The ancient origins of a mysterious jawbone dredged by commercial fishers off the coast of Taiwan have been revealed: it came from an enigmatic, extinct type of human called a Denisovan.
Ancient protein fragments surviving in the chunky mandible, dubbed "Penghu 1", show it belonged to a male Denisovan, who lived sometime between 10,000 and 190,000 years ago.
Denisovans, a long-dead cousin of modern humans, are represented by only a handful of fossils.And yet traces of their genes can be found in present-day people, including in Asian, Melanesian and Australian Aboriginal populations.
The latest discovery, published today in Science, adds to our understanding of Denisovan anatomy, and confirms they travelled far and wide, adapting to all kinds of environments.
Proteins help identify owner
When Penghu 1 was unveiled in 2015, researchers believed it came from a primitive species of human, but did not know which.
Yousuke Kaifu of the University of Tokyo, who was part of the original discovery and is a co-author on the new study, said the fossil was a "very thick mandible and large teeth", quite different to jawbones belonging to Neanderthals and modern humans, Homo sapiens.
But four years later, the discovery of the first Denisovan jawbone in the Baishiya Karst Cave on the Tibetan Plateau cast Penghu 1 in a new light.
"The Tibetan mandible was surprisingly similar to Penghu 1," Professor Kaifu said.
The researchers set out to see if Penghu 1's genome was also Denisovan. Unfortunately, ancient DNA easily degrades, and try as they might, the team could not extract DNA from the fossil.
But DNA is a blueprint for proteins. Just as species have genetic signatures unique to them, they also have unique protein variants.
The next step was to try to find ancient proteins, which survive in fossils longer than ancient DNA, in Penghu 1.
Professor Kaifu and colleagues teamed up with researchers at the University of Copenhagen, who had helped identify the Tibetan fossil, to extract and analyse protein fragments in Penghu 1's bone and dental enamel.
"We wanted to make sure by a molecular biological approach if that Taiwan mandible belonged to a Denisovan," Professor Kaifu said.
And it turns out it did. Their analysis found two protein variants known to only be present in Denisovans.
In addition, a protein coded for on the Y chromosome was found in tooth enamel in the fossil, revealing the Penghu 1 Denisovan was male.
Unfortunately, the researchers were unable to date the fossil directly, although other evidence told them Penghu 1 could be either be 10,000 to 70,000 years old or 130,000 to 190,000 years old.
Helping to pin down these windows of time were associated animal remains and known periods of low sea level when the Penghu 1 Denisovan could have lived on mainland Asia in what is now the Penghu Channel area.
An answer, but many questions remain
Identifying Penghu 1 as Denisovan solved one question, but raised many others.
Denisovans had larger teeth and jawbones than their sister-group, the also-extinct Neanderthals. But despite these apparent primitive traits, they also showed a more modern trait: a lack of wisdom teeth.
"It's an interesting combination," Professor Kaifu said, adding it was unknown how this unique morphology evolved.
Palaeoanthropologist Andy Herries of La Trobe University, who was not involved in the study, welcomed the latest addition to the scant fossil record for Denisovans.
"It's nice to have another jaw that is comparable to the Baishiya Karst Cave [Tibetan Plateau] one," Professor Herries said.
Penghu 1's robust mandible may be an anatomical relic of another ancient hominin, he added.
"It could be that Denisovans retained the morphology of the common ancestor of Neanderthals and Denisovans," he said.
Although, without better pinning down the age of the mandible, he said it would be hard to know.
Denisovans had 'remarkable adaptability'
Bastien Llamas, of the Australian Centre for Ancient DNA at the University of Adelaide, said the two Denisovan protein variants found in Penghu 1 were compelling evidence.
"[Those particular variants] don't exist in the Neanderthals. They don't exist in humans. They've only been detected so far in Denisovans," Dr Llamas says.
"Two doesn't sound like much. But when we're talking about proteins … it's pretty convincing."
So far, confirmed fossils of Denisovans are limited to a few fragments in the Denisova cave in Siberia, along with the Tibetan jawbone and a rib from the same site.
Dr Llamas said the findings of the latest study had implications for our understanding of the spread of this enigmatic ancient human, and their ability to cope with different environments — from cold, dry and high-altitude environments to hot and humid areas.
"The fact that we have Denisovans in Siberia, in Tibet and now in Taiwan really shows that they have a pretty remarkable adaptability potential," he said.
The findings are also consistent with modern human DNA research, suggesting Denisovans mixed with modern humans and their genes were carried into the islands of South-East Asia and beyond.
Dr Llamas points to research showing people on the island of Luzon in the Philippines, close to Taiwan, have the highest level of Denisovan DNA in their genome.
"To have Denisovan [fossils] in Taiwan and then to have the humans carrying the most Denisovan DNA in their genomes in the next island south, I found that quite interesting."
