Erich Jarvis

"I think sign language is a language. It’s a form of learned communication that involves movement, but not the movement of the larynx.

"Although, I wanted my colleagues to separate out the difference between moving the oral musculature and signing to try to pull apart what’s going on in the brain with those two behaviors. 

"When I talk to people who study sign language, they say it’s almost impossible. It’s hard to not move the mouth and sign at the same time. And so I think because of that, there is a connection behaviorally and evolutionarily in speech and signing in humans.

"What’s interesting is when you teach gorillas and chimpanzees to sign, I don’t see good evidence that they’re moving their oral musculature. And so in them, maybe it’s not as connected, evolutionarily —but in humans, it is. But that would mean that the oral movement part in humans came after the signing. And why might that be the case?

"It goes back to what I was saying earlier, the brain pathways for producing spoken language are embedded in brain pathways controlling learned movement, including, I believe, signing. 

"Some people say they actually intertwine. 

"And I think this happened by a whole brain pathway duplication where the whole motor-learning circuitry that controls the gesturing and other body movements, right, replicated itself and got connected to the vocal organs. And this is also partly why they’re connected."