What difference does eight dead gorillas make? As Newsweek reported exactly a year ago, when poachers slaughtered eight of the world’s last remaining mountain gorillas—including in Congo's supposedly protected Virunga National Park—it cruelly highlighted a threat that conservationists thought was behind them: illegal hunting. It did something else, too. Scientists had been considering reclassifying the mountain gorilla (Gorilla beringei beringei) from critically endangered to endangered. But the brutal killings underscored just how fragile even the best-intentioned wildlife-conservation efforts are, and the mountain gorilla therefore remains critically endangered. (Click here to see a gallery of the world's vanishing primates.)

For at least two decades the dogma in the conservation world had been that habitat destruction, not hunting, posed the gravest threat to the world’s rare animals. But the slaughter in the jungle was just the most notorious wake-up call that poaching was back, and in a way that threatened to send an unknown number of species into extinction. After the murders of the Congo gorillas, scientists realized that the proposed reclassification would be premature: mountain gorillas are still critically endangered and, according to the first comprehensive review in five years of the world’s primates, they are not alone. Monkeys, apes and other primates are disappearing from the face of the Earth. And as with Congo’s gorillas, a key cause is hunting. Our cousins are being eaten into extinction.

A report issued this evening from the 22nd International Primatological Society Congress in Edinburgh, Scotland, concludes that 48 percent of Earth’s primates are in danger of going extinct. Of the 634 known primate species and subspecies, 303 are at risk of extinction. Ninety-seven species and subspecies are vulnerable, the least-bad of the worst; 137 are endangered, and 69 are critically endangered on the IUCN Red List of Threatened Species, meaning they are on the brink of vanishing forever in the wild.

The worst areas are Asia and Africa. In Asia, 71 percent—120 primate species or subspecies—are vulnerable, endangered or critically endangered, meaning they could soon become extinct. In just Vietnam and Cambodia, 90 percent of primate species are at risk of extinction. Numbers of gibbons, leaf monkeys, langurs and others have fallen due to both habitat loss and hunting—for food as well as trade in traditional Chinese medicine and pets.

Yes, habitat destruction, particularly from the burning and clearing of tropical forests, remains a big reason for the declining populations of primates, but hunting has become worse than in anyone’s nightmares. (Note to hunters: when I write about the threat that hunting poses to wildlife I get flooded with letters from furious hunters claiming that they are not to blame, that poaching is the problem. Unfortunately, much of the animal killing going on is perfectly legal—which makes it hunting, not poaching.)

“We have solid data to show the situation is far more severe than we imagined,” said Russell A. Mittermeier, president of Conservation International (CI) and chairman of the IUCN Species Survival Commission’s Primate Specialist Group. “Tropical forest destruction has always been the main cause, but now it appears that hunting is just as serious a threat in some areas, even where the habitat is still quite intact. In many places, primates are quite literally being eaten to extinction.”

In Africa, 37 percent of primates—63 species and subspecies—are vulnerable, endangered or critically endangered. Eleven of the 13 kinds of red colobus monkeys assessed are critically endangered or endangered. The Bouvier’s red colobus (Procolobus pennantii bouvieri) has not been seen in 25 years and may already be extinct; no living Miss Waldron’s red colobus (Procolobus badius waldroni) has been seen by a primatologist since 1978, and may also be history.

The successes are few, but two notable ones are of Brazil’s black lion tamarin (Leontopithecus chrysopygus) and the golden lion tamarin (Leontopithecus rosalia), which in 2003 were downlisted to endangered from critically endangered. That small bit of progress—we’re supposed to be encouraged that a species is “only” endangered—reflects three decades of conservation efforts to save fragments of their forest and, crucially, to create corridors connecting one fragment to another. It won’t last unless reforestation expands.

Click here to see a gallery of the world's vanishing primates.