The animal’s placement in the tomb suggests it was kept as a pet. Such a practice could have been devastating to the species, and may explain why it went extinct, said Dr. Cheyne, an associate lecturer in primate conservation at Oxford Brookes University.

The gibbon probably would have been captured as a youngster, she said, because they are particularly “small and cute and fluffy.” That could have meant killing its mother, “potentially impacting the social structure of entire group, which may not survive the loss of an adult,” Dr. Cheyne said. “So each live individual being kept as a pet certainly represents a bigger loss of individuals from the wild.”

Its presence in the tomb strongly suggests that humans played a role in the species’ extinction, she said.

The field of gibbon research has taken off in recent years, Cheyne said, with eight new species of living gibbons discovered since 2000 and two just in the last two years. “It goes to show how much we still have to learn about these animals,” she said.

The 20 known living species include, in China, the recently discovered Skywalker gibbon, and the Hainan, which is found only in a small part of Hainan Island, off the southern tip of China. It is considered the most endangered mammal on earth, with fewer than 30 known individuals left in the species, she said.

Dr. Cheyne said that conserving a species requires a multipronged effort, including the end of hunting and the preservation of habitat.

Jo Setchell, a professor of anthropology at Durham University and president of the Primate Society of Great Britain, who was not involved in the work, said the discovery provided new insights.

“The broader message is that we might have underestimated the number of primate extinctions caused by humans in the past,” she wrote in an email. “Understanding past extinctions will help us to predict how vulnerable current species are, and therefore help us to protect them more effectively.”