The 2,200-year-old tomb of Lady Xia, the grandmother of China’s first emperor Qin Shihuang, basically contains a zoo. Originally found in 2004, the tomb's dozen pits hold the bones of cranes, a lynx, a leopard, and an Asiatic black bear, among other animals. It also contained an unusual gibbon skull. Now, reports Maya Wei-Haas at National Geographic, researchers have determined that the gibbon is from an unknown and now extinct genus of the animal.

In 2009, gibbon expert Samuel Turvey of the Zoological Society of London (ZSL) was touring a museum in China when the fossil skull caught his eye, reports Karen Weintraub at The New York Times. It looked slightly different from the skulls of other gibbons, so the ZSL and University College London evolutionary biologist Helen Chatterjee decided to take a closer look to figure out just what species of gibbon it was.

The team created a database of skull and teeth dimensions from the four known genera of gibbons, the world’s smallest apes, measuring 789 teeth and 477 skulls. They then compared the Chinese fossil with that database, finding that in the Chinese fossil, the brain is larger, the cheekbones are narrower and that the grinding surface of its molars are larger.

The differences are enough to declare the animal a previously unknown genus and species of gibbon, dubbed Junzi imperialis. The description appears in the journal Science.

Colin Barras at Nature reports that other than those characteristics, researchers know nothing about the gibbon, like where it fits in the gibbon family tree, whether it came from China and why it went extinct.

Not everyone is convinced, however, that Lady Xia’s gibbon represents a new species. Terry Harrison, a biological anthropologist at New York University, tells Barras that it’s possible the "new species" was a captive animal. Captive animals often show growth abnormalities.

DNA analysis might have been able to solve the mystery, but the fossil was too fragile and museum officials would not allow the team to take a sample.

If it is a new species, its extinction is its most significant trait. Though ape species are under tremendous pressure from human activity, researchers believe we have not yet caused the extinction of any ape. It’s possible that Junzi changes that. “Our discovery and description of Junzi imperialis suggests that we are underestimating the impact of humans on primate diversity,” lead author Turvey says in a press release. “These findings reveal the importance of using historical archives such as the archaeological record to inform our understanding of conservation and stress the need for greater international collaboration to protect surviving populations of gibbons in the wild.”

Wei-Haas reports that the fossil suggests that gibbons may have lived in central China 2,000 years ago. According to historical records, they disappeared from the area around 300 years ago, when their forest habitats were cut down to create more agricultural land. Trapping and the pet trade may have also doomed them. “There was the idea that apes in the past have been somewhat resilient to anthropogenic pressures and incidental habitat loss,” co-author Alejandra Oritz of Arizona State University says. However, the loss of Junzi proves otherwise.

Many of the same pressures are pushing the remaining gibbon species, which live in eastern and southeast Asia including China, to the brink as well, with deforestation and the pet trade having huge impacts. “What we can start to see is that [modern gibbon species are] a relic of what was perhaps a much wider radiation of gibbons and primates across Asia,” James Hansford of the Zoological Society of London tells Wei-Haas. “We’ve lost more and more and more of them. We can't even quantify what we lost because we don't have the records of it.”

“The broader message is that we might have underestimated the number of primate extinctions caused by humans in the past,” Jo Setchell, president of the Primate Society of Great Britain tells Weintraub. “Understanding past extinctions will help us to predict how vulnerable current species are, and therefore help us to protect them more effectively.”

In fact, the 20 extant species of gibbons are some of the most endangered animals on Earth. The Hainan gibbon, discovered two years ago on China’s Hainan island, is the rarest mammal in the world, with only 25 animals remaining. The Skywalker gibbon, also native to China, was just discovered in the Gaoligong mountains last year and is also considered endangered.