One of the biggest scientific mysteries of the Denisovans, a branch of ancient humans that disappeared roughly 50,000 years ago, has been solved with the discovery of a tooth inside a mountain cave.
Since 2010, when Denisovan teeth and finger bones were first discovered, it has been revealed that the enigmatic hominins were among the ancestors of people alive today in Australia and the Pacific.
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.
Denisovans are in the path of modern humans who arrived in Southeast Asia tens of thousands of years later.
Laura Shackelford, a paleoanthropologist at the University of Illinois, is a co-author of the new study.
In 2008 Dr. Shackelford joined a team of French and Laotian colleagues on an expedition to the Annamite Mountains in northern Laos, and they have been digging up fossils ever since. The oldest evidence of modern humans in Southeast Asia can be found in one of the many caves that riddle the mountains.
Children from a nearby village told the researchers at the end of the field season that there was a cave with bones. Her Laotian colleagues warned her that the cave was a favorite spot for cobras, but she decided to go inside.
A team of experts went to the site first, and then Dr. Shackelford went into a closet-sized hole where the children claimed to have found bones. She didn't see anything when she looked at the cave floor.
She said that she could see bones and teeth in the ceiling of the cave. They were everywhere.
Dr. Shackelford and her colleagues started working full-time in the new cave, which they dubbed Cobra Cave, despite never having encountered a snake. They chiseled rocks the size of soccer balls out of the walls and soaked them in acid. The harder fossils were left behind as the rock crumbled.
Most of the fossils were from extinct mammals, such as pigs, deer and pygmy elephants. The marks on the bones showed how the animals ended up in a mess in the cave.
The scientists found a tooth that looked like a human child's tooth. Dr. Shackelford said that the features of the molar suggested it was not a human.
They were thrilled when geologists looked at the cave wall to determine the age of the tooth. The tooth was too small to analyze, but the researchers found fossils and minerals nearby that were radioactive. The tooth was thought to be between 164,000 and 131,000 years old by the researchers.
The oldest modern humans that Dr. Shackelford and her colleagues have found in the region are about the same age as the cobra Cave tooth. The tooth's age suggested that it was from an extinct relative of modern humans. Which one?
About six million years ago, the ancestors of Chimpanzees split from humanity. They evolved into big-brained meat-eaters over four million years. Some relatives moved from Africa to Europe and Asia. The Homo erectus species has spread east as far as Indonesia.
Evidence from fossils and ancient DNA show that another wave of early humans left Africa. About half a million years ago, the population split between Europe and Asia. The eastern population became Denisovans, while the western population became Neanderthals.
Neanderthal fossils were first discovered in Germany and Belgium in the late 19th century and have since been found across Europe, the Middle East and Siberia. Neanderthals were found to be stocky, chinless humans. Tools and other remains offer a glimpse into their minds. They made their necklaces from eagle talons.
Neanderthal fossils were linked to living humans. Neanderthals were interbred with modern human ancestors in the Middle East after they left Africa.
The ancient migration of the Denisovans has been difficult to reconstruct. The Denisova Cave in Siberia was the only place where these ancient humans were fond of. It was hard to imagine how Denisovan could have ended up in Australia, New Guinea and nearby islands.
The Denisovan samples were limited to a few teeth and finger bones. Scientists were able to extract Denisovan DNA from the dirt on the cave floor.
The evidence gathered so far shows that Denisovans lived in the cave 300,000 years ago and in the surrounding area until about 50,000 years ago. They left behind stone tools in the cave.
Scientists were confident that they would find Denisovan fossils elsewhere. That was the case in 2019. The jaw of a 160,000-year-old human was found in a Tibetan cave with teeth that were similar to those found at the Siberia site. The range of Denisovans was greatly expanded by that discovery in Tibet.
The jaw gave more information about the Denisovans. For one thing, the proportions of the jaw and teeth implied that they were strong and tall like football players. They would have had to survive on low oxygen in the air and cold weather in the Tibetan plateau.
Scientists are wondering how the two groups could have interbred after 1,100 miles separated them.
Dr. Shackelford and her colleagues didn't know if the tooth was from Denisovans, Neanderthals, or Homo erectus. They have been analyzing the tooth for clues over the past four years.
They wanted it to contain ancient DNA. They had colleagues at the University of Copenhagen look at other mammal fossils from the same sample of the Cobra Cave wall. They didn't look for any in the specimen because it yielded no DNA.
They had better luck when they looked for fragments in the tooth. The chemical makeup of the fragments has only been found in the teeth of modern humans, Neanderthals and Denisovans.
The researchers can't say which one the molar came from because the protein composition is the same in all three groups of hominins.
The juvenile tooth had a piece of information that belonged to a girl. The Y chromosome is only carried by males and it lacks a specific enamel protein.
The scientists were able to compare the structure of the molar to that of living and extinct humans by using a high-resolution scanning device. The Denisovan jaw from Tibet is what the cobra cave specimen most closely resembled.
The conclusion was sound, according to a paleoanthropologist at New York University who studied the Tibetan jaw but was not involved in the new study.
Dr. Bailey acknowledged that some people may wonder how a single tooth can reveal so much. The shape of the landscape of the teeth is largely determined by genes, making them a rich source of information about evolution.
Dr. Bailey said that teeth are the unsung heroes of paleoanthropology.
The Denisovans were exactly where they needed to be to be with modern humans who arrived in Southeast Asia thousands of years later.
Dr. Bailey and Dr. Shackelford agree that more Denisovan fossils will be found elsewhere. Recent studies have shown a small amount of Denisovan DNA in East Asians, which may have been acquired through a separate interbreeding. Some ancient teeth that were discovered in China and Taiwan may have a Denisovan shape and need a new look.
Bence Viola, a paleoanthropologist at the University of Toronto who was not involved in the study, said that the cobra Cave tooth provided some new clues about the Denisovans.
He said thatDenisovans were able to deal with a lot of snow and low winter temperatures, but also with humid tropical environments. They were very similar to modern humans.
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