It’s one of the great mysteries about primates, and scientists think they may have found a major clue that can help them solve it. It’s long been debated why monkeys don’t have the ability to talk in any meaningful way, but new research suggests that perhaps they can.

The researchers were surprised to discover the macaques are an interesting candidate because of their vocal anatomy, which is similar to humans in terms of speech, according to the study, which was published in the journal Science Advances. It’s an indication that perhaps what makes humans special is the ability to control our vocal chords, and not having vocal chords.

Scientists had previously suggested that the shape, size or structure of the vocal anatomy might be preventing htem from speech, so they looked at those structures.

The finding that macaque monkeys may indeed have a vocal tract capable of a spoken language could lead to breakthroughs in understand how we evolved the ability to speak from our ancestors.

“Now nobody can say that it’s something about the vocal anatomy that keeps monkeys from being able to speak — it has to be something in the brain. Even if this finding only applies to macaque monkeys, it would still debunk the idea that it’s the anatomy that limits speech in nonhumans,” author Asif Ghazanfar, a Princeton University professor of psychology and the Princeton Neuroscience Institute, said in a statement. “Now, the interesting question is, what is it in the human brain that makes it special?”

Thore Jon Bergman, an assistant professor of psychology and ecology and evolutionary biology at the University of Michigan, said the research is important for figuring out human speech.

“It looks like mainly neuro-cognitive — as opposed to anatomical — differences contribute to the broader range of sounds we produce relative to other primates,” said Bergman, who is familiar with the research but was not involved in it.