In order for human language to evolve, we had to master our vocal cords. Because this ability hasn't been observed in any non-human primates, researchers believed that the ability may have evolved after we diverged from the great apes.

But then along came Rocky.

Rocky is an orangutan who lives at the Indianapolis Zoo. Researchers worked with him in a game of "do-as-I-do," asking him to copy the sounds they made in exchange for treats. They varied the pitch and tones of the sounds they made, and Rocky was able to copy them, making vowel-like calls.

So what does he sound like? Watch Rocky as he works with a researcher, mirroring human speech:

But how did researchers know what they were hearing?

The research team compared those sounds to a database containing more than 12,000 hours of observations of more than 120 orangutans from 15 wild and captive populations. They found that the sounds Rocky made were different, demonstrating that he was able to learn new sounds and was able to control his voice in a "conversational" way.

"Our results show that a nonhuman great ape can achieve levels of volitional voice control qualitatively comparable to those manifested in humans; notably, real-time, dynamic and interactive vocal fold control beyond the species-specific vocal repertoire," the researchers concluded in the study, which was published in the journal Nature.

Previously, researchers thought great apes had no control over the noises they made because they were driven by arousal, and that they were unable to learn new sounds.

“But our research proves that orangutans have the potential capacity to control the action of their voices," coauthor Dr. Adriano Lameira of the Department of Anthropology at Durham University in North East England, said in a statement. "This indicates that the voice control shown by humans could derive from an evolutionary ancestor with similar voice control capacities as those found in orangutans and in all great apes more generally.”

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.