Animal Handedness and Language
Humans are unique in using speech, and the shapes of our brains reflect the fact that on the left side is a bulge corresponding to the generation and decoding of language. The fact that most people are also right handed - meaning their left brain hemisphere is dominant - has led researchers to speculate that language goes hand in hand with handedness. And that struck a chord with Adrien Meguerditchian early on in his career when he was out studying our close relatives, the baboons…
Adrien - We watched the baboons making communicative gestures and I remember having these baboons come towards me and start to threaten me. He was a young baboon. He was slapping his hand towards me and looking in my eyes and would repeatedly slap his hand. I was not moving away, I was just staring at him and he just kept doing it. This message was not working obviously because I was still around and he started to shake his head, stand up and all this display was very silent. He was only using his hand, his body posture to try to make his point. This study started from there. I remember from my PhD starting to measure which hand the baboons were using when they were communicating with his hand. I remember having an expected surprise that when the baboon was using the hand for communication, something happened in the hand preferences. You can be right-handed or left-handed when you manipulate objects, but here, when they started using gestures for communication, the use of the right hand was exposing. It was a very strange phenomena and we were wondering 'why?' It was a case even in left-handed baboons, they were using more of the right hand for communication.
And that’s what Adrien and his student Yannick Becker have since gone on to study, confirming that they principally use their right hands when they make gestures and other signals, and also that their brains too are asymmetrical towards the left, like ours. So might this be a tantilising glimpse into the evolutionary origins of our own ability to communicate? Yannick takes up the story…
Yannick - There's something peculiar about language in the human brain. It's processed more in the left side of the brain than the right. When we want to study the evolution of language, we have to look at our primate relatives or cousins; which in this case is the baboon and particular species of all monkeys. Adrien, my supervisor, has observed that these baboons use their right hand for communication more than the left hand and the right side of the body is controlled by the left hemisphere. Because language in humans is controlled by the left hemisphere, there was suddenly the hypothesis that maybe, in these baboons, it might also be the left hemisphere that is controlling communication, especially gestural communication.
Chris - That's intriguing; to think that you've got that potential asymmetry going on in these close human relatives. Do they have right hand dominance like we do though? Because in humans, 90% of the population are right-handed and the vast majority of those right-handers are controlling their right hand with the left side of their brain, which is also where language is, which is why we have this hypothesis about left side dominance of the brain. Is that the same with handedness in these baboons?
Yannick - It's very close. We divide handedness into two different ways of assessing handedness. There's this what natively would call handedness maybe in the humans that we use actions with our hands. For this baboons are, on a population level, a little bit right handed. But then when we assess handedness for communication, special gestures that they make to communicate with each other, they are much more baboons right-handed for communicative gestures. Even those that have been left-handed in these manual actions will become right-handed in communicated gestures.
Chris - What sorts of gestures do they make then? How do they signal to each other with their right hands?
Yannick - There's this hand slapping gesture. They slap one hand to the ground in order to intimidate; it's really a threatening gesture. If you see this, you immediately understand that this baboon tells you to go away. There's a whole body posture that comes with it.
Chris - Is your sort of inference then that, given that gestures are all about communication, and in humans a chief mode of communication is speech, and we know speech is in the majority of us is on the left side of the brain, that perhaps this is one step along the road to which our brains evolve the ability to speak because they had inherited this particular use of one side of the body for communication that we've taken it a step further?
Yannick - That's exactly the point. Yeah. Thank you. We have inherited something in our evolutionary history from our ancestors, and this can be also found in these baboons. This asymmetry for this kind of communicative gesture might be a precursor of what we call language now.
Chris - Have you done the experiment you would do on a human? If we put a human in an MRI scanner and asked them to speak, we can see relative activation on the left side of the brain where they're creating that speech. Can you do a similar sort of thing in your baboons to see if they are similarly using that part of the brain asymmetrically when they're doing this communication?
Yannick - Yeah. That's the holy grail to get functional brain scannings. It's very difficult to do in animal primates, so what we've done is we're scanning these baboons, but they were asleep, we have anatomical scans, and then our function would be the behaviour that we have recorded.
Chris - And how does the anatomy compare left and right in the animals?
Yannick - We can measure some regions of interest in both hemispheres, and then we can compare them and calculate what we call an asymmetry quotient. We have done this study for a very special area that was shown to be an equivalent area to this famous speech production area in the human brain, which is called 'broca's area'. Compared to the left and the right hemisphere, we saw that this fold will be deeper in the left brain for those subjects that were communicating with the right hand, and reversibly, it was deep in the right atmosphere for those that are communicating with the left hand.
Chris - That suggests that there is this specialisation that does seem to be associated with communication, but going back to the humans again, we know that about 10% of humans are left handed, but they still do language a lot of the time on the left side of the brain. Did you find any baboons that bucked the trend like that and were lefties, but also appeared to have the specialisation for possible language still on the left?
Yannick - Very, very interesting question. We have variation in our data, but I think the most important point for us is that in humans, we find this for this manual action handedness. The hypothesis is that we would find this less if we assess handedness for communicative gesture also in humans, but this has never been done yet.