For decades, fishermen sailing off the coast of Taiwan have occasionally discovered bones from other large mammals that lived tens of thousands of years ago, such as elephants, buffaloes, and other large mammals.

However, in 2010, Taiwanese paleontologists were presented with a particularly strange discovery. This fossil looks like half the jaw of a gorilla. Scientists have been baffled by it ever since.

The mystery of the underwater jaw has now been solved. Wednesday, the researchers team made an announcement that it was part of Denisovan, a member of the mystical human lineage associated with the Neanderthals. This finding greatly expands the range of well-identified denisovan fossils previously known from Siberia and Tibet.

“Indeed, Denisovan was east all the way to the coast,” says Frido Welker, a molecular anthropologist and author of the new study.

Chun-hsiang Chang, a paleontologist at the National Museum of Natural Sciences of Taiwan, first learned about the jaws from a private collector in 2010. After examining it, he quickly determined that it did not belong to a gorilla, as gorillas and other apes have U-shaped jaws. Instead, the fossil jaws were angled outward from the jaw, just like ours.

However, the jaws were missing the prominent jaws seen in modern-day humans. “At the time, I thought it looked human, but not like modern-day human,” Dr. Chang said. “I thought it was very important, so I pushed the private collector to lend it to my museum.”

Over the next five years, Dr. Chang studied the jaw anatomy, working with an international group of scientists. Its shape resembles the jaws of extinct relatives of humans known to have lived in Asia for over a million years. However, Penghu 1 also had distinctive features, including large teeth.

Determining the age of Penghu 1 was also a challenge, as they were not sure exactly where Dr. Chang discovered it on the seabed. He and his colleagues analyzed the chemistry of the jaw and discovered that it resembles that of fossils of hyena species that evolved in East Asia about 400,000 years ago.

At that time, Taiwan was separated from the mainland by water. However, 190,000 years ago, sea levels fell sufficiently to create a land bridge that lasted until 130,000 years ago. The ocean then rose again until 70,000 years ago. This pattern continued until 10,000 years ago when another land bridge formed. Dr. Chang and his colleagues believe that Penghu 1 lived in one of these periods when sea levels were lower.

One possibility was that Penghu 1 belonged to a mysterious group of humans called Denisovans. Researchers discovered Denisovan in 2010 while examining fossils from the Siberian Denisova Cave. The bones of teeth and fingers contain ancient DNA with unusual mutations, revealing previously unknown human strains.

Subsequent research shows that Denisovans, Neanderthals, and modern humans share common ancestors who lived in Africa about 600,000 years ago. The Neanderthal and Denisovan ancestors migrated from Africa, and then these two lineages split about 400,000 years ago, with the Neanderthals spreading to Europe in the west.

Denisovan’s spread was difficult to chart. For years, the only known Denisovan fossils have been teeth and bone fragments found in Denisova caves. However, a valuable clue comes from living humans. Many people in East Asia and the Pacific today have small amounts of Denisovan DNA, suggesting that Denisovans must have interbred with East Asian Homo sapiens before their extinction and lived far beyond Siberia.

Dr. Chang and his colleagues noticed that the teeth in the Penghu 1 jaw resemble the teeth found in the Denisova Cave. But those clues were not enough to link them. They attempted to search for DNA in their jaws but found nothing. This was not surprising given that the Penghu 1 fossils had been sitting on the seabed for thousands of years.

After Dr. Chan’s team revealed their analysis in 2015, Penghu 1 became an even more enigmatic, human-like fossil at the museum. “Our research was stagnant,” Dr. Chan said.

Over the next few years, Dr. Welker and other researchers pioneered ways to recover ancient proteins from fossils. They discovered that even if a fossil loses all its DNA, it could still retain protein fragments.

Using such methods, Dr. Welker studied the 160,000-year-old jaws found in a high-altitude cave in Tibet. In 2019, the team reported that Tibetan fossils contained fragments of ancient collagen and other proteins.

These proteins resemble modern human proteins but also exhibit differences indicating that the jaw belonged to a Denisovan.

With that discovery, Dr. Welker searched other Asian fossil scientific literature for jaws resembling the Tibetan jaws that could be tested for proteins.

“That’s when the lower jaw of Penghu came on my radar,” he said.

Both the Tibetan and Taiwanese jaws had very large teeth. Dr. Welker and his colleagues reached out to Dr. Chang. In 2023, Dr. Chang and his team flew to Copenhagen with their jaws. Analysis of the results revealed protein fragments that were only present in Denisovans.

“Since the first publication of Penghu’s lower jaw, many of us thought it might be Denisovan, mainly based on being in the right place at the right time,” said Bens Viola, a paleontologist at the University of Toronto, who was not involved in the study. “But of course, assumptions are assumptions, and we need actual data to validate them.”

Another clue came from fragments of enamel protein in the teeth. Modern humans carry enamel genes on the X chromosome, while men carry slightly different enamel genes on the Y chromosome. Denisovan was carrying the Y-chromosome version, indicating that it belonged to a male adult.

The two Denisovan jaw anatomy may turn out to be a hallmark of male Denisovan. Women’s Denisovan may have a thinner anatomy, but scientists will need more evidence to know for sure.

The discovery of other Denisovan fossils could expand the scope of humanity. In 2022, researchers found teeth from 160,000 years ago in a cave in Laos. They resemble the teeth of the jaw in Tibetan Denisovan. Anyone who lived there would have had to survive in the tropical forests far from Siberia.

However, Laos’ teeth do not contain DNA, and the protein fragments did not clarify the type of human it belonged to. However, the cave and its neighbors still retain many teeth that have not been fully analyzed. Other traces of Denisovan may not have been found in the museum yet.

But so far, evidence reveals that Denisovans were able to thrive for thousands of miles and in a variety of environments. Janet Kelso, a paleontologist at the Max Planck Institute for Evolutionary Anthropology, was impressed by the differences between the Penghu 1 protein and the protein found in Tibet. As Denisovans expanded throughout different environments, they adapted and became genetically distinct populations.

“There’s still a lot to learn about Denisovan,” she said.