• Researchers have found that monkeys' gestural communication shares some interesting traits with human language, according to our partner The Conversation.
  • For example, monkeys produce gestures that fully meet the criteria of so-called "intentional communication," a central property in language development in preverbal children.
  • This analysis was conducted by Yannick Becker, postdoctoral researcher in primatology at the Max Planck Institute for Human Cognitive and Brain Sciences (Leipzig, Germany) and Adrien Meguerditchian, primatologist, CNRS researcher in comparative psychology at Aix-Marseille University (AMU).

Bicêtre Hospital, Paris, mid-nineteenth century: Dr. Paul Broca meets an extraordinary new patient. He seems quite normal, but when the doctor asks him about his ailment, he simply answers: "tan"!

It turns out that "Mr. Tan," as he would later be called, fully understands what he is being told, but it is impossible for him to respond with any other word than "tan," which he repeats endlessly. After his death, Broca discovered the reason for this loss of speech production: a lesion of the left frontal brain.

Since this discovery, "Broca's area" has gone down in history as the area of speech production. And more recently, the function of this region has been refined and now includes other properties such as semantics, motor planning in gesture or syntax.

Syntax concerns the grammatical rules that structure a sentence, but also the so-called "motor" syntax, i.e. any sequence of nested actions, such as gestures, the use and manufacture of tools or driving a car for example. For all these actions, your Broca area is activated!

Such motor functions at the heart of this key area of language are questioning. Are they not traces of our ancient system of communication inherited from our ancestors?

A gestural origin of language?

Answering this evolutionary question is not easy. Even if soft tissues such as the brain do not fossilize, it is possible to speculate on the cognitive functions of our ancestors by relying, as paleoanthropologists do, on the archaeological traces they have left including the fossils of their bones, including those of their skull (inside which their brain left precious imprints). their tools and other creations.

Another complementary approach, prized by us primatologists, is to rely on the study of our primate cousins. We share, in fact, with them relatively recent common ancestors in the history of evolution. Imagine that we discover common points, we could thus reconstruct the cognitive traits inherited from these famous common ancestors.

In this so-called "comparative" approach between species, researchers have discovered that monkeys' gestural communication shares some interesting traits with language. For example, monkeys produce gestures that fully meet the criteria of so-called "intentional communication," a central property in language development in preverbal children.

Indeed, if the monkey who hits his hand on the ground to threaten a congener, does not receive the expected response, he tends to repeat his signal or even vary it (widening his eyes, raising his eyebrows, shaking his head, jumping with both feet ...) like a teacher who will reformulate a statement to make himself better understood.

In our team, we have been closely studying this behavior for years with a hundred baboons of all ages living in social groups at the CNRS Primatology station near Aix-en-Provence.

We discovered that the majority of baboons (Papio anubis) preferentially use their right hand when communicating with their hands, in greater proportions than when handling objects.

In other words, we described not only that baboons could be right-handed or left-handed for complex object manipulation tasks, but also that communicating with the hands changed these manual preferences, especially in favor of the right hand.

This discovery put us on an interesting track: since the right hand is controlled by the left hemisphere, and language also mobilizes the left hemisphere, does monkeys' gestural communication mobilize the same hemispheric specialization as human language? Is it in similar brain structures, including Broca's famous area?

To explore this bold thesis, we were missing an essential piece of the puzzle: the brain. But here's the thing: the brain is the most protected organ in an organism.

MRI is also for monkeys

Fortunately, thanks to the democratization of non-invasive magnetic resonance imaging (MRI) brain imaging techniques, which even affected our psychology lab, we caught ourselves dreaming...

Not to put a willing baboon in the narrow, noisy tube of an MRI, let alone ask it not to move an inch to produce pointing gestures on command – we would have had a hard time finding volunteers. But rather to take anatomical brain images from monkeys asleep for the occasion, like a 3D photograph of their brains.

The idea is to study not brain activation in communication situations, but the morphology of homologous brain structures of language, including that of Broca's area, in order to compare it with that of humans.

And so every week, we left the song of the cicadas of the countryside to go to the MRI center in Marseille in the company of our dear baboons. After drawing a portrait of their brains and making sure that they woke up once out of the machine, we hurry to bring them back to the primatology station so that they can find their peers in their social group.

And it is with impatience that we unveiled on computer one by one of these brain images collected from 50 baboons to analyze them and try to delimit the size of the homologous zone of Broca's area.

The equivalent of this brain area in monkeys is visible by following the natural demarcation of one of the folds, called the arched groove, located in the frontal part of the brain.

Thanks to software, we were able to extract and represent this furrow in 3D in each of the two hemispheres – like cement poured into a mold – to measure its dimensions from every angle, including the depth of the portion of the furrow representing the edge of Broca's area.

For each baboon, we were able to quantify the differences in this border between the two hemispheres in order to determine the cerebral asymmetries of Broca's area. We then compared these measurements between two groups of baboons: baboons preferring to communicate with their right hand and those preferring to communicate with their left hand.

Similarities between monkey and human brains

Among the 28 baboons that prefer to communicate with their right hand (shown blue on the graph), it turned out that the border of their Broca's area (colored red in the figure) was deeper in the left hemisphere than that of the 22 baboons preferring their left hand (orange on the graph), and vice versa.

On the other hand, when a baboon manipulates objects without a communicative purpose (for example, when trying to remove a treat from a tube), its manual preference is not related to this cerebral asymmetry. Thus only communicative gestures are associated with the homologous zone of Broca's area!

This discovery in baboons suggests that key regions of human language may be specialized in gestural communication in monkeys!

We hypothesize that language and its asymmetric cerebral organization may have been inherited from the gestural communication system of our distant ancestors shared with baboons and thus precede the origin of speech.

This system would thus go back, not to the emergence of hominids, but rather to the much older common ancestor of Old World apes and humans, 25-35 million years ago.

But then multiple questions arise: what about the other brain regions involved in human language? At what age do baboons develop such a brain-gesture connection? Is it an early organization of the brain that leads to the development of gestures or the emergence of gestural communication that affects the hemispheric specialization of the brain?

OUR "BRAIN" DOSSIER

To try to answer these questions, we started a new project to track the development of baboons' gestural behaviors from birth within their social group, while regularly collecting images of their developing brains. We'll tell you more soon.

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This article is produced by The Conversation and hosted by 20 Minutes.

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