On the trail of the Denisovans

ORIGIN

DNA has shown that extinct humans thrived around the world, from cold Siberia to high-altitude Tibet – perhaps even on the Pacific islands.

Neanderthals may have disappeared 40,000 years ago, but they are no strangers to us today. Their stocky skeletons shine in museums around the world. Their imagined personas star in television advertising. When Kevin Bacon noted on Instagram that its morning habits resemble those of a Neanderthal, but he didn’t stop to explain that our ancient cousins ​​interbred with modern humans who expanded from Africa.

But no such prominence exists among the Denisovans, a group of people who split from the Neanderthal lineage and survived for hundreds of thousands of years before becoming extinct. That’s largely because we have so few of their bones. In a new overview article anthropologists count all fossils clearly identified as Denisovan since the first discovery in 2010. The whole list consists of half a broken jaw, a finger bone, a skull fragment, three loose teeth and four other pieces of bone.

“The bits of Denisovan we have, it’s almost nothing,” says Janet Kelso, a paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who helped write the review.

Nevertheless, many scientists are becoming increasingly fascinated by Denisovans. Like us, they were extremely resilient, perhaps even more so than Neanderthals. “I find Denisovans much more interesting,” says Emilia Huerta-Sánchez, a geneticist at Brown University.

What the Denisovans lack in fossils, they make up for with DNA. Geneticists have managed to extract bits of genetic material from teeth and bones that are 200,000 years old. They have found genetic clues in the dirt of cave floors. And billions of people on Earth carry Denisovan DNA, inherited through interbreeding.

The evidence offers a picture of remarkable people who were able to thrive across thousands of miles and in diverse environments, from cold Siberia to high-altitude Tibet to the forests of Laos – perhaps even on the Pacific islands. Their versatility is comparable to ours.

“What we found about the Denisovans is that from a behavioral perspective, they were much more like modern humans,” says Laura Shackelford, a paleoanthropologist at the University of Illinois.

The Denisovans take their name from the Denisova Cave in Siberia, where their remains were first identified. Russian paleontologists had been extracting pieces of bone from the cave floor for years when Dr. Kelso and other investigators offered to search them for DNA.

A molar tooth somewhere between 122,700 and 194,400 years old contained Neanderthal-like genes. But the tooth’s DNA was clear enough to suggest it came from a separate branch of human evolution. A finger bone that was 51,600 to 76,200 years old belonged to the same lineage, showing that it had been around for tens of thousands of years, if not longer.

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 belonged to a Denisovan-Neanderthal hybrid, showing that the two groups sometimes interbred.

Dr. Kelso and her colleagues soon began to suspect that the Denisovans had not been confined to Siberia. The researchers found that some stretches of DNA from ancient humans closely matched genetic material carried by people in East Asia, Native Americans, Aboriginal Australians and people in New Guinea and other islands nearby.

When modern humans expanded out of Africa some 60,000 years ago, the Denisovans must have been on their way to interbreed and introduce some of their genes into our lineage. But it wasn’t until 2019 that scientists found the first fossil trace of Denisovans outside Siberia, in a high-altitude cave in Tibet.

Researchers there found part of a jaw that is more than 160,000 years old and resembles Denisovan. It also contained proteins with a molecular structure that you would expect from a Denisovan based on their genes. The following year, researchers reported that the cave floor had been eroded contained Denisovan DNA.

In 2022, Dr. Shackelford and her colleagues made a discovery that could expand the Denisovan area into Southeast Asia, right on the path of modern humans in their early waves from Africa. In a cave in Laos, they found a tooth about the age of the Denisovan jaw, which matched the tooth inside.

However, the Laotian tooth yielded no DNA, so researchers began scouring sediments in nearby caves. “We have a lot of DNA,” said Dr. Shackelford. “But we don’t know yet what all that DNA stands for.”

Other researchers are examining the Denisovan DNA inherited by living humans. The pattern of mutations documented so far suggests that several genetically distinct groups of Denisovans interbred with our ancestors. Furthermore, none of these Denisovan groups were closely related to those who occupied Denisovan Cave.

Some of the most intriguing results come from studies of people in New Guinea and the Philippines. They show signs of repeated cases of interbreeding with Denisovans that were different from what happened in mainland Asia. Dr. Kelso and other experts on Denisovans suspect that when sea levels were low during the last Ice Age, Denisovans may have wandered to New Guinea and the Philippines, where they lived for thousands of years before modern humans arrived.

Taken together, these findings suggest that Denisovans thrived in vastly different environments. They endured the harsh winters of Siberia and the thin air of the Tibetan plateau. In Laos, Dr. Shackleford and her colleagues discovered that Denisovans lived in open forests with herds of pygmy elephants and other mammals to hunt. And they may have lived in rainforests of New Guinea and the Philippines.

That flexibility is in stark contrast to Neanderthals, who adapted to the cold climate of Europe and western Asia but did not expand elsewhere.

The Denisovans’ versatility may have kept them going for a long time. People in New Guinea may have inherited some Denisovan DNA through interbreeding only 25,000 years ago.

Dr. Shackelford said such findings have raised the possibility that Denisovans and modern humans coexisted and interacted with each other for tens of thousands of years — although it is unclear whether they communicated with each other. “That really goes down the rabbit hole,” said Dr. Shackelford.

After the Denisovans disappeared, their genetic legacy lived on. Certain genes of Denisovans are more common because they provide an evolutionary advantage in modern humans. In Tibethave Dr. Huerta-Sánchez and her colleagues found a Denisovan gene that helps people survive at high altitudes. She found that too Native Americans carry a Denisovan gene for a mucus protein, although its benefit remains a mystery.

In New GuineaSome Denisovan genes are preferred by people living in the lowlands, while others are preferred in the highlands. The lowland genes appear to help fight infections. It is possible that the high rates of malaria and other diseases make these genes valuable.

But in the highlands, the Denisovan genes with the evolutionary advantage are active in the brain. Michael Dannemann, an evolutionary geneticist at the University of Tartu in Estonia who led the New Guinea study, speculated that people at high altitudes in New Guinea may face periods of food shortages. “You may have to modify parts of the body that use a lot of energy, and one part that uses a lot of energy in humans is the brain,” he said.

Dr. Shackelford predicted that the search for more Denisovan fossils would be difficult because the humid conditions in places like Laos do not favor the survival of skeletons. “I’m begging for bones,” she said. “But I’ve been wanting bones for a long time.”