A tooth found in a mountain cave in Laos has solved one of the greatest scientific mysteries of the Denisovans, a branch of ancient humans who disappeared about 50,000 years ago.
Since 2010, when Denisovan teeth and finger bones were first discovered, DNA testing has shown that the enigmatic hominins were among the ancestors of the humans living in Australia and the Pacific today.
But scientists failed to understand how the Denisovans, whose scant remains had only been found in Siberia and Tibet, could interbreed with the group of humans who had expanded eastward from Africa through Southeast Asia before entering Australia, New Guinea and others. reached islands. in the Pacific Ocean.
The discovery of a girl’s molar in Laos, 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 Denisovans should 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 and a team of French and Laotian colleagues went on an expedition to the Annamite Mountains in northern Laos in 2008, and they’ve been excavating fossils ever since. In one of the many caves that crisscross the mountains, they 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 about another cave that contained bones. Her Laotian colleagues warned her that the cave was a favorite place for cobras, but she decided a trip inside was worth the risk.
A team of caving experts first explored the site, then Dr. Shackelford made his 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 on my flashlight and looked up,” she recalls. “All you could see were bones and teeth, embedded in the walls and ceiling of this cave. They were just everywhere.”
dr. Shackelford and her colleagues began working full-time in the new cave, which they named Cobra Cave (despite never having encountered a snake). They cut stones the size of footballs out of the walls and dipped them in a mild acid. The rock gradually disintegrated, leaving behind the harder fossils.
Upon closer inspection, most of the fossils turned out to be bones from extinct mammals, such as pigs, deer and dwarf elephants. Chew marks on the bones revealed how they all ended up in a hodgepodge in the Cobra Cave: Porcupines probably carried them there, chewing the bones to grind their teeth.
By sorting through the gnawed bones, the scientists found a surprise: a molar that resembled the tooth of a human child. But some features of the molar suggested it wasn’t quite human. “We were so amazed and so excited,” said Dr. Shakelford.
They were even more elated when geologists examined the cave wall to determine the tooth’s age. The tooth itself was too small to analyze, but the researchers found nearby fossils and minerals containing radioactive elements that were breaking down at a regular rate. By measuring those elements, the researchers estimate that the tooth was between 164,000 and 131,000 years old.
In other words, the Cobra Cave tooth is about twice as old as the oldest modern human Dr. Shackelford and her colleagues in the region. The tooth’s advanced age hinted that it belonged to an extinct relative of modern humans. But which?
Humans’ lineage split from the ancestors of chimpanzees about six million years ago in Africa. Over the next four million years, they evolved into upright, large-brained meat eaters. At that time, some relatives began to move from Africa to Europe and Asia. One species, called Homo erectus, spread east as far as Indonesia.
Evidence from fossils and ancient DNA indicates that a new wave of early humans traveled from Africa even later. As the population spread across Europe and Asia, it split about half a million years ago. The western population became Neanderthals, and the eastern ones became Denisovans.
Paleoanthropologists first discovered Neanderthal fossils in the mid-1800s in Germany and Belgium and have since found an abundance of bones marking the group’s range in Europe, the Middle East and Siberia. The fossils revealed Neanderthals as stocky, chinless humans. Tools and other remains offered a glimpse of their minds: They were adept hunters who could stalk both rhinoceroses and dolphins. They buried their dead and made necklaces of eagle claws.
DNA in Neanderthal fossils also linked them to living humans. Shortly after modern human ancestors left Africa, they met and interbred with Neanderthals in the Middle East — probably on several occasions.
Scientists have had a much harder time reconstructing the ancient migration of the Denisovans. For years, the only place they loved these ancient people was the Denisova Cave in Siberia. It was hard to imagine how people thousands of miles away, in Australia, New Guinea and nearby islands, could have ended up with Denisovan DNA.
The Denisovan samples from Siberia were limited to a few teeth and finger bones. Fortunately, scientists found an abundance of DNA in these specimens and even extracted Denisovan DNA from dirt on the cave floor.
The evidence gathered so far indicates that Denisovans occupied the cave 300,000 years ago and occasionally inhabited the area until about 50,000 years ago. They also appear to have left stone tools in the cave.
Given that Denisovans lasted for hundreds of thousands of years, scientists were confident they would find fossils of Denisovan elsewhere. That turned out to be the case in 2019. Chinese researchers announced the discovery of a 160,000-year-old jaw in a Tibetan cave with teeth similar to those found at the Siberian site. That discovery in Tibet, more than 1,400 miles south of Siberia, dramatically expanded Denisovans’ known range.
The jaw also gave a few more clues about the Denisovans themselves. For starters, the proportions of the jaw and teeth implied that they might have been tall and sturdily built, like soccer players. To survive in the harsh environment of the Tibetan Plateau, they had to withstand low oxygen levels in the air and cold weather.
Still, some 1,100 miles would have separated the Denisovans in Tibet from the modern human ancestors in Southeast Asia, leaving scientists to wonder how the two groups could interbreed.
While examining the tooth from Cobra Cave in Laos, Dr. Shackelford and her colleagues aren’t sure whether it came from Denisovans, Neanderthals, Homo erectus, or some other unknown human species. Over the past four years, they’ve been analyzing the tooth for clues.
Initially, they hoped it would contain ancient DNA. But before destroying part of the tooth to find out, they had colleagues from the University of Copenhagen look at other mammal fossils found in the same sample from the Cobra cave wall. Those specimens didn’t yield any DNA, so they decided not to look for them.
But they had better luck looking for protein fragments in the enamel of the molar. The chemical composition of the fragments has only been found in the teeth of modern humans, Neanderthals and Denisovans, but not those of other mammals.
Unfortunately, the protein composition in all three groups of hominins is the same, which means that the researchers cannot say which molar the molar originated from.
But the enamel of the juvenile tooth had something else to offer: it belonged to a girl. It lacked a specific enamel protein encoded by a gene on the Y chromosome, which is only carried by males.
The scientists created a high-resolution scan of the molar’s surface and interior, allowing them to compare the fine anatomical structure with more than 400 molars from living and extinct humans. Of those teeth, the Cobra Cave specimen most resembled a molar lodged in the Denisovan jaw from Tibet.
Shara Bailey, a paleoanthropologist at New York University who has studied the Tibetan jaw but was not involved in the new study, said this conclusion was correct. “I agree 100 percent with the analysis,” she said.
dr. Bailey acknowledged that some people may wonder how a single tooth could reveal so much about its Paleolithic owner. But the cusps and ridges of teeth are complex landscapes, and the shape of that landscape is largely determined by genes, making teeth a wealth of information about evolution.
“Teeth are the unsung heroes of paleoanthropology,” said Dr. Bailey.
The discovery of Denisovans in Laos shows that they were exactly where they needed to be to interbreed with modern humans who arrived in Southeast Asia thousands of years later.
dr. Bailey and Dr. Shackelford agreed that more Denisovan fossils are waiting to be discovered elsewhere. For example, recent studies have found a small amount of Denisovan DNA in East Asians, who may have obtained their ancestors through a separate crossing. And some old teeth already discovered in China and Taiwan now seem to have a Denisovan shape, justifying a fresh look.
But Bence Viola, a paleoanthropologist at the University of Toronto who was not involved in the study, said the Cobra Cave tooth itself provided new clues about the Denisovans.
“These guys could handle extensive snow cover and very low winter temperatures, but at the same time with humid tropical environments,” he said. “Denisovans were very adaptable — probably more than Neanderthals. In the end, they were most like modern humans.”