There are four living genera of gibbon. Researchers now think they’ve found a fifth.Credit: AY Images/Getty

An ape skull found in a Chinese tomb might belong to a previously unknown gibbon — one that was driven to extinction within the past two millennia, researchers report on 21 June in Science1. Although several other kinds of primates have disappeared during this time, the gibbon would become the first ape known to have vanished since the last ice age ended, 12,000 years ago.

In 2004, archaeologists discovered a large tomb near Xi’an in Shaanxi province, central China, dating to between 2,200-2,300 years old. They speculated that it belonged to Lady Xia, grandmother of China’s first emperor, Qin Shihuang — whose impressive mausoleum, with its Terracotta Army, lies around 50 kilometres to the northeast.

Inside the Xi’an tomb were the remains of an ancient menagerie that included skeletons of leopards, bears and cranes — and the skull and jaw of a gibbon. It’s unusual to find ancient gibbon bones in China, so a team led by conservation biologist Samuel Turvey at the Zoological Society of London and Helen Chatterjee, an evolutionary biologist at University College London, looked at the bones to see which gibbon species it belonged to.

A careful examination of the shape of the skull showed that the braincase is much larger, and the cheekbones narrower, than those of any of the four known living genera of gibbon and the closely related, but larger, siamang (Symphalangus syndactylus). The molars of the ancient gibbon are distinct, too, with grinding surfaces bigger than those of most gibbons, yet smaller than those of siamangs.

The skull of Junzi imperialis was found in an ancient tomb in China.Credit: Samuel Turvey/ZSL

These differences, Chatterjee says, “are significant enough, we argue, to warrant a new genus and species”. Her team named the gibbon Junzi imperialis, which means imperial man of virtue or noble character — a reference to the high regard in which gibbons have traditionally been held in China.

The team can say little else about J. imperialis: there are no clues as to where it fits in the gibbon evolutionary tree, or even where else it lived, or for how long. If J. imperialis was native to China, it’s possible that its extinction was related to environmental pressures caused by humans, says Chatterjee, because human populations in China have expanded rapidly over the past two millennia.

Urbanization increased quickly in China, and historical sources from the last few hundred years indicate that gibbons have experienced drastic range contractions as a consequence, says Chatterjee. Junzi imperialis might have been a casualty of those contractions.

Other researchers, however, are not yet convinced that the remains deserve to be put into a new genus. The fossil record of gibbons and siamangs is scant, says Ulrich Reichard, an anthropologist at Southern Illinois University in Carbondale. That means it’s difficult to know how the four living genera — Hoolock, Hylobates, Nomascus and Symphalangus — might have evolved through the millennia. There are three species of Nomascus gibbon in China, including the black crested gibbon (Nomascus concolor), and J. imperialis could instead belong in the same genus as them.

Terry Harrison, a biological anthropologist at New York University, says a new gibbon would be of “immense importance”, but he isn’t even sure that the specimen belongs to a distinct species. It’s possible the ancient ape was a captive animal, given that it was found in a tomb. Animals reared in captivity often have bone problems and experience abnormal growth, he adds. “This could account for all of the facial differences that the authors indicate as distinctive.”

Turvey is open to the possibility that the gibbon was a captive animal. But “it could equally have been a wild animal that was killed for the tomb menagerie," he says.

China is still home to some 24 species of primate, says Paul Garber, an anthropologist at the University of Illinois in Urbana-Champaign, but 80% of them are under threat. This includes the Hainan gibbon (Nomascus hainanus), thought to be represented by about 26 individuals squeezed into less than 20 square kilometres of forest on China’s Hainan Island. Evidence that a Chinese gibbon species has been driven to global extinction in the past two millenia would be important for highlighting the conservation issues that China faces today, he says.