Chimpanzees may not be able to recite Hamlet or giving rousing speeches but there is no doubt that they are excellent communicators. They exchange a wide variety of sophisticated calls and gestures that carry meaning and can be tailored to different audiences.
The sophistication of chimp communication doesn't stop there. Jared Taglialatela from the Yerkes National Primate Research Center has found that chimp signals and human speech are both strongly influenced by the same area in the left half of the brain - a region called the inferior frontal gyrus (IFG).
In humans, the left IFG is home to 'Broca's area', one of a few areas that are vitally important for our ability to speak coherently (even for those of us who rarely do so). As we speak, this area becomes highly active and it even lights up when we plan to say something or use sign language. People who suffer damage to Broca's area often lose the ability to form articulate sentences even though they can perfectly understand the spoken word and are often aware of their inability to voice their thoughts.
Like us, a chimp's left IFG is noticeably different form its counterpart on the right and it's perhaps unsurprising that many of them are distinctly right-handed when it comes to gestures. In earlier work, Taglialatela previously found that individuals that were most strongly right-handed also had the most asymmetrical IFGs.
Taglialatela watched the brains of three chimps at work, as they signalled their desire for food. A human experimenter placed a meal outside each chimp's enclosure, just out of reach, and each individual persistently called and signalled to the researcher as they walked in and out of view.
While all this was going on, the chimps had unwittingly knocked back a drink spiked with a mildly radioactive sugar molecule. These quickly became incorporated into the cells of the energetically demanding brains as they leapt into action. Taglialatela measured the radioactivity emitted by the sugars using a technique called positron emission tomography, more familiarly known as the PET scan. In this way, he could voyeuristically peer into the brains of communicating chimps without damaging them, and reveal the location of the most active parts.
He compared these results with a second set of scans taken when the chimps were given food within easy reach and didn't have to signal for their meals. This comparison accounted for areas of the brain that are simply involved in general motor skills and leaving behind those with a more specific role in communication.
The IFG was one of these, and the left IFG particularly so. When Taglialatela flipped the brain scans and compared the activity of equivalent regions on the left and right hemispheres, the left one showed much stronger activity.
The results suggest that the chimp IFG plays a very similar role to Broca's area. Not only does that highlight the remarkable sophistication of chimp communication, but it also suggests that the parts of our brain that underlie our own conversational skills may have been present in the last common ancestor that we shared with chimps.
A few questions remain unanswered though. The researchers have yet to see if the chimp IFG contains the same unique type of cell that makes up Broca's area in humans. And Taglialatela freely acknowledges that with a small sample size of just three chimps, the findings lack a statistical punch. Even so, a PET scan isn't the easiest technique to pull off on a chimp and the consistent nature of the results across the three individuals lend some strength to the data.
For more on chimp gestures, have a look at this post from the old site on the role of gestures in language evolution
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Wow. This is so awesomely, totally, cool.
I'd be interested to know what the homolog of the left IFG does in other animals. A quick web search turned up an unreliable source saying that it contains mirror neurons in birds, i.e., this is an area that lights up both when they perform an action and when they see another bird performing it.
If true, I would guess that this is involved in learning, and that language used that as a convenient springboard to evolve from.
Actually, I'm not sure what to make of this study. The control condition seems very strange to me. The experimental condition is as you describe it, but the control condition involved having the chimps perform a task in which they pick up 20 small stones and hand them to the experimenter, who gives them verbal praise and a food reward.
I'm not sure what's being controlled for here, but the two conditions seem different enough that I'm not sure we can draw any solid conclusions about what the additional activity in the IFG is doing. I'd like to see a control where the food is out of reach, but no human is present. In the experimental condition, the chimp is experiencing the desire and frustration for the food that is out of reach, and those mental states are not present in the control condition they carried out.
Derek, that's a really good point. I think the logic behind the control task was to create actions that would use the same motor skills but without the same communicative intent, but you're right about the added emotional element.