A fossil jaw originally netted by fishermen off the ocean floor near Taiwan’s west coast belonged to a member of a mysterious hominid population known as Denisovans, scientists report in the April 11 Science.
Their new findings indicate that Denisovans, known from their ancient DNA and a handful of bones found at a couple of Asian sites, spread over a larger area than previously thought.
DNA from fragmentary bones and teeth excavated in Siberia’s Denisova Cave first identified Denisovans as close relatives of Neandertals in 2012. Researchers determined that Denisovans visited the Siberian site from around 300,000 to 50,000 years ago. Although their evolutionary status remains unclear, Denisovans mated with Neandertals, and some modern East Asian populations have inherited Denisovan genes as a result of interbreeding with ancient Homo sapiens.
The newly identified Denisovan jaw, along with a range of ancient animal fossils, was netted by commercial fishermen before ending up in an antique shop. A Taiwanese man bought the hominid fossil in 2008 and then donated it to his country’s National Museum of Natural Science, where scientists began studying it.
Researchers who first examined the Taiwan fossil dubbed it Penghu 1 and classified it as an unknown Homo species.
Two lines of evidence now put Penghu 1, a right lower jaw that retains four cheek teeth and a partial canine tooth, in the Denisovan camp, say biological anthropologist Takumi Tsutaya and colleagues. Tsutaya, who did the work while at the University of Copenhagen, now is at the Graduate University for Advanced Studies, in Hayama, Japan.
First, in an analysis of 4,241 protein residues extracted from the fossil, two displayed a chemical structure previously reported as common among Denisovans but absent in Neandertals and rare in people today, the researchers report.
Penghu 1 has not yielded any DNA. But chemical changes to proteins, which are produced by genes, occur infrequently. Thus, a mere pair of Denisovan-related protein changes detected in the Taiwan jaw were enough to indicate that it came from a Denisovan, Tsutaya’s group says. In addition, the investigators found a protein marker of male sex in tooth enamel from the Taiwan jaw.
Retrieving any information about protein variation from a fossil of unknown origin yanked out of the ocean “is an important step we couldn’t have taken even eight or nine years ago,” says paleoanthropologist Sheela Athreya of Texas A&M University in College Station, who was not part of Tsutaya’s team.
A second line of evidence comes from Penghu 1’s anatomy, which resembles a Denisovan jaw found on the Tibetan Plateau in Xiahe, China. Both jaws sat low in the mouth and featured thick bones, large molars and distinctively shaped tooth roots.
Attempts to pinpoint Penghu 1’s age have failed. Prolonged exposure to seawater and loss of bone collagen have prevented direct dating of the fossil. The researchers suspect the find dates to either of two time periods when glacier formation due to cold temperatures lowered sea levels enough to connect Taiwan to mainland Asia. One of those periods ran from 70,000 to 10,000 years ago. The other ranged from about 190,000 to 130,000 years ago.
Penghu 1’s Denisovan debut shows that this now-extinct population adapted not only to long, cold winters at Siberia’s Denisova Cave and to thin air atop the Tibetan Plateau but to mild, rainy conditions about 4,000 kilometers southeast of Denisova Cave, Tsutaya says.
One member of Tsutaya’s team, paleoanthropologist Yousuke Kaifu of the University of Tokyo, suspects that Denisovans occupied most of Central and East Asia. Existing Asian fossils suggest that Denisovans displayed regionally distinctive looks, Kaifu says.
But no scientific consensus exists on what Denisovans looked like, whether they represented a separate Homo species or how they may have contributed to the evolution of present-day people.
Paleoanthropologist Xiujie Wu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing considers Penghu 1 and other Denisovan fossils members of a new species, Homo juluensis. Fossils from two other Chinese sites, dating to between roughly 200,000 and 105,000 years ago, provide the best evidence for this species, Wu says, including exceptionally large braincases.
Athreya considers that proposal premature. Scientists have found too few, mostly fragmentary fossils classified as Denisovan to conclusively identify Denisovan skeletal features, she says. And the most complete fossils assigned to Denisovans have not yielded ancient DNA. Those fossils might have belonged to another Asian species, such as Homo erectus, she cautions.
“Until we know who the fossils called Denisovans were, we can’t know their fate or their relationship to Homo sapiens,” Athreya says.