Origin
DNA has shown that early humans evolved all over the world, from frigid Siberia to high-altitude Tibet – perhaps even on the Pacific islands.
March 2, 2024
Neanderthals may have disappeared 40,000 years ago, but they are not strangers to us today. Their sturdy skeletons fascinate in museums around the world. Personalities imagined themselves becoming television commercial stars. When Kevin Bacon noted on Instagram that his morning habits were like Neanderthals, he didn’t stop to explain that our ancient cousins interacted with modern humans who evolved from Africa.
But there was no such familiarity with the Denisovans, a group of humans who descended from the Neanderthal line and survived for hundreds of thousands of years before becoming extinct. That’s mostly because we have so few of their bones. In a new review paper, anthropologists tallied all the fossils that have been unambiguously identified as Denisovan since the first discovery in 2010. The complete list consists of a broken half jaw, a finger bone, a skull fragment, three loose teeth and four other bone fragments.
“The Denisovan pieces that we have, they’re almost non-existent,” said Janet Kelso, a paleoanthropologist at the Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who helped write the review.
However, many scientists are increasingly fascinated by Denisovans. Like us, they were extremely tough, perhaps more so than Neanderthals. “I find Denisovan much more interesting,” said Emilia Huerta-Sánchez, a geneticist at Brown University.
What the Denisovan fossils lacked they made up for with DNA. Geneticists have been able to extract pieces of genetic material from teeth and bones dating back 200,000 years. They have found genetic clues on the cave floor. And billions of people on Earth carry Denisovan DNA, inherited from interbreeding.
This evidence offers a picture of an extraordinary human being able to thrive across thousands of miles and diverse environments, from frigid Siberia to high Tibet, to the jungles of Laos – perhaps even the Pacific islands. Their flexibility rivals our own.
“What we found about Denisovans is that, from a behavioral standpoint, they were much more similar to modern humans,” said Laura Shackelford, a paleoanthropologist at the University of Illinois.
Denisovans got their name from Denisova Cave in Siberia, where their remains were first identified. Russian paleontologists had been removing bone fragments from the cave floor for years when Dr. Kelso and other researchers offered to search for DNA in them.
A molar tooth between 122,700 and 194,400 years old contains Neanderthal-like genes. But the tooth’s DNA was different enough to suggest it came from a different branch of human evolution. A finger bone dating from 51,600 to 76,200 years old belongs to the same lineage, indicating that it has been around for tens of thousands of years, if not more.
Since then, researchers have found more Denisovan fossils in the cave, and they have also collected loose genetic material from the cave floor. The samples date from 200,000 to 50,000 years ago. A 90,000-year-old bone fragment comes from a Denisovan-Neanderthal hybrid, suggesting that the two groups were sometimes related.
Dr. Kelso and his colleagues quickly suspected that the Denisovans were not limited to Siberia. The researchers found that some pieces of ancient human DNA closely matched genetic material carried by people in East Asia, Native Americans, indigenous Australians, people in Papua New Guinea and other islands in the region.
When modern humans evolved from Africa about 60,000 years ago, Denisovans were certainly on their way to interbreeding and introducing some of their genes into our lineage. But it wasn’t until 2019 that scientists discovered the first traces of Denisovan fossils outside Siberia, in a high-altitude cave in Tibet.
Researchers there discovered part of a jaw dating back more than 160,000 years with Denisovan-like teeth. It also contains proteins with the molecular structure one might expect from a Denisovan, based on their genes. The following year, researchers reported that the cave floor contained Denisovan DNA.
In 2022, Dr. Shackelford and his colleagues made a discovery that could extend the Denisovan range into Southeast Asia, right in the path of modern humans in the initial wave out of Africa. In a cave in Laos, they found teeth about the same age as the Denisovan jaw, and matched the teeth embedded in it.
The Laotian tooth did not provide any DNA, but researchers have begun sifting through sediment in nearby caves. “We have a lot of DNA,” says Dr. Shackelford. “But we don’t yet know what all that DNA represents.”
Other researchers are surveying the Denisovan DNA inherited by living people. The mutation patterns documented so far suggest that several genetically distinct groups of Denisovans interacted with our ancestors. Moreover, none of these Denisovan groups had close relations with those who inhabited the Denisovan caves.
Some of the most interesting results come from studies on people in Papua New Guinea and the Philippines. They show signs of repeated events related to interbreeding with Denisovans that are different from those that occur in mainland Asia. Dr. Kelso and other Denisovan experts suspect that when sea levels were low during the last ice age, Denisovans may have made their way to Papua New Guinea and the Philippines, where they lived for thousands of years before modern humans arrived.
Thus, these findings suggest that Denisovans evolved in a very different environment. They survive the harsh winters of Siberia and the thin air of the Tibetan plateau. In Laos, Dr. Shackleford and his colleagues had discovered that Denisovans lived in open forests with herds of pygmy elephants and other mammals to hunt. And they may also have lived in the rainforests of Papua New Guinea and the Philippines.
Their flexibility differed from that of Neanderthals, which adapted to the cold climates of Europe and West Asia but did not expand elsewhere.
The Denisovans’ flexibility may have helped them survive for so long. People in Papua New Guinea may have inherited some Denisovan DNA from interbreeding just 25,000 years ago.
Dr. Shackelford said such findings raise the possibility that Denisovans and modern humans coexisted and interacted for tens of thousands of years – although it remains unclear whether they communicated. “It really goes down and out of the rabbit hole,” says Dr. Shackelford.
After the Denisovans disappeared, their genetic legacy lived on. Certain Denisovan genes are becoming more common because they provide an evolutionary advantage to modern humans. In Tibet, Dr. Huerta-Sánchez and his colleagues discovered a Denisovan gene that helps people survive at high altitudes. He also discovered that Native Americans carried the Denisovan gene for a mucus protein, although its benefit remained a mystery.
In Papua New Guinea, some Denisovan genes have been favored by people living in the lowlands, while others have been favored in mountainous areas. Low-datran genes appear to help fight infections. It’s possible that high rates of malaria and other diseases make those genes valuable.
But in mountainous areas, Denisovan genes with an evolutionary advantage are active in the brain. Michael Dannemann, an evolutionary geneticist at the University of Tartu in Estonia who led the study in Papua New Guinea, speculates that at high altitudes in Papua New Guinea, people may have faced periods of famine. “You may have to adapt parts of the body that use a lot of energy, and one that uses a lot of energy in humans is the brain,” he said.
Dr. Shackelford predicts that the search for more Denisovan fossils will be difficult, because the damp conditions in places like Laos do not favor skull survival. “I beg for a bone,” he said. “But I will still want bones for a long time.”
2024-03-02 12:23:24
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