Where is the mind? Some would say it's in the head, of course. In the brain. However, according to some researchers, not only: the mind, or rather the cognitive process, can escape the boundaries of the skull and involve objects in the environment. This is the extended cognition hypothesis, proposed more than 20 years ago. This hypothesis was not restricted to humans and it is already said that spiders, plants and even unicellular organisms are capable of extending their cognition. In an article published last week, we go further: we propose that extended cognition may be much more common than previously thought, and several different organisms would also have their cognitions leaking into the environment.
Legend has it that under the palace of King Minos of Crete, there was a labyrinth where a terrible monster, half man, half bull, was imprisoned: the Minotaur. Every seven years, King Minos demanded from Athens, the city he subjugated, a sacrifice of seven men and seven maidens to enter the labyrinth and be devoured by the Minotaur. When the time came to send a third batch of victims, Theseus, the Greek hero, wishing to put an end to this injustice, volunteered to be one of the men to be sent to Crete. His plan was to enter the labyrinth, find the Minotaur and kill him, freeing the Athenians from their gruesome tribute.
When Theseus arrived at the palace of Minos, Ariadne, the king's daughter, fell in love with him. She admired his courage, but feared that even if he killed the Minotaur, Theseus would not make it out of the labyrinth, and would be lost forever. So she gave him a ball of thread, instructing him to tie the end of the thread to the entrance to the maze and unspool the ball as he went. When it was time to return, it would be enough to follow the thread he had left behind him. Theseus did so. When he got to the heart of the labyrinth, he found the Minotaur and, after an epic fight, he killed the monster. Then, Theseus followed the thread back to the labyrinth exit and Ariadne's arms.
Theseus could never get out of the labyrinth using only his mind, no matter how smart he was. The labyrinth was huge, the paths tortuous, full of forks and dead ends. His memory would never be able to remember the entire journey. Yet, thanks to Ariadne, he employed a trick: he used the thread to remember the path. Somehow, the line was his memory of the path taken, practically as if part of his mind was discharged into the environment and was outside his body, stretched along the entire route. Could we say that, in a sense, his mind was extended to the line?
Mechanisms to externalise memory are not uncommon for us. We do this all the time almost unconsciously. In a much less epic way than Theseus, we write shopping lists, stick post-its around the house to remind us of an appointment, activate notifications and alarms on our cell phones to notify us of meetings, use GPS to tell us how to navigate an area of the city that we don't know. What does all this say about how the mind works? According to some scientists, the use of these resources is evidence that our mind is not confined to our head, but that it extends to the environment. In a way, the mind does not fit only in the body and ends up involving objects around us.
This bold hypothesis, known as the extended cognition hypothesis, was proposed by the philosophers of mind Andy Clark and David Chalmers in 1998. According to them, the mind is extended, and objects that are manipulated by people to help in the cognitive process become an active and integral part of this process. Thus, paper and pen when doing a mathematical calculation would be part of our cognitive system as much as the neurons in our brain. It goes without saying that this is a very controversial hypothesis and that it has generated great discussions in the academy since it was proposed, mainly in the field of philosophy of mind, still generating debates today. After all, is Ariadne's thread or is it not part of Theseus' mind?
This debate, however, did not prevent the extended cognition hypothesis from spilling over into other fields of the natural sciences. In 2017, together with the evolutionary biologist Kevin Laland, the professor at the Federal University of Bahia (Brazil) Hilton Japyassú published an article in which, after carrying out an extensive literature review, they concluded that spiders also extend their cognition to their webs. This would explain, for example, how animals with such small brains would be able to display such complex behaviours, like sophisticated hunting strategies or the very construction of structures as large and complicated as their webs. According to the researchers, spiders would use webs not only to capture prey, but also to detect them (that is, to perceive them, to be aware of them). In addition, they would use the webs to remember the path they have taken, as Theseus did, and even to acquire information on how to weave the famous spiral orb webs. Who would have thought that such a small animal would have such large cognition!
By the way, from now on I will no longer speak about the mind, as the concept is closely linked to the idea of human consciousness. I prefer to use the more general term cognition, although the human mind is a product of our cognition.
Shortly afterwards, inspired by the work of Japyassú and Laland, I published, together with behavioural ecologist Monica Gagliano and my master's advisor, professor Gustavo Maia Souza, an article in which we proposed that plants are also capable of extending their cognition. After all, plants have a large number of senses and exhibit very complex behaviours such as decision-making, memory and learning, communication, anticipation, etc. And in the case of plants, they don't even have brains or neurons to help with the process! It is still a big mystery how plants are able to produce cognitive behaviours like those mentioned. However, if some of the burden of processing and using environmental information is offloaded onto the environment itself, it could add a piece to this fascinating puzzle.
But it didn't stop there. In addition to spiders and plants, another group of organisms got a ticket to the extended cognition club. This time, a group of organisms that is as unknown as unexpected: the slime moulds. These creatures look like goo that crawls along the forest floor devouring organic matter and bacteria. They are visible to the human eye, but can go unnoticed, although some species are vibrantly coloured, like yellow or orange. They don't have brains, neurons, nor even individual cells. However, they are surprisingly intelligent, able to solve problems such as finding the fastest way through a maze and avoiding traps. When studying these organisms more deeply, researchers observed that when crawling on the ground, they release a mucus that serves, like Ariadne's thread for Theseus or the web for spiders, as an external memory of the places where they have already been. Thus, slime moulds avoid going through the same place twice, optimising their search for food and avoiding getting stuck in dead ends. For this reason, researchers Matthew Sims and Julian Kiverstein have proposed that slime moulds are another example of non-neural organisms that extend their cognition.
At this point, it has been proposed that humans, spiders, plants and slime moulds extend their cognition. So different organisms, each at one end of the tree of life, all sharing this characteristic. This got us thinking: what if extended cognition isn't all that uncommon after all, on the contrary? What other organisms would be able to extend their cognition? By what mechanisms can cognition be extended?
With these questions in mind, Monica Gagliano, marine biologist Steve Whalan (both from the Southern Cross University, Australia), biologist Gabriela Gubert (then at the Federal University of Santa Catarina, Brazil) and I delved into the literature looking for evidence that could corroborate our hypothesis. Our suspicion fell mainly on organisms capable of modifying and manipulating the environment, either physically or through chemical substances. That is, to build niches to act and inhabit. And so, we published in the scientific journal Frontiers in Ecology and Evolution the article Ariadne’s thread and the extension of cognition: a common but overlooked phenomenon in nature?. In it, we present a series of examples that demonstrate that it is possible that extended cognition is common, although further studies are needed to corroborate this hypothesis.
Examples of extended cognition in nature
We have already talked about external memory, which is perhaps one of the most common phenomena: just like Theseus and the thread, spiders and their webs, other organisms could use the environment to store their memories. A curious example is that of the arabidopsis plant, from the mustard family. Plants, in general, secrete various chemical substances through the roots, called exudates, which modify the environment around them. Thus, they manage to manipulate the environment, for example by favouring the growth of some bacteria in the soil and inhibiting the growth of others. What has recently been discovered is that this manipulation also serves to store in the soil memories of diseases they've had, in a similar way to what our immune system does when it stores memories of illnesses we've had or vaccines we've taken (which is called immunological memory.)
In a study by Jun Yan and colleagues, arabidopsis plants were contaminated with a bacteria that causes leaf disease. After a while, the researchers removed the plant from the pot and planted another one in its place, repeating the procedure five times. Finally, when they planted a sixth arabidopsis, they found that the plant was much more resistant to this disease than its predecessors, as a memory of the leaf disease was stored in the soil through the composition of the bacterial community, which was altered by the plants. When the new plants accessed this immunological memory, they were able to prepare themselves physiologically to face a disease they never had in their lives. Planting five generations of diseased plants in a row may seem exaggerated, but a more recent study has shown that just one infected generation is enough to modify the soil and generate this memory. A memory that can not only be used by the plants that produced them, but also be shared with other plants that have never been sick.
Root exudates can also serve to detect objects at a distance and understand the environment around the roots. Studies carried out in the United States and Israel have shown that some plants secrete these substances to perceive objects that are out of the roots' reach. Suppose that in the direction the root is growing there is a stone. The exudates will no longer diffuse through the soil, but build up between the root and the stone to a point where the root stops growing and may eventually die. As a consequence, the processing of the information "stone ahead" takes place outside the plant body, between it and the stone. This causes the plant to grow roots in the other direction, avoiding the obstacle before it even gets close to it, resulting in a smarter root distribution. Some authors even claim that cognition is what generates intelligent behaviour, which reinforces the idea of extended cognition in these cases.
Another way to detect obstacles ahead is through what are called self-generated chemotactic gradients, a phenomenon studied in single-celled organisms and even cancer cells. These gradients are generated by cells when they degrade a substance that attracts them and is present in the environment. If there is an obstacle between the source of the attractive substance and the cells, they will consume the substance, creating a kind of "chemical vacuum" between them and the obstacle. In this way, the cells are able to direct their movement to distant areas of this void, towards the source of the substance.
To test how this might work in practice, UK researchers placed slime moulds or pancreatic cancer cells inside mazes. They observed that, by generating the chemotactic gradients around them, the cells were able to perceive dead ends even before entering them, so that they were able to find the exit the maze thanks to this perception from a distance. This effect was nullified when the scientists provided the cells substances they were unable to metabolise, or after they mutated cells to be unable to generate the gradients. This study is fascinating because not only demonstrates that even unicellular organisms would be able to extend their cognition, but it could generate knowledge about how cancer cells move through the body during metastasis, which could contribute to finding solutions to fight the disease.
Other organisms that potentially extend their cognition are social insects such as termites, ants, and bees. These animals are known to build very complex structures from their collective work. But how do they know how to build their nests? How do termites build such tall towers, full of galleries, and bees their hives with perfectly geometric combs? Part of the answer may lie, perhaps, in extended cognition.
In our work, we discuss the case of termites. For a long time, it was believed that when building their nests, termites left pheromones in the soil pellets. These pheromones would encourage more termites to deposit pellets alongside the previous one, adding to the pheromones as well, which would encourage even more termites to deposit the pellets there, and so on until, in the end, large structures are built because of these feedback loops. However, to date no such pheromone has been found, while other explanations seem more likely. For example, it has been found that, when building, termites are much more sensitive to terrain geography than they are to pheromones, as they tend to deposit soil pellets in high places. The curious thing is that when termites arrive on virgin territory, they walk in a row. This walk makes the trail they walked through become lower than the surrounding terrain, which encourages them to deposit the soil pellets on the edges of the path. They will do this successively until the first tunnel of the new nest is built. Afterwards, they build over this tunnel, and so on, until they generate the huge termite mounds that we know. Therefore, it is clear that the raw material for the information on how to build the nest is not in the termites' heads, but in the ground. The interaction of termites with the terrain modified by themselves is what generates the information to build the nest, which becomes a kind of collective memory of their work and the basis for future work. Daniel Calovi and colleagues from the University of Cambridge (UK) and Syracuse (USA) go further and suggest that, in fact, the termite mound and the termites form a single cognitive entity in which the termites would be a "liquid" and dynamic component, and the termite mound, the crystallized memory of this cognitive system.
Finally, another curious example of extended cognition is not in the external environment, but in the internal environment. That is, our gut microbiota. The intestine is populated by an enormous amount and diversity of bacteria and fungi that live in symbiosis with us and without which we cannot survive. They help to digest food, but recently it has been discovered that they also release substances such as neurotransmitters that have an effect on our brain and modify our own cognition, affecting, for example, our emotions. This connection between the brain and the gut is known as the brain-gut axis and recent studies in mice, rats and humans have confirmed the importance of the gut microbiota for the correct functioning of our cognitive processes. For this very reason, some authors have suggested that the gut microbiota is also part of our extended cognition, even if it is extended inwards!
To get an idea of the importance of the microbiota in cognitive processes, a very interesting, albeit somewhat sad, study was carried out by a French group from the Pasteur Institute. They took healthy mice and applied an unpredictable stress protocol, upsetting the animals to the point where they started showing depression-like symptoms. The researchers analysed the gut microbiota of these mice and saw that this had changed the composition of gut bacteria. So, they transferred faeces from the "depressed" mice into the intestines of healthy mice, which caused them to develop depressive symptoms, lower neurogenesis in the brain and less activity of the endocannabinoid system, which in humans is related to mood, appetite, memory and pain. These mice were then given either healthy mouse faeces or probiotic food supplements, and this significantly restored their "good mood", new neuron formation and endocannabinoid activity, reversing the depression-like effects. This study suggests that problems with the gut microbiota may be related to depression in humans, but more than that: maybe without realising it, Chevalier and colleagues have shown that mice's cognition may extend to their gut microbiota.
The studies mentioned above, reviewed in our article, give an idea that the extension of cognition may be much more common than one imagines. Apparently, primates, mice, plants, slime moulds, termites, cancer cells and perhaps fungi extend their cognition, which means that other niche-building organisms such as corals, sea sponges, birds, bats and others could extend their cognitions as well. If this phenomenon occurs so often, it is conceivable that it may be related to the evolution of cognition itself. By manipulating objects in the environment, organisms would have increased their ability to process and use information, which clearly has evolutionary advantages. Thus, the ability to extend cognition was maintained throughout all branches of the tree of life.
All of this, it is always good to remember, is a hypothesis. Despite being based on already published studies, if we are to say with confidence that an organism extends its cognition, we will need studies designed specifically to detect extended cognition. However, it is fascinating to conjecture that our minds and cognitive processes might be not just in our heads and bodies, but extending into the environment around us. What effect would this have on our relationships with objects, the environment, and even other people? We are yet to see where Ariadne's thread will lead us.
This work was partly supported by the Templeton World Charity Foundation.
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