A long-forgotten species of ape has been found buried in a 2,300-year-old tomb. It's a type of gibbon, which scientists named Junzi imperialis.
Gibbons are the smallest apes, chatty and as lanky-limbed as Kermit the Frog. They're also more closely related to humans than they are to any monkey.
And humans, the scientists say, are the likely agents of these gibbons' extinction.
Archaeologists excavated the burial site, in the ancient Chinese capital city of Chang'an, now part of modern Xi'an, in 2004.
The tomb is about 2,300 or 2,200 years old, and is possibly the final resting place of Lady Xia, grandmother of the Qin dynasty's first emperor.
The tomb contained several dead exotic animals in 12 pits, including a leopard and a bear, befitting a member of the ancient Chinese elite. Among these remains, excavators found a small jawbone and skull with prominent canine teeth.
The gibbon bones wound up in a museum drawer until Samuel Turvey, at the Zoological Society of London, plucked them out of obscurity.
"It's just luck that Sam found this specimen and immediately suspected it was a gibbon," Chatterjee said.
Turvey scanned the gibbon bones and sent the images to Chatterjee. With their students, the scientists began to pick apart the gibbon's features. Their analysis "revealed it to be significantly different from living gibbons," Chatterjee said.
Junzi imperialis had a steeper forehead than other gibbons, narrower cheekbones and more slender brow ridges, said Alejandra Ortiz, an anthropologist at Arizona State University and a co-author of the report. Its molars were unusually sized, too.
All of these features combined, the authors say, make a strong case that the gibbon is not just a new species but a new genus. (A genus, you'll recall, ranks above a species - it's the Homo in Homo sapiens.) Living gibbons are split into 20 species over four genera.
"There's good reason to believe this represents a new species of gibbon," said anthropologist Paul Garber, a professor emeritus at the University of Illinois who has studied gibbons in China and was not involved with this report.
Whether it's a new genus is tough to say, he said, based on one specimen.
What's more critical, in Garber's mind, is the gibbon's extinction.
In China, wild gibbons stick to the dense forest canopies of the southwest. One species, the Hainan gibbon, lives at the nation's southernmost point; there are only 30 of these apes left, making them one of the rarest mammals alive.
The Chang'an tomb, in the central province of Shaanxi, is 750 miles (1,200 km) from the nearest known gibbon habitat.
Shaanxi is mountainous, Garber said, and though macaques and snub-nosed monkeys live there, no gibbons do.
It's possible, Ortiz said, that "Lady Xia's gibbon was transported to Chang'an as a trade item or tribute."
(Ortiz pointed to old Chinese texts referring to the animals as "elegant" and symbols of "gentlemen.")
But the study's authors say Junzi imperialis could have been a local. Except for the gibbon, the other mammals found in the tomb still occur in Shaanxi.
"Gibbons had much wider ranges in the past," Chatterjee said.
"It is unlikely specimens such as Junzi would have traveled this far just by humans." Chatterjee and her colleagues suspect there are more Junzi bones in the area, waiting to be found. "We are keen to find them."
The scientists cannot say with certainty that humans wiped the gibbons off the planet. They just think it's the most likely hypothesis. (The current study of this species, after all, depended on its cultural value to long-dead humans.)
And although we might think of ecological loss as a modern problem, ancient Chang'an had a dense human population. "We have been a threat for quite a while," Ortiz said.
"Probably more than any country in the world, China has transformed its landscape," Garber said. Two thousand years ago, the Han dynasty had an estimated population of 60 million people, a quarter of the world's total.
Primate habitats shrank dramatically in China over the past two millennia. In September, Garber published a paper based on historical records of snub-nosed monkeys, taken from texts as old as 1 BCE.
As the population of China boomed from the 1700s onward, references to snub-nosed monkeys in eastern and central China vanished completely.
Gibbons, who consume mostly fruits, are especially ill-equipped for shrinking forests. Because they rarely descend from the canopy, when forests splinter, the apes remain boxed in.
Their ability to cross open gaps to between habitats, Ortiz said, is "extremely limited".
"The Junzi find is a sobering lesson in the devastating effects that humans can have on the natural world," Chatterjee said.
"Nature cannot keep up."
The primate vanishing act has not stopped with Junzi. "Unless things dramatically change over the next 25 to 75 years, there will be a major primate extinction crisis," Garber said.
"Worldwide, 60 percent of primates are threatened, endangered or critically endangered."
China still has the opportunity to enact better policies that protect living primates, he said. But that window won't stay open forever.
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