A Fossil Hunter Stumbled Upon a Jawbone. It Just Might Rewrite the Story of Human Migration.
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A jawbone fossil from a previously unknown hominin was finally identified as belonging to the elusive Denisovans—a species whose only other fossils have come from Siberia and Tibet.
Though DNA sequencing of the fossil (known as Penghu 1) was not possible because of a lack of collagen, its proteome—all the proteins that occur in an organism or tissue—showed links to Denisovans.
Penghu 1 is the first fossil evidence that Denisovans did not only inhabit regions with cold climates, but spread throughout East and Southeast Asia.
When amateur fossil hunter Kun-Yu Tsai was browsing an antique market in Southern Taiwan in 2008, he eyed a jawbone fossil that looked suspiciously (but not exactly) human. Told it had surfaced from the waters of the Penghu Channel in fishermen's nets, Tsai felt compelled to buy the fossil, and donated it to Taiwan's National Museum of Natural Sciences. He was right about the mandible belonging to a human ancestor, but what he didn't know was that he had stumbled on an ancient human enigma.
That same year, archaeologists searching for fossils in the Altai Mountains of southern Siberia found a finger bone and molar embedded in the permafrost of Denisova Cave. Sequencing of the finger's mitochondrial DNA (mtDNA) revealed that it was from neither a Neanderthal nor modern human, but there was still not enough information to determine the species. Only in a later study was most of the genome was sequenced, and the species was determined to be completely new to science. These new hominins were called Denisovans.
Almost twenty years after Tsai's chance find, the mysterious jawbone he picked up has now also been identified as Densiovan. It is one of very few Denisovan fossils to have ever been unearthed, along with those discovered in Denisova Cave (more fragments were found since the finger bone and molar) and another mandible and rib from Baishya Karst Cave in Xiahe County, China. Researchers from the Graduate University for Advanced Studies in Japan and the University of Copenhagen were finally able to identify the jawbone as Denisovan through paleoproteomics, which involves analyzing ancient proteins.
'Mandibular bone and dental enamel were identified as the most promising tissues for extraction, with richer and better preserved protein profiles,' the research team, led by Takumi Tsutaya, wrote in a study recently published in Science.
Previous attempts to study the DNA of the specimen—now known as Penghu 1—were unsuccessful, as there was not enough collagen for carbon dating. Uranium dating also failed as a result of interference from uranium deposited by seawater. Instead, the researchers looked at the specimen's proteome (all the proteins expressed by an organism), and were able to identify 4,2421 amino acid residues from 51 proteins. There turned out to be five proteins showing variations that were either specific to Denisovans or had phylogentic associatiations with them. The only reference genome sequence for Denisovans comes from Denisova 3, the finger bone specimen from Siberia, and Penghu 1 was phylogenetically close.
Tooth enamel revealed that Penghu 1 came from a male individual, and both Penghu 1 and Xiahe 1 may have something to do with the way Densiovan morphology is perceived. Densiovans have been characterized as having a robust and low jaw, but the only mandibles known to exist are male. Jawbones in Neanderthal fossils are higher and more gracile. It is possible that some other fossils thought to be Denisovan candidates could be females, and there may be others which have been misidentified as other hominin species because of a more gracile jaw. The finger bone and molar from Siberia are both female—but no jawbone was ever found.
The age of Penghu 1 remains unknown, except that it is probably younger than 450,000 years. Because it emerged from the ocean, there was no way to study the surrounding sediment and determine an age, so the only way to guess at what time period it could be from is using what is already known about ancient sea levels. During the Pleistocene, sea levels were low enough for Penghu 1 to have on been part of the mainland between either 130,000 and 190,000 years ago or 10,000 and 70,000 years ago.
Submerged areas that used to be above water could explain why Densiovan DNA has been found in peoples of mainland and island Southeast Asia, and even as far as Australia. Aboriginal peoples of Australia carry Densiovan DNA, suggesting a widespread migration, but there was no fossil evidence of these hominins having lived in warmer regions until now. Densiovans were thought to be endemic to cold climates when the only fossils we had found were those from Siberia and Tibet, but Penghu 1 speaks to their ability to adapt to a much wider range of environments—from frozen caves to lush, humid tropics to parched desert lands.
'The identification of Penghu 1 as a Denisovan mandible confirms the inference from modern human genomic studies that Denisovans were widely distributed in eastern Asia,' Tsutaya's team said in the same study.
Next time you find yourself meandering around an antique shop, you might want to ask about any fossils that appear even remotely human. Apparently, there's a chance that's where more of our ancient history is hiding.
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