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Is there any common ground between the evolutionary history of apes and humans? It seems logical to answer yes, because we both belong to the great family of primates. But to what extent are we connected through evolution?
Adrien Meguerditchian, a researcher at the Cognitive Psychology Laboratory, is interested in the properties of our primate cousins’ modes of communication and their links with certain properties of human language. For his work on the implications of gesture in the evolution of language, he received the 2021 Bronze Medal from the CNRS.
So, are you ready to start a little discussion on … Language? But first, a little presentation is in order.
Adrien Meguerditchian studies primate communication systems from a comparative perspective. With his thesis in hand at the end of 2009, a Fyssen grant enabled him to continue his work as a postdoc on the trail of wild chimpanzees in Senegal, with the American anthropologist Jill Pruetz. He then worked in Atlanta (USA) in the laboratory of primatologist William D. Hopkins to study chimpanzee communication and its links with brain structures.
At the end of 2012, thanks to ANR funding, he set up his own research group and joined the Cognitive Psychology Laboratory in Marseille within the "Comparative Cognition" team directed by Joël Fagot. He then joined the CNRS in 2014 before being awarded an ERC starting grant in 2016 to continue his work on the gestural origins of language.
Language in the research spotlight
When we talk about language, you probably think of speech, i.e. the sounds that humans make with their vocal apparatus? Well, you’re right … But that’s only part of the definition!
Language, defined as the use of speech and the development of a linguistic system, would therefore be the prerogative of Homo Sapiens, without equivalent in the animal kingdom. This theory, which is becoming less and less tangible, is in fact the subject of a multidisciplinary scientific debate. Indeed, across the spectrum of cognitive sciences, language does not correspond to the single property of speech, but rather to a system that integrates many of these properties!
For Adrien Meguerditchian:
Language is the conductor of a set of properties.
Just think of babies pointing. These gestures are the first intentional signals of communication and the basis for language learning. And yes, when a baby points to an object, the parents will assign a word to that object that the child will integrate: the gesture is, therefore, a property of language. But underneath the communicative gestures, there is also a very specific syntax. We speak of syntax as the sequential organization of action, i.e., setting up units of action to achieve a goal.
This new conception of language leads to a reflection, do you perceive it? In this set of properties that constitute language, some animal species may have acquired one or more of them! And it is precisely at this junction that Adrien Meguerditchian’s research work is situated. He is investigating the origins of certain properties of language and how, over the course of evolution, they began to interact with each other to work together to form the language of Homo Sapiens. The researcher and his team are conducting ‘comparative‘ research between species to detect any common properties inherited from a common ancestor.
Do primates communicate with gesture?
To explore these common properties of language between human and non-human primates, the researcher and his collaborators studied more specifically the gestural communication of the baboon. But how does one go about this?
After hours and hours of observation, the researcher and his team collected data to describe the gesture repertoire of baboons. This gestural repertoire is made up of about sixty gestures. What conclusions can be drawn from these hours of observation?
- A major discovery was the ability of baboons to point to objects in the environment and thereby direct a social partner’s attention to them. Researchers call these abilities ‘referential’ properties. These properties are found in humans, particularly in children, who, before becoming oral, will use pointing to represent a need, an idea, or a representation. The challenge is to understand whether the baboon uses pointing for the purpose of intentional communication. And it is notably through a test that puts the baboon in a situation where a source of food is inaccessible to it, that the researchers have shown that the baboon will communicate its desire for food by a pointing gesture!
- The flexibility of the baboon’s gestural system has also been observed, in comparison with vocalisations that are more fixed. Indeed, these animals do not only use their hands to create their gestures, they use their whole body, which offers them a very wide range of gestures… to learn?
- Because yes, the other surprising discovery is the ability of these animals to learn new gestures throughout their lives. Let’s take an example: instead of using the very common threatening gesture, which consists of slapping her hand on the ground, a young female started clapping to intimidate her peers. This is called ritualisation, i.e. the integration of gestures that were previously absent from the gestural repertoire. This phenomenon of ritualisation is also observed, albeit more rarely, in the natural environment.
Adrien Meguerditchian and his collaborators highlight the link between gesture and language, which reinforces the theory of the evolutionary continuity of language. But what about the brain structures involved?
Is gestural communication also lateralized in primates?
Through observation, disparities between the use of the left and right hands according to the gestures performed emerged (communicative gestures or gestures to perform an action without social interaction).
These observations are called manual asymmetry or lateralization.
The behavioral studies that the researcher conducted during his doctoral thesis, supervised by Jacques Vauclair, professor of psychology at Aix-Marseille University, showed that the use of the right hand is much more important (even in left-handed people) when communicating through gestures than when performing simple tasks such as manipulating objects. The communicative gesture is more lateralised in favour of the right hand.
To test the argument of evolutionary continuity (primate-human) between gesture and language intentionality, we need to zoom into the brain! In humans, the right hand is controlled by the left hemisphere, and this hemisphere has a predominant role in most language functions. Adrien Meguerditchian, therefore, hypothesises that baboon gestures and human language share the same asymmetric neuronal bases. The researcher is using anatomical MRI to verify that gestural communication in primates involves dominance of the left hemisphere in cerebral areas that are homologous to the language areas in the human species.
Broca's and Wernicke's areas
The left hemisphere of the human brain is considered to be the seat of language, in particular, Broca's and Wernicke's brain areas. This representation now seems somewhat outdated, in view of recent advances highlighting the extent and complexity of the neural network involved in language, including the right hemisphere. Nevertheless, it is still accepted that the left brain plays a predominant role in most language functions in the vast majority of human beings.
Towards evolutionary convergence
Using the brain images collected from the baboons, we used a computer to measure the anatomical brain asymmetries between the two hemispheres in areas related to language in the human species. For each baboon, we measured the dimensions of three key language areas in each hemisphere: the anterior part of the insula, Broca’s area, and the planum temporale – an area that overlaps with Wernicke’s area.
His team discovered that the planum temporale is much larger in the left hemisphere of the baboon! The argument for continuity is therefore increasingly certain, as this asymmetry is considered to be a specific anatomical brain signature of the appearance of language in our species.
Even more surprisingly, with his doctoral student Yannick Becker, whom he co-directs with Olivier Coulon, director of research at the Institut de Neuroscience de la Timone, they discovered that the homologous area of the famous “Broca’s area” in baboons presented anatomical cerebral asymmetries correlated with the manual preferences of gestural communication. In other words, baboons that communicate with the right hand have a larger Broca’s area in the left hemisphere and vice versa.
This research questions the origin of language itself and challenges a number of assumptions. The direct link between gestural communication in monkeys and homologous language areas in humans can no longer be ignored. Let’s think about this a little further. This means that it was not with Homo Sapiens that the brain was organised in this way, i.e. that the precursors of this evolution do not go back 350,000 years, but rather to around 25 to 40 million years ago in baboons! These fascinating discoveries lead to many other questions. Adrien’s group is now focusing on the following question:
Are these cerebral asymmetries present, as in the human baby, at a very early stage?
To do this, they study the manual behavior of young individuals during their development, from birth. Their brain development is also studied using non-invasive MRI acquisitions. Babies are observed daily in their social group to document the development of gestural communication and the emergence of an associated manual preference. They are also exposed to more conventional manual lateralisation tasks, for example, a game with a tube filled with peanut butter. The predominant hand that the monkey uses is then a good indicator of whether it is right or left-handed.
The first results have just come in: like human babies, the brain volume of the planum temporale region of newborn baboons also shows an asymmetry in favour of the left hemisphere… and seems to predict the later development of manual preferences associated exclusively with hand communication that emerges at the age of 6-7 months. In contrast, manual preferences associated with tube manipulation, which emerge at the same age, are correlated only with central sulcus asymmetries in the motor area of the hand, exactly as in the human species.
Now it remains to be seen how the manual preferences evolve during development. Will they be stable? Will manual preferences change? Does the anatomy of the language homologous areas also change during development?
Stay tuned to follow the progress of Adrien Mergueditchian’s research on this subject!