Three-rooted molars: A rare dental trait lives on

A rare dental trait that is more common in Asian and Native American populations – three-rooted molars – could have its origins in trysts with our archaic relatives, the Denisovans, according to new research.

Few people probably give much thought to the subterranean shape of the grinding teeth in their lower jaw, but palaeoanthropologists look to teeth – often the only surviving fossil remains of our ancient relatives – for clues to our prehistoric family tree. 

Recently, a lower jawbone found in a Tibetan cave was identified as being at least 160,000 years old and belonging to a member of the group known as the Denisovans. It bears a molar with three roots.

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A three-rooted lower second molar from a Denisovan found in Xiahe, China.

Credit: Jean-Jacques Hublin.

Another jawbone, found off the coast of Taiwan, and belonging to an archaic human – possibly a Denisovan – has a three-rooted molar, too.

Three-rooted molars are oddities in most modern dental practices. Molars generally have just two roots, but occasionally a third, smaller root grows. 

In Europe and Africa, fewer than 3.5% of people have such teeth.

But rates upwards of 40% have been found in surveys of archaeological specimens from northern China and islands in the Bering Sea that were once part of a land bridge connecting Asia and North America.

Indeed, the high frequencies of three-rooted molars in these populations is a key feature that points to the Asian origins of Native Americans.

Surveys of modern Asian populations also have higher rates of the dental anomaly – up to nearly a third in some studies.

When a Denisovan genome was sequenced from a scrap of bone found in the Siberian Denisova cave, it became evident that Denisovans met and intermingled with our own prehistoric ancestors.

Modern-day populations across Asia, New Guinea and Australia retain snippets of Denisovan DNA in their genome.

In the case of present-day Tibetans, one snippet inherited from Denisovans helps them to live in the low oxygen environments of the Tibetan Plateau.

The new study, published in the journal PNAS, suggests that the three-rooted molars in modern-day people also derive from Denisovans. 

“We now have very clear evidence that gene flow between archaic groups and Homo sapiens resulted in the transfer of identifiable morphological features,” the authors write. 

“The [three-rooted molar] is an Asian-derived character that we can definitively trace to Denisovans,” they say.

Palaeoanthropologist Tanya Smith from Griffith University, who wasn’t involved in the study, takes a more cautious view.

“It is a very interesting suggestion,” she says, but adds that “without genetic evidence, I think it is premature to declare that this one fossil provides compelling morphological evidence of Denisovan admixture in Asian-derived populations”.

Before concluding that three-rooted molars in modern humans came from Denisovans, scientists first need to be sure that most Denisovans had this trait, given that the trait can readily pop up due to mutation alone. That’s a hard ask given the small number of Denisovan molars identified so far.

Identifying the genes that cause a third root in modern people’s molars, and mapping that back to regions of the genome inherited from Denisovans would also make the link more air-tight, says Smith.

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