By the time they got to the orangutan, it was already dying.

In the Batang Toru forest, on the western flank of Sumatra, orangutans will often venture from the jungle to pick fruit from nearby gardens—a habit that puts them in conflict with villagers. In November 2013, the conservationist Matthew Nowak got word of one such conflict, and his veterinary colleagues went to investigate. They arrived to find a male orangutan, badly beaten, his face and hands riddled with cuts. Despite the team’s efforts, he died from his injuries eight days later.

With just 120,000 orangutans left in the wild, the loss of any one is a tragedy. But this particular ape has a significance that will transcend his death. Based on a close analysis of his skeleton, and a study of several orangutan genomes, Nowak and his colleagues think that the dead individual belongs to a different species of orangutan than those that we’re familiar with. If they’re right, there are actually three species of these orange-haired apes. And the newly described one would be the most endangered great ape alive.

When I was a child, an orangutan was an orangutan was an orangutan. But in 2001, after years of debate, scientists formally agreed that there actually two species—one from the Indonesian island of Borneo, and the other from neighboring Sumatra. The Sumatran species is slimmer and paler, with fur that’s closer to cinnamon than maroon. It spends more time in trees (perhaps because Sumatra, unlike Borneo, has tigers). And it’s rarer, with about 14,000 remaining individuals, compared with 105,000 in Borneo.

Most of the Sumatran orangutans live on the northern part of the island. But there’s another small group that lives in Batang Toru—100 kilometers to the south, on the other side of the sizable Lake Toba. A few obscure reports from the 1930s hinted at the existence of this splinter cell, but the group was only formally described in 1997, by a team led by the conservationist Erik Meijaard. These orangutans always seemed a little unusual. They live in more mountainous forests, and they eat different kinds of food.

Their genes are also distinct. In 2013, Michael Krützen, from the University of Zurich, analyzed the DNA of 123 Sumatran orangutans, and found that, in at least one part of their genome, the Batang Toru (or Tapanuli) orangutans were distinct. If anything, they seemed more closely related to the Bornean orangutans on a different island than the Sumatran ones just a day’s walk to the north. “We didn’t expect that,” says Krützen. “It was peculiar, but we needed more data.”

He later mentioned this peculiarity while giving a talk at a conference, where both Meijaard and Nowak happened to be in the audience. The three talked, and, suspecting that these orangutans might belong to their own distinct species, they teamed up to test that idea.

Krützen’s team analyzed the entire genomes of 37 orangutans, including two from Batang Toru. This more thorough analysis confirmed that these animals are indeed genetically distinct from both the Bornean and Sumatran species—and closer to the former than the latter.

They think that the ancestors of all modern orangutans traveled from mainland Asia into Sundaland—a continuous landmass that includes what is now Sumatra, Borneo, and other islands. Around 3.4 million years ago, these ancestral apes split into two populations, one of which gave rise to the current Batang Toru lineage. The other group spread throughout Sundaland; around 670,000 years ago, they split again into two new lineages, which we now know as the Bornean and Sumatran orangutans. The Batang Toru population occasionally crossbred with their Sumatran cousins, but those interspecies shenanigans stopped almost completely 100,000 years ago, when an erupting volcano cut them off.

It’s often said that humans differ from chimps by just 1 percent of our genome, so I wondered how close the three orangutan species are. Krützen disabused me of that question; the number depends on the history of each pair of species, and there’s little to be gained by comparing between different pairs. “There’s no magic number where, beyond that level, it’s a different species,” he says. And besides, genetic differences aren’t the only line of evidence the team has.

At the time Krützen was analyzing orangutan DNA, Nowak got word of the dying male who had been attacked in Batang Toru. And when his team compared its skeleton to those of 33 other orangutans, whose remains are housed in museums, they found clear differences in the shapes of its skull, teeth, and jaw. Even from the outside, they look distinctive, with frizzier fur and a more prominent mustache, and beards on the females. They behave differently too: They eat different plants than the northern populations, and the males have a higher-pitched call.

Not everyone is convinced, though. “It’s premature to consider this a separate species based on one cranium and two genetic samples,” says Rebecca Stumpf, from the University of Illinois at Urbana-Champaign. “I’d suggest that more evidence is needed before adopting separate species designations.”

But Nowak and Krützen argue that it’s the weight of all their evidence that matters. The physical measurements “are based on a sample size of one, which is a pity but we can’t change that,” says Krützen. But there’s also the genetic data, and the unique behaviors. Besides, other primates have been billed as distinctive species on the basis of much less. Bonobos, for example, were proposed as a distinct species from chimpanzees based on a single female specimen and five skulls—without any supporting genetic information to begin with. “There’s no one smoking gun that this is a species,” says Nowak, “but we kept looking, and we kept on finding unique things.”

“Time will tell,” Krützen says. “We’ll continue to work on this, and it might be that in 10 years’ time, we’ll say we have new data that doesn’t support a species status. That’s okay. That’s scientific progress.”

The apes might not have 10 years, though. The Sumatran orangutans were already critically endangered, and now their population might be even smaller than anyone suspected. The newly identified Batang Toru orangutan is rarer still, with an estimated 800 individuals left. Like the other two species, they are killed as agricultural pests, hunted for the pet trade, and rendered homeless as their forests are felled.

The good news is that since 2006, biologist Gabriella Fredriksson from the Sumatran Orangutan Conservation Program has been pushing the local government to spare the Batang Toru forest from logging. Thanks to her efforts, around 85 percent of the forest is now at least partially protected.

The bad news is that the unprotected 15 percent includes land that’s being set aside for a hydroelectric dam. If built, the dam would cut off two large chunks of forest where the Batang Toru orangutans live, splitting this already small population into even smaller factions. That would be devastating. “It’s probably one of the most endangered great apes we know,” says Krützen. “With just 800 individuals, there’s not much leeway for any mistakes.”

Marc Ancrenaz, co-director of the Kinabatangan Orangutan Conservation Project, is hopeful, though. “I hope that this new status will foster conservation efforts to make sure that the population doesn’t go extinct shortly after being described,” he says. “It’s definitely good news in these times where conservation is more often than not gloom and doom.”