In recent years, scientists have discovered that chimpanzees, our closest relatives, are capable of all sorts of human-like behaviors that go far beyond tool use.

They self-medicate, eating roughage to clear their intestines of parasites. Baby chimps use human-like gestures to convey their needs to adults. Studies suggest even that chimps have a seemingly innate sense of fairness and go through mid-life crises.

Now, new research indicates that chimps’ vocalized communications are a bit closer in nature to our own spoken languages as well. A new study published in PLOS ONE shows that, when chimps warn each other about impending danger, the noises they make are much more than the instinctive expression of fear—they’re intentionally produced, exclusively in the presence of other chimps, and cease when these other chimps are safe from danger.

This not might sound like much, but linguists use intentionality as a key hallmark of language. Those who argue that apes aren’t capable of language—and that the apes who’ve been trained in sign language are merely engaging in rote memorization, not true language acquisition—point to a lack of intentionality as one of the reasons why. So the study shows that, in their natural environment, chimps do use vocalizations in a way more similar to language than previously thought.

The researchers, led by Anne Marijke Schel of the University of York, studied a community of 73 chimps that lives in Uganda’s Budongo Forest Reserve. To simulate danger, they used the skin of a dead African Rock Python—one of the chimps’ natural predators—to create a fake python, with fishing line attached to its head so they could make it move realistically.

Over the course of nearly a year in the field, they repeatedly placed this artificial predator in the forest with a camera rolling, waiting for unsuspecting chimps—sometimes alone, sometimes with other chimps—to come upon it so they could closely study their response. Typically, when the chimps saw the snake, they were startled, and made one of two different vocalizations, which the researchers identified as ‘huus’ (softer calls, with less alarm) or ‘waas’ (louder, more alarmed calls).

When the researchers analyzed the specific responses, they found that when other chimps were around, the startled chimps were much more likely to make the ‘waas’ rather than ‘huus.’ Moreover, the chimps clearly observed the location of other chimps and whether they were paying attention, and kept sounding the alarm until the others had fled and were safe from danger. The length of time they sounded the alarm, meanwhile, wasn’t linked with their own distance from the snake, further supporting the idea that the call was an intentional warning to others.

The researchers also took note of the pre-existing relationships among chimps (within the social hierarchy, some are closer than others) and found that closer relationships were more likely to trigger alarms. “It was particularly striking when new individuals who had not seen the snake yet, arrived in the area,” Schel said in a press statement. “If a chimpanzee who had actually seen the snake enjoyed a close friendship with this arriving individual, they would give alarm calls, warning their friend of the danger. It really seemed the chimpanzees directed their alarm calls at specific individuals.”

The authors argue that these characteristics—specifically, the fact that alternate vocalizations were employed in different circumstances, that they were made with the attention of the audience in mind and that they were goal-directed, continuing until they’d successfully warned other chimps so they fled—show that the noises are more than reflections of instinctive fear. Rather, they’re a tactical, intentional form of communication.

This observation, the authors say, may also tell us something about the evolution of human language. Gestural theories on the origin of language contend that spoken language evolved from hand gestures, and cite the fact that non-human primates (a model for primitive hominids) exclusively use gestures for true communication, merely making vocalizations based on engrained instinct, rather than calculated intention.

But this discovery of intentional warnings in chimps seems to upend that idea, suggesting that primitive hominids too were able to communicate via both vocalizations and gestures. This indicates, the researchers say, that spoken language may have evolved from multiple different sources, both gestures and vocal calls.