The tooth, found in a mountain cave in Laos, has solved one of the biggest scientific mysteries of the Denisovans, an ancient human arm that disappeared about 50,000 years ago.
Since 2010, when Denisovan teeth and finger bones were first used discoveredDNA testing revealed that the enigmatic hominins were among the ancestors of people living in Australia and the Pacific today.
But scientists don’t understand how Denisovans, whose scarce remains are found only in Siberia and Tibet, were able to mate with the group of humans that spread eastward from Africa to Southeast Asia before reaching Australia, New Guinea and other islands. in the Pacific.
Now, the discovery of a girl’s molar in Laos, published The paper, published Tuesday in the journal Nature Communications, puts Denisovans on the path of modern humans who arrived in Southeast Asia tens of thousands of years later.
“We knew the Denisovans had to be here,” said Laura Shackelford, a paleoanthropologist at the University of Illinois and co-author of the new study. “It’s nice to have 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’ve been digging up the fossils ever since. In one of the many caves that pierced the mountains, they uncovered the human being. skull fragments Dating around 75,000 years ago, making them the earliest evidence of modern humans in Southeast Asia.
At the end of the researchers’ 2018 field season, children from a nearby village met Dr. They told Shackelford and his colleagues about another cave containing bones. His Laotian colleagues warned him that the cave was a favorite place for cobras, but he decided it was worth the risk to go inside.
First, a team of caving experts explored the area, and then Dr. Shackelford made his way to a closet-sized space where the boys claimed to have found bones. When he looked at the floor of the cave, he saw nothing.
“But then I turned on my flashlight and looked up,” he recalled. “All you could see were bones and teeth embedded in the walls and ceiling of this cave. They were just kind of everywhere.”
Dr. Shackelford and his colleagues have embarked on full-time work in the new cave they call the Cobra Cave (although they have never encountered any snakes). They chipped soccer ball-sized stones from the walls and soaked them with a mild acid. The rock gradually disintegrated, leaving behind harder fossils.
A closer look revealed that most of the fossils were bones from extinct mammals such as pigs, deer, and pygmy elephants. The gnawing marks on the bones revealed how they all became a mess in the Cobra Cave: Porcupines likely carried them there and chewed the bones to sharpen their teeth.
By extracting the gnawed bones, the scientists found a surprise: a molar, resembling the tooth of a human child. However, some features of the molar indicated that it was not fully human. Dr. “We were very surprised and very excited,” Shackelford said.
They were even more delighted when the geologists examined the cave wall to determine the age of the tooth. The tooth itself was too small to analyze, but researchers found nearby fossils and minerals containing radioactive elements that decay at a regular rate. By measuring these elements, the researchers estimated that the tooth was between 164,000 and 131,000 years old.
In other words, the Cobra Cave female, Dr. It’s about twice as old as the earliest modern human that Shackelford and colleagues found at the site. The tooth’s great age hinted that it belonged to an extinct relative of modern humans. But which one?
Humanity’s lineage diverged from the ancestors of chimpanzees in Africa about six million years ago. Over the next four million years, they evolved into upright, big-brained meat eaters. At this point some relatives began to move from Africa to Europe and Asia. A species called Homo erectus spread as far east as Indonesia.
Evidence from fossils and ancient DNA suggests that another wave of early humans traveled from Africa later. It split about half a million years ago as the population spread across Europe and Asia. The western population became Neanderthals and the eastern population became Denisovans.
Paleoanthropologists first discovered Neanderthal fossils in Germany and Belgium in the mid-1800s, and have since found numerous bones that mark the group’s range in Europe, the Middle East and Siberia. Fossils reveal Neanderthals as stocky, jawless humans. Tools and other relics caught a glimpse of their minds: They were master hunters who could track both. rhinos and dolphins. Them buried their dead and fashion necklaces eagle claws.
DNA in Neanderthal fossils tied them up to living people. Shortly after modern human ancestors left Africa, they encountered and interbred with Neanderthals in the Middle East—possibly several times.
Scientists have had a much harder time reconstructing the ancient migration of Denisovans. For years the only place they loved these ancient people was Denisova Cave in Siberia. It was difficult to understand how people in Australia, New Guinea, and nearby islands a few thousand miles away could have Denisovan DNA.
Denisova samples from Siberia were limited to a few teeth and finger bones. Fortunately, scientists found copious amounts of DNA in these samples and even extracted Denisovan DNA. dirt on the cave floor.
Evidence collected so far indicates that Denisovans occupied the cave 300,000 years ago and inhabited the surrounding area until about 50,000 years ago. They also left stone tools in the cave.
Considering that Denisovans lasted for hundreds of thousands of years, scientists were confident they would find Denisovan fossils elsewhere. in 2019, that’s how it happened. Chinese researchers announced that they discovered a 160,000-year-old jaw in a Tibetan cave, and its teeth match those found in the Siberian region. This expedition in Tibet, 1,400 miles south of Siberia, greatly expanded the known Denisovan range.
Chin also provided a few more clues about the Denisovans themselves. First, the proportions of the jaws and teeth implied that they might be tall and solidly built, like football players. To survive in the harsh environment of the Tibetan plateau, they would have to endure the low oxygen in the air and the cold weather.
Still, about 1,100 miles would separate the Denisovans in Tibet from their modern human ancestors in Southeast Asia, leaving scientists to wonder how the two groups might have become intertwined.
Examining the tooth found in the Cobra Cave in Laos, Dr. Shackelford and his colleagues did not know whether the tooth came from Denisovans, Neanderthals, Homo erectus, or other unknown human species. For the past four years, they’ve been analyzing the tooth for clues.
Initially, they hoped it would contain ancient DNA. But before they destroy part of the tooth to find out, they asked their colleagues at the University of Copenhagen to examine other mammal fossils found from the same specimen of the Cobra Cave wall. Since there was no DNA in these samples, they decided not to look at the DNA.
But they were luckier when they looked for bits of protein in the enamel of the molars. The chemical makeup of the fragments was found only in the teeth of modern humans, Neanderthals, and Denisovans, but not in the teeth of other mammals.
Unfortunately, the protein composition is the same in all three hominin groups, and its researchers cannot tell which molar it comes from.
However, the enamel of the young tooth had one more piece of information to offer: It belonged to a girl. It lacked a specific enamel protein encoded by gene on the Y chromosome carried only by males
The scientists made a high-resolution scan of the surface and interior of the molar, allowing them to compare its fine anatomical structure with more than 400 molars from living and extinct humans. Of these teeth, the Cobra Cave specimen was most similar to a molar implanted in the jaws of Denisovans from Tibet.
This conclusion is solid, said Shara Bailey, a paleoanthropologist at New York University who studies the Tibetan jaw but was not involved in the new study. “I agree 100 percent with the analysis,” she said.
Dr. Bailey acknowledged that some people may wonder how a single tooth was able to reveal so much about its Paleolithic owner. But the cusps and ridges of teeth are complex landscapes, and the shape of these landscapes is largely determined by genes, making teeth a wealth of information about evolution.
Dr. “Teeth are the unsung heroes of paleoanthropology,” Bailey said.
The discovery of Denisovans in Laos indicates that they were exactly where they needed to be to mate with modern humans who arrived in Southeast Asia thousands of years later.
Dr. Bailey and Dr. Shackelford agreed that more Denisovan fossils await discovery elsewhere. Recent studies have found small amounts of Denisovan DNA, for example, in East Asians, which their ancestors may have acquired in some genetic way. separate hybridization. And some ancient teeth already discovered in China and Taiwan now seem to have a Denisova shape that warrants a new look.
However, the Cobra Cave female itself provides some new clues about Denisovans, said Bence Viola, a paleoanthropologist at the University of Toronto who was not involved in the study.
“These guys were able to cope with dense snow cover and very low winter temperatures, as well as humid tropical environments,” he said. “The Denisovans were very adaptable—probably more so than Neanderthals. In the end, they were the most similar to modern humans.”