealed the presence of a three-rooted molar, Dr Shara Bailey of New York University reports in Proceedings of the National Academy of Sciences.
We already know that people of East Asian, Melanesian, and Indigenous Australian background have more Denisovan DNA than those whose origins lie elsewhere in the world. This appears to have some influence on immunity to diseases and a propensity to allergies. Gene varieties inherited from Denisovans are also thought to have contributed to Tibetan's high-altitude adaptations.
It's new, however, to find something that can be seen, albeit only in the dentist's chair or after death.
It had been thought the appearance of three-rooted second molars was a relatively recent development in humanity's history. Notably, we haven't seen this feature in the jaws of early Asian Homo Sapiens, other than one from Taiwan of uncertain age. However, it must have preceded the settlement of the Americas, and the paper notes it was one of the things that was used to confirm the Asian origins of indigenous North Americans decades ago.
A single example doesn't entirely disprove the recent origins theory. It's theoretically possible that most Denisovans didn't have three-rooted second molars either, and it just happens the one specimen we have was an exception who left no genetic trace. However, it is much more likely the third root was common among Denisovans and some people inherited it through interbreeding that occurred. This would explain why it is more common in peoples with a greater Denisovan contribution to their ethnic heritage.
While Denisovan inheritance is high in China, it's higher still among natives of New Guinea and Australia, where three-rooted second molars are rare. Bailey thinks this is because it was useful where diets required greater chewing, creating an evolutionary advantage.