Tooth of an ancient girl fills gap in human family tree

NEW DELHI: A tooth found inside of a mountain cave in Laos has solved one of the biggest scientific mysteries of the Denisovans, a branch of ancient humans that disappeared roughly 50,000 years ago. Since 2010, when Denisovan teeth and finger bones were first discovered, DNA testing has revealed that the enigmatic hominins were among the ancestors of people alive today in Australia and the Pacific. But scientists didn’t understand how the Denisovans, whose scant remains had been found only in Siberia and Tibet, would have been able to interbreed with the group of humans who expanded east from Africa through Southeast Asia before reaching Australia, New Guinea and other islands in the Pacific.

Now, the discovery of a girl’s molar in Laos, published on Tuesday in the journal Nature Communications, puts Denisovans right in the path of modern humans who arrived in Southeast Asia tens of thousands of years later. “We knew that Denisovans should be here,” said Laura Shackelford, a paleoanthropologist at the University of Illinois and a co-author of the new study. “It’s nice to have some tangible evidence of their existence in this area.”

Dr. Shackelford joined a team of French and Laotian colleagues on an expedition to the Annamite Mountains in northern Laos in 2008, and they have been digging up fossils ever since. In one of the many caves that riddle the mountains, they have unearthed human skull fragments dating back about 75,000 years, making them the oldest evidence of modern humans in Southeast Asia.

At the end of the researchers’ 2018 field season, children from a nearby village told Dr. Shackelford and her colleagues of another cave that contained bones. Her Laotian colleagues warned her that the cave was a favourite spot for cobras, but she decided a trip inside was worth the risk. A team of caving experts scouted the site first, and then Dr. Shackelford made her way into a closet-sized cavity where the children claimed to have found bones. When she inspected the cave floor, she saw nothing. “But then I turned my flashlight on, and I looked up,” she recalled. “All you could see were bones and teeth, embedded in the walls and in the ceiling of this cave. They were just sort of everywhere.”

Dr. Shackelford and her colleagues started working full-time in the new cave, which they dubbed Cobra Cave ( despite never encountering a snake). They chiselled rocks the size of soccer balls out of the walls and soaked them in a mild acid. The rock gradually disintegrated, leaving the harder fossils behind. On close inspection, most of the fossils turned out to be bones from extinct mammals, such as pigs, deer and pygmy elephants. Gnaw marks on the bones revealed how they all ended up in a jumble in Cobra Cave: Porcupines likely carried them there and chewed on the bones to hone their teeth.

Sorting through the gnawed bones, the scientists found a surprise: a molar that resembled a human child’s tooth. But some features of the molar suggested it was not quite human. “We were so amazed and so excited,” Dr. Shackelford said. They were even more delighted when geologists examined the cave wall to determine the age of the tooth. The tooth itself was too small to analyse, but the researchers found fossils and minerals nearby that contained radioactive elements that broke down at a regular pace. By measuring those elements, the researchers estimated the tooth was between 164,000 and 131,000 years old.

Zimmer is a journalist with NYT©2022

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