Ever since the 1940s, when a couple attempting to raise a pair of chimpanzees like children failed to impart the ability of speech, scientists have assumed language to be a uniquely human evolutionary adaptation.

The thinking went that apes have little to no control over vocalizations and breathing-related behaviors. What sounds they do make, researchers posited, are largely involuntary -- a reflexive reaction to their environment.

But a new study by Marcus Perlman, now a postdoctoral researcher at the University of Wisconsin, suggests apes aren't as far removed from the ability to speak as scientists think.

For several years, Perlman studied at the Gorilla Foundation in California. Much of his time was spent observing Koko, the 40-year-old gorilla who famously learned sign language and has spent her life interacting with humans.

While working at the foundation, Perlman noticed Koko seemed to have more control over her breathing and vocal chords than one would expect.

In reviewing hours and hours of footage of Koko's vocalizations and sociable behaviors, Perlman and his research partner Nathaniel Clark, of the University of California, Santa Cruz, identified nine different, voluntary behaviors requiring Koko to exercise control over her vocalization and breathing.

When Koko wants a treat, she blows into her hand. She can also blow her nose and play wind instruments. When Koko wants a clean glass, she breathes heavily on the crystal before wiping it off. And, rather playfully, she sometimes makes a series of unintelligible grunts while pretending to talk on a toy phone.

"She doesn't produce a pretty, periodic sound when she performs these behaviors, like we do when we speak," Perlman said. "But she can control her larynx enough to produce a controlled grunting sound."

Koko also coughs on command, which requires her to shut her larynx.

"The motivation for the behaviors varies," Perlman explained. "She often looks like she plays her wind instruments for her own amusement, but she tends to do the cough at the request of Penny and Ron."

Perlman, whose observations are detailed in the journal Animal Cognition, says Koko is proof some of the abilities that enabled speech to evolve in humans were present in apes.

Her behaviors aren't proof of her exceptionality, just her potential given the right set of circumstances -- in this case 40 years of immersion in a mostly human world.

"Koko bridges a gap," Perlman says. "She shows the potential under the right environmental conditions for apes to develop quite a bit of flexible control over their vocal tract. It's not as fine as human control, but it is certainly control."