Wednesday, July 10, 2019

Speaking of Science: An odd tooth spills ancient secrets

Speaking of Science
Talk nerdy to us

A three-rooted lower first molar and the jaw it belonged to, from a modern human. (Christine Lee/California State University)

An old, unusual tooth discovered in China reinforces the theory that Homo sapiens and an extinct human species, the Denisovans, swapped genetic material — and physical traits — thousands of years ago.

The tooth, a molar with three roots, belonged to a jawbone found remarkably high in a mountain cave on the Tibetan Plateau. Anthropologists announced the discovery of the jaw, which predated modern human settlement of the region by 100,000 years, in May. A new paper, published this week in the Proceedings of the National Academy of Sciences, is the first to describe the jaw's three-root tooth in detail.

Three roots in a lower molar are rare in modern humans. The overall prevalence in non-Asian people is about 3.5 percent. In Asian and Native American populations, though, the proportion of three-rooted molars rises to about 40 percent. Given this, scientists had predicted that the characteristic arose recently in human history, as people dispersed into Eurasia. (A person who lived in the Philippines almost 50,000 years ago had such a tooth.)

Yet the discovery indicates humans may have inherited the feature from Denisovan ancestors. "Its presence in a 160,000-year-old archaic human in Asia strongly suggests the trait was transferred to H. sapiens in the region through interbreeding with archaic humans in Asia," said study author Shara Bailey, an anthropologist at New York University, in a statement.

This new study doesn't offer direct genetic evidence of hybridization; the scientists have been able to pry ancient proteins, but not DNA, out of the mandible. But the feature is a strong sign of interbreeding, the scientists said. "We now have very clear evidence that gene flow between archaic groups and H. sapiens resulted in the transfer of identifiable morphological features," they wrote in the new study.

The Denisovans were first identified as a species based on a finger bone in 2010 found in Siberia. Later genetic tests showed they mated with Homo sapiens, as well as Neanderthals. As my colleague Sarah Kaplan wrote last year, some scientists compare the planet during this age of prehistory to J.R.R. Tolkien's Middle Earth: "Except instead of hobbits, dwarves and elves, there were different kinds of humans."


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