“There are distinct species that have been around for millions of years, even though they look to our eyes very similar,” said Dr. Yoder. Unfortunately, she noted, new species revealed by DNA often turn out to exist in perilously low numbers.

The new research was not all bad news for primates.

“Some species are doing O.K.,” said Katherine C. MacKinnon, an anthropologist at Saint Louis University and a co-author of the study. “The ones that are doing O.K. are the ones that aren’t super-specialists, the ones that are most flexible.”

But most species are not so flexible. Every species of ape (including gorillas, chimpanzees, bonobos, orangutans and 19 species of gibbons) is threatened, while 87 percent of lemur species are. Other species that are critically endangered include the brown-headed spider monkey of Ecuador, the Niger Delta red colobus, and the crested macaque, an Indonesian species famous for having taken a selfie with a photographer’s camera.

“It’s worse than we thought 10 years ago,” Dr. MacKinnon said.

She and her colleagues identified a number of human activities pushing primates to the edge, such as hunting. In West Africa, for example, there is a strong demand in local markets for primate meat.

“The forests are still standing, but they’ve shot everything out of it,” said Dr. Rylands.

The incentives to kill primates are not only local, though. A lot of primate meat is making its way to China, along with body parts falsely believed to have healing powers.

“They’ll import enormous amounts from around Southeast Asia,” said Dr. Rylands. “They’re a driving force through the whole region.”

Primates are also threatened by the wholesale destruction of forests to make way for agriculture. In the Amazon, the jungle is being converted to cattle ranches and soybean fields, while in Madagascar, rice paddies are taking the place of lemur forests.