Secret Mind of Slime

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The Secret Mind of Slime

PBS Airdate: September 16, 2020

NARRATOR: It creeps! It crawls!

TANYA LATTY (University of Sydney): It’s this big, blobby yellow thing.

NARRATOR: It’s slime mold, a slithering shapeshifter, like no other on this planet.

AUDREY DUSSUTOUR (French National Centre for Scientific Research [CNRS], Toulouse): It reminded us of this alien organism from a ‘50s movie that ate people.

MOVIE ANNOUNCER: The Blob.

NARRATOR: This one-celled wonder has no brain.

TANYA LATTY: No brain, no organs, no neurons, yet it’s able to do some of the behaviors we normally associate with animals with brains.

NARRATOR: How does it solve a maze…

TOSHIYUKI NAKAGAKI (Hokkaido University): It is amazing.

NARRATOR: …escape captivity, compete for resources, build efficient networks?

AUDREY DUSSUTOUR: You start to think, “Does this organism possess intelligence?”

MICHAEL LEVIN (Allen Discovery Center at Tufts University): All of these are fundamental components to being smart.

NARRATOR: No brain? No problem.

MICHAEL LEVIN: Clearly now, we can see that this can be done by one cell.

NARRATOR: Can this ancient organism shed new light on the evolutionary origins of intelligence?

TANYA LATTY: We can think of them as… “how intelligence started way back.”

NARRATOR: The Secret Mind of Slime, next, on NOVA.

Lurking in the forest’s shadowy undergrowth lies an unlikely predator. It creeps along, stalking its prey, yet it is not an animal. It has a fondness for dark, humid places, yet it is not a fungus, nor a plant. It’s a single-celled organism, but not a bacterium. It’s known as “slime mold.”

It has no eyes, no mouth, no stomach, no legs, yet it can “see,” “smell,” and move around, through its pulsating network of veins, gorging itself on bacteria, fungi, yeasts and growing exponentially.

This strange yet humble creature holds secrets that have biologists, neuroscientists and mathematicians scratching their heads, because this organism, without a brain or nervous system, seems capable of making choices, solving complex problems and devising strategies. And it is forcing scientists to rethink intelligence as something that does not imply the need for a brain. Tanya Latty is a researcher, at the University of Sydney, who studies slime mold.

TANYA LATTY: You can find slime mold all over the place. They’re in leaf litter, they’re in the soil, they’re on bits of wood. They’re small structures that almost look like mushrooms.

NARRATOR: Not only is Tanya an expert on slime mold, she’s one of its biggest fans.

TANYA LATTY: How could you not love them? It’s this big, blobby, yellow thing, you know, that’s just fundamentally cool. I stand by that!

There it is.

NARRATOR: She even follows them on Facebook.

TANYA LATTY: The spores are gorgeous. They’re beautiful.

The slime molds might be one of the most abundant things in the soil.Their spores are everywhere.

NARRATOR: Biologists have puzzled over where exactly this creature fits into the great tree of life.

TANYA LATTY: Slime molds branched off from that tree before the plants, fungi and the animals.They’re single-celled organisms, but the type of cell they have is quite different from bacteria.

There’s at least 900 different species of slime mold that we know about, and that number is probably a massive underestimate.

The slime mold most people think about is the Physarum polycephalum, bright yellow, grows really fast and has become a kind of lab rat most scientists are studying, probably because it’s the easiest one to grow in the lab.

Some of them have great names, like the “dog’s vomit.” They also come in reds and grays and browns and white.

NARRATOR: Tanya was a latecomer to slime mold research. She began her career studying the behavior of insects.

TANYA LATTY: I’m an entomologist by training. I work with ants, bees and other invertebrates.

NARRATOR: But ten years ago, while researching bees at the Hokkaido University, she had a chance encounter with slime mold, through one of the most respected experts in the field, Professor Toshiyuki Nakagaki.

Slime mold is a serious subject of study in Japan, one going back generations.

TOSHIYUKI NAKAGAKI: The Emperor Hirohito, himself, was a learned biologist who established a taxonomy of slime mold and even discovered a new species of the organism. In my research, I focus on this single, simple cell through the disciplines of physics and math.

TANYA LATTY: So, I was able to see how he was doing this and learn a little bit about the slime mold.

NARRATOR: Toshiyuki had been studying how this curious organism would react when faced with a challenge traditionally posed to animals.

TANYA LATTY: He did some of the first really awesome experiments on slime molds with solving mazes.

NARRATOR: Could a slime mold find food placed at the far end of a labyrinth?

TANYA LATTY: I had never even heard of slime mold before I took this job.

So, here you have this organism that has no brain, no organs, no neurons of any kind, yet it’s able to do some of the behaviors we normally associate with animals with brains.

TOSHIYUKI NAKAGAKI: It is mysterious. Astounding. Solving a maze is amazing. How can a single-celled organism have such a capacity?

NARRATOR: Toshiyuki has devoted his career to studying this question.

TANYA LATTY: I mean, when you think about it, 99 percent of the living things on our planet are brainless, but they need to find food; they need to find partners to reproduce often; they may need to hide from predator. How do you do all of that when you don’t have a brain?

When I came back to Australia, I thought it would be awesome to have one of these as a lab pet. It lived in my desk for a few weeks. I started to notice that it was doing things a lot like my ants were doing.

NARRATOR: So, she introduced Physarum to one of her colleagues, Audrey Dussutour.

AUDREY DUSSUTOUR: I was a post-doc, in Australia, studying nutrition in ants.

It looked like an old omelet. The next day, it had escaped from the box we put him in. It reminded us of this alien organism from a ’50s movie that ate people, and grew and grew as it ate.

TANYA LATTY: It’s unstoppable, it just, kind of, keeps coming.

MOVIE ANNOUNCER: The world could fall, before the bloodcurdling threat of The Blob.

AUDREY DUSSUTOUR: Physarum doesn’t eat people, it eats oatmeal. But it’s really a glutton. It doubles in size every day. I nicknamed it “Blob.”

NARRATOR: Physarum began consuming their time and attention, and soon Tanya and Audrey shifted the focus of their research to this remarkable creature, so radically different than any other they’d studied with abilities beyond what could be expected from a single cell, beginning with an unmistakable knack for finding food.

TANYA LATTY: Slime molds have receptors all over the cell body. And that allows them to detect different chemicals in the environment. It’s very similar to our sense of smell. We have receptors in our noses, and we’re actually detecting chemicals coming off of our food in the air. Slime molds are doing the same thing, but they’re doing it through the soil or through a liquid medium.

NARRATOR: Like the surface of a petri dish.

TANYA LATTY: They’re able to sense that chemical cue, and that’s what draws them towards the food.

NARRATOR: Slime mold’s receptors sense a wide range of cues in its environment.

AUDREY DUSSUTOUR: It can detect moisture and perceive the pH. It can perceive light, because it has photoreceptors. So, even though it is a single cell, it is, all at once, an eye, nose, ear and so on.

NARRATOR: But it was how Physarum used these senses that captured their attention: an awareness of its environment, an ability to navigate and how it moved with seeming purpose.

AUDREY DUSSUTOUR: Here’s an organism that knows how to react and adapt to its environment, consistently and efficiently.

NARRATOR: They knew that Physarum’s receptors allowed it to react to different stimuli.

AUDREY DUSSUTOUR: It will move towards a substance like sugar, but it will also move away from certain substances, like salt.

NARRATOR: But was Physarum simply reacting to these stimuli, or was it using these receptors to do something more complex? Was it taking in this information about its environment and making choices?

So, for their first experiment, they would present Physarum with one of the most basic choices facing any living organism: what to eat.

TANYA LATTY: We can offer it a choice between different food sources.

AUDREY DUSSUTOUR: The purpose of this experiment was to see how Physarum regulates its nutritional needs.

NARRATOR: But before they could even begin, they’d have to determine what Physarum’s nutritional needs were.

In nature, Physarum feeds on a diverse diet of micro-organisms from the forest floor. But in laboratories around the world, they’re fed a single food: oats.

TANYA LATTY: Slime mold inexplicably love rolled oats. And that’s good for them, but for this experiment, we had to design different diets.

AUDREY DUSSUTOUR: So, we had to invent special recipes for Blob.

NARRATOR: …making their Australian laboratory seem more like a British bakeoff.

TANYA LATTY: We had to do a lot of different mixing and cooking to figure out the ratio of proteins to carbohydrates that the slime mold did the best on, so the ones that maximize their growth.

AUDREY DUSSUTOUR: We created these sort of crème brûlées. We called them custards. Thirty-five different recipes; each had a particular ratio of protein-to-sugar.

NARRATOR: To determine which of these recipes was best, the judge in this test kitchen was Physarum.

They placed Physarum on each of these 35 mixtures of protein and carbohydrates—basically, sugar—and found that Physarum needed both to survive. But too much protein, and Physarum would split apart; too much sugar proved fatal. With just the right balance, Physarum grew and remained healthiest.

Audrey and Tanya had found Physarum’s optimal diet, now they were ready to test if Physarum could find it, when presented with multiple options.

AUDREY DUSSUTOUR: So, we offered Physarum a buffet. In this buffet, Physarum had a variety of custards to choose from, each with a different ratio of protein-to-sugar.

NARRATOR: Could Physarum find one with the proper balance, among these seven different ratios of nutrients? There was one, slight problem.

AUDREY DUSSUTOUR: Physarum is slow. It is extremely slow…

NARRATOR: Way slower than a slug; not even half the speed of the minute hand on this watch. Physarum is slow.

AUDREY DUSSUTOUR: ... at least 24 hours for a single experiment.

NARRATOR: So, to observe and record their slime mold experiments, Tanya and Audrey relied on time-lapse photography. In images taken over periods of hours, and even days, Physarum comes to life, unfurling its tendril-like extensions called pseudopods.

TANYA LATTY: It’s detected that food cue and extended a pseudopod out. I get the impression they don’t super like that lighter blue food source.

NARRATOR: In fact, it contains levels of sugar that would be lethal to Physarum, and so, it heads elsewhere.

TANYA LATTY: You can see it’s sort of dividing into a bunch of pseudopods. It’s contacted that one in the middle. I’m not sure if it’s actually chosen that one.

NARRATOR: It’s the best ratio it’s sensed so far.

It begins eating, and continues searching.

TANYA LATTY: Slime mold can, and so it does. It will first engulf the food and then send out pseudopods to start looking, because you never know what’s around the corner.

NARRATOR: The slime mold soon abandons that source.

TANYA LATTY: It’s sort of shifted. The slime mold clearly likes the left side better.

NARRATOR: On this side, is the best nutritional balance.

Given seven options, Physarum found the optimal ratio of protein to sugar.

AUDREY DUSSUTOUR: Blob headed towards the custard that would maximize growth. It will always try to optimize both protein and sugar.

NARRATOR: But had Physarum made a decision? Was this truly a choice?

TANYA LATTY: We have to be very careful using words like “choice” or “decide.” We’re not saying that they have feelings or that they’re thinking or doing those decisions in the same way that we do. What we are saying is that slime molds are able to make choices, because if you give them an option between two or more items, they will consistently make certain choices over others.

NARRATOR: Choice or not, they had conclusively shown that Physarum had effectively taken in information about its environment and turned that into action, action that, in itself, is mystifying.

AUDREY DUSSUTOUR: It begs the question, “How does a cell move, if it has no appendages, no legs?” It’s a system quite unique in nature. If you look at it closely, you see that it is crisscrossed with a network of veins.

TANYA LATTY: The veins of slime mold are constantly pulsing, and that drives it forwards.

NARRATOR: In Bremen, Germany, Hans-Günther Döbereiner and his team study the mechanics of Physarum’s movement.

JONGHYUN LEE (University of Bremen): There is a stream of what we call protoplasm. You can see, here, that there’s a flow within these veins.

NARRATOR: That flow is produced by proteins that wrap around the veins and rhythmically squeeze and relax, propelling the protoplasm within.

HANS-GÜNTHER DÖBEREINER (University of Bremen): And that liquid is driven through the veins to the “growth front” of Physarum.

NARRATOR: The growth front is Physarum’s forward-moving branches, fed by these veins. But how does Physarum coordinate this movement in order to navigate towards its next meal?

TANYA LATTY: In our bodies, we have a brain that tells the rest of our body what to do. That’s not the case in the slime mold, at all. It’s totally decentralized, and so, every bit of slime mold is essentially deciding for itself.

NARRATOR: What “every bit” would be in a single-celled organism is confusing, even for the people that study Physarum. So, to better understand what the “parts” are that make up this single cell, Hans-Günther’s team shredded Physarum into thousands of tiny fragments. The veins, the pseudopods, all of the familiar structures are gone. What remains are random bits of protoplasm, the goo that Physarum is made of.

For Physarum, this is a minor setback. Viewed under a microscope, these bits are already pulsing. And as they watch, Physarum begins forming back together.

HANS-GÜNTHER DÖBEREINER: These separate, small objects join up to form an interconnected network. You can see more and more of them coming together.

NARRATOR: Physarum is reassembling itself into its familiar network of veins, fanning out in every direction. Hans-Günther has found that Physarum’s parts are interchangeable.

HANS-GÜNTHER DÖBEREINER: A dog is always a dog. A bacterium is always a bacterium. But Physarum is more like... a Transformer. It’s a natural, biological Transformer™.

TANYA LATTY: Any part of the slime mold can pretty much recreate any other part of the slime mold. And that’s because, if you think of a slime mold as being almost like a soup, there is all these different components of the cell that do different jobs. They can organize and pack tightly to form a vein, they can be less packed together to form the free-flowing cytoplasm.

It will just reorganize itself and become a slime mold again, with all the normal slime mold bits. So it can totally reorganize itself.

NARRATOR: These individually pulsing bits are the key to its movement. If certain areas begin beating faster than others, Physarum will move in that direction.

TANYA LATTY: Cytoplasm will flow in the area where there’s the most pulsing.

NARRATOR: And one thing that controls these pulses is Physarum’s receptors.

TANYA LATTY: Once their receptor detects the food, that triggers that part of the slime mold that’s closest to the food to start pulsing faster. And so the slime starts to flow in the direction of the food.

NARRATOR: Hans-Günther has been mapping Physarum’s patterns of structure and movement, and says that, in many ways, it resembles the flow of traffic on human transportation networks.

In Japan, members of Toshiyuki’s team have been studying this.

DANIEL SCHENZ(Hokkaido University):Physarum is particularly interesting, because it is, basically, an adaptive transportation network.

NARRATOR: In this experiment, Physarum is meticulously placed over the entire surface of the labyrinth. At the entrance and exit are oat flakes, to see how Physarum will handle the presence of two food sources. One by one, it eliminates unproductive pathways, until there’s just one vein left, linking the two food sources by the shortest path. By reconfiguring itself in this way, Physarum has optimized its intake of nutrients.

But two points are easy. Toshiyuki began to test Physarum’s ability to optimize its form over many more points, eventually increasing the number to 37. The number of possible ways to connect 37 points is somewhere in the neighborhood of an eight, followed by 54 zeros.

Can Physarum find the optimal way?

And to make it interesting, Toshiyuki and his team gave Physarum a real-world problem, one that human engineers had already solved: find the most efficient transportation network in the greater Tokyo area.

Using oat flakes to represent principal towns and cities, Physarum is placed where Tokyo would be. Can Physarum create an efficient network?

Physarum explores its environment and, as it discovers the oat flakes, reconfigures its network of veins, reinforcing the links between the food sources and making the others disappear.

TANYA LATTY: What we are getting is something that looks an awful lot like the actual Tokyo metro system, far more like it than we would expect by chance.

NARRATOR: Physarum has created an optimal network, something it’s done successfully for millions of years, long before humans came along.

TANYA LATTY: We noticed, almost always, when they were searching a plate, they would never go back over where they had been before. And that was unusual.

NARRATOR: Unusual, because that would seem to imply that Physarum would have to remember.

TANYA LATTY: We assumed that it didn’t have any classic memory, that it wasn’t remembering where it had been, which left the question, “Well then how is it doing it?”

NARRATOR: The answer came from Physarum itself. It had been leaving a trail of clues, written in slime.

TANYA LATTY: This grey stuff, that’s the slime trail, which is kind of like the slime trails that we see behind slugs.

AUDREY DUSSUTOUR: So, you have to ask, “Does Blob use its trail to memorize its environment?”

NARRATOR: Ants emit trails of pheromones to mark their paths to where they’ve found food. Then, other members of the colony use that trail as an “external memory,” to navigate to the food and back.

TANYA LATTY: Because we worked with ants, we were kind of familiar with this idea of having an external form of memory. It’s not encoded in a brain, it’s marked on the environment.

NARRATOR: In Physarum’s case, its trail is a signal to keep away.

AUDREY DUSSUTOUR: That slime is, in fact, a repellent. Slime mold never wants to go back to where it has left its own slime.So, we set up an experiment to see if this slime was being used as a kind of external memory: the “u-shaped trap.”

NARRATOR: The “u-shaped trap” was an experimental challenge, used in early machine learning research, to test things like a robot’s navigational memory. A robot is given a goal, which it can sense, and a u-shaped barrier, which it can’t.

TANYA LATTY: I mean, if you don’t have a system of memory or some other mechanism you just keep running into the wall. And that’s not a good thing.

NARRATOR: Robots without a system of memory would often remain trapped in this loop.

In this experiment, Physarum’s goal was food.

AUDREY DUSSUTOUR: Blob can spot the food source at a distance, because it is diffusing into the environment.

TANYA LATTY: If the slime mold just follows that smell cue, it’s going to hit a wall. And without some sort of memory or some other system, we expect it to just keep smashing itself against that wall, and it won’t escape.

AUDREY DUSSUTOUR: So, what will Blob do?

TANYA LATTY: So, the slime mold would go down, hit the wall, but when it goes to do it the second time it detects that it’s already been there and sort of takes a step back and you can almost see it tracing the wall and then coming up the side and zipping down and finding the food source really quickly.

AUDREY DUSSUTOUR: So, once we’d shown that Blob was able to do that, we performed the experiment a second time, but this time, we covered the environment with slime. We made Blob think it had already explored the area.

TANYA LATTY: The slime mold can still detect there was a food cue and it still moved towards it, but it was unable to tell the difference between where it had been and where it hadn’t been yet. And so, because of that, when it got to the bottom of the U, it just gets stuck and can’t quite figure out how to get out.

AUDREY DUSSUTOUR: It was no longer able to find the food source.

NARRATOR: And so, Tanya and Audrey concluded that Physarum’s trail does, in fact, serve as an external memory.

For slime mold researchers, the mounting evidence of these behaviors lead them to pose a question that makes many scientists nervous.

AUDREY DUSSUTOUR: When you see what Physarum polycephalum can do—it can get through a labyrinth, create optimized networks, escape from a trap, balance its own diet—you start to think: “Does this organism possess intelligence?”

TANYA LATTY: So, that word is tricky…

MICHAEL LEVIN: Defining intelligence is not an easy thing, and, certainly, many people have different definitions for it.

STEFANO MANCUSO (University of Florence): The question of intelligence depends a lot on how we define intelligence.

FRANTIŠEK BALUŠKA (University of Bonn): Due to the fact that it is so tightly linked to the human beings, it is very difficult now to speak about intelligence in other organisms.

TOSHIYUKI NAKAGAKI: A commonly accepted idea is that the only living organisms who display intelligence are those with a nervous system and brain. I say, with certainty, that I believe this idea is wrong. In my opinion, I call this intelligent behavior.

NARRATOR: For centuries, scientists believed the trait of intelligence, the ability to reason and think, was unique to Homo sapiens. It wasn’t until the mid-20th century that researchers started to document cognitive abilities in our primate cousins, as well as other animals, farther and farther from us on the evolutionary tree: chimpanzees and even crows making and using tools; whales and dolphins using sound to communicate complex ideas; birds demonstrating memories of places, things and past events: sea lions that can grasp logic; and evidence that many animals can actually count.

Audrey and Tanya now found themselves among the small but growing ranks of researchers pushing the boundaries of intelligence beyond the animal kingdom altogether, searching for signs of intelligence in organisms with no brains at all. The first conclusive evidence was soon found in plants.

In Florence Italy, the idea of plant intelligence has become the focus of some serious scientific study, thanks to these two biologists: Stefano Mancuso and František Baluška, who founded the Society for Plant Signaling and Behavior.

The notion of intelligence in plants is not new. Charles Darwin, himself, faced ridicule by his peers, when, afterextensive experimentation, he proposed that plants possessed a cognitive organ and that it resided in their root tips.

STEFANO MANCUSO: Darwin says that at the end of a root, there is the precise equivalent of a small brain, like the brain of an insect, that guides the plant.

FRANTIŠEK BALUŠKA: We started to argue that this theory is really not crazy theory.

NARRATOR: They came to this conclusion after conducting their own root-tip experiments.

STEFANO MANCUSO: It was thought that a root arrived at an obstacle and did something like that [Gestures with hands, mimicking a root tapping it’s way around an obstacle]to find a way out. Instead, what I saw was different. The roots turned well before reaching the obstacle, they were able to sense what was around them and calculate the shortest path in response.

NARRATOR: Stefano thought that this kind of sensing behavior would require coordination of the cells in the root tip. He discovered that a specific region was sending out electrical signals to the surrounding area. Could this be the “brain” that Darwin hypothesized?

When this section was cut off, the root could still grow, but it lost the ability to sense its environment and adapt accordingly. From this, Stefano concluded that Darwin was correct: plant roots did appear to contain a kind of sensory organ.

STEFANO MANCUSO: Plants are like upside-down humans. They have their head underground, and the part we can see is the reproductive part. And that’s how we should look at plants: something like this pot.

NARRATOR: Since plants now did seem to have some form of sensory ability, Stefano and František began searching for more complex behavior, which might conclusively demonstrate intelligence: memory and learning.

One of the most basic types of learning in brained creatures can be seen in these pigeons, who’ve learned not to fear humans. They even tune them out. “Habituation” as it’s called, enables organisms to focus on the important information, while filtering out the noise in the world around them.

To test for this type of learning in plants, Stefano and his colleagues turned to this intriguing specimen: Mimosa pudica, which reacts to disturbances by folding its leaves.

If this reaction is purely mechanical, they should respond this way every time they’re disturbed, even after numerous disturbances that don’t prove to be harmful.

So, they repeatedly dropped plants from a height of six inches, not particularly harmful for pudica. And after only five or six drops, the plants stopped reacting. And yet, when they were disturbed in other ways, they still responded by folding their leaves.

STEFANO MANCUSO: We showed that Mimosa pudica was able to memorize different stimuli, and to differentiate between a dangerous stimulus and one that wasn’t dangerous and then respond accordingly.

NARRATOR: This was, indeed, quite a sensation.

Stefano and his colleagues had documented intriguing evidence of memory and learningin a brainless organism. For Audrey, this raised a tantalizing question.

AUDREY DUSSUTOUR: So, with Physarum we asked, “Are we going to find this kind of ability that can be found in plants?”

NARRATOR: She and Tanya had proven that Physarum used a kind of external memory, but was it possible that Physarum possessed some true internal memory?

AUDREY DUSSUTOUR: Is slime mold capable of remembering? Is it capable of learning? That question is the Holy Grail.

NARRATOR: Proving that Physarum had these capabilities could show that a single-celled creature was capable of intelligence and might very well force scientists to reconsider the idea of intelligence itself.

AUDREY DUSSUTOUR: So, we set up an experiment to see if Blob learns things: a habituation experiment.

NARRATOR: The objective was to try and habituate Physarum to a substance it was repulsed by, in this case, salt.

Between this Physarum and a tasty snack, is a bridge a few centimeters long. Normally, Physarum makes the crossing in about an hour.

For her experiment, Audrey coated the bridge in salt, not enough to harm Physarum, but enough to leave a “bad taste.”

AUDREY DUSSUTOUR: It took Blob 10 hours to cross the bridge. The next day, we’d do it again.

NARRATOR: By day two, that same Physarum was moving faster over the salt bridge.

AUDREY DUSSUTOUR: This time it takes eight hours to cross the bridge.

NARRATOR: Audrey repeated this for five days, and each day, Physarum moved more quickly across the salt bridge.

AUDREY DUSSUTOUR: By the fifth day, the blob takes no more time than our “control” blob that crossed the bridge with no salt. So, Physarum ends up getting used to a substance it doesn’t like.

NARRATOR: It seemed that Physarum learned to tolerate, or habituate to the salt. Had Physarum actually learned that the salt wasn’t harmful? If this were true, it would mean that somewhere, inside Physarum, there was a formof memory.

AUDREY DUSSUTOUR: Of course, some were skeptical, because we were up against the dogma that learning was strictly reserved for organisms with a brain.

NARRATOR: So, Audrey wasn’t leaving things to chance.

AUDREY DUSSUTOUR: If you want to prove that a single-celled creature can learn, you’ve got to be sure you can convince your colleagues.

NARRATOR: On each of these tiny salt bridges, they repeated the experiment, again and again.

AUDREY DUSSUTOUR: Four-thousand times, on 4,000 different blobs.

NARRATOR: The results were consistent: Physarum had altered its behavior through its past experiences, a fundamental definition of learning.

Audrey had demonstrated learning and memory in a single-celled organism. A creature this simple demonstrated the ability to learn.

But how do they do it?

MICHAEL LEVIN: Audrey showed very nicely, in her work, that Physarum clearly has memory and learned from its experience to adaptively make decisions about where it’s going to go and how it’s going to explore its environment. All of these are fundamental components to being smart.

NARRATOR: Director of the Allen Discovery Center, Michael Levin, listens in on the electrical signals exchanged between cells.

MICHAEL LEVIN: My lab focuses on bioelectricity. All cells are electrically active and these electrical signals enable cells to join into networks that store memories and process information and make decisions.

NARRATOR: It was previously thought that these cognitive abilities came from the conversations between many cells, but Physarum had just proven that wrong.

MICHAEL LEVIN: So, Physarum is one single cell. So, this takes us away from questions of cell to cell communication, all these things that multicellular organisms do, that we think of as really important for cognition. But clearly now we see that this can be done by one cell.

NARRATOR: Michael learned a lot about cellular communication from his work with a curious multi-celled creature called a planarian, a small aquatic worm with the unusual ability to re-grow missing or damaged parts of its body.

MICHAEL LEVIN: They regenerate every part of the body. So, if cut into pieces, every piece of a planarian knows exactly what a standard planarian body should look like.

NARRATOR: When a planarian is cut in half, its cells regenerate to rebuild a head at one end and a tail at the other.

MICHAEL LEVIN: Everything that’s missing in the correct location… and it stops when it’s done.

NARRATOR: But how? Michael and his team began to tinker with planarian cells to see if they could affect how these worms controlled their regeneration.

MICHAEL LEVIN: We’ve discovered that part of this control is an electric circuit that allows these cells to store this kind of information. And what we found is that if we temporarily, just for 48 hours, alter this electric circuit and, in essence, “rewrite” the finely encoded pattern, memory of these tissues, when they regenerate they can regenerate as two-headed animals.

NARRATOR: Michael and his team did this without any kind of gene editing. It was done purely through the manipulation of the signals between its cells.

MICHAEL LEVIN: And so, this tells you that the information that is needed to specify what kind of body you are going to make is not entirely specified in the genome.

NARRATOR: Michael says that cells function like electronic processors.

MICHAEL LEVIN: Bioelectricity is the software that is run on these cells, and it can be re-written.

NARRATOR: Michael wondered if Physarum was using bioelectricity as its system of memory.

MICHAEL LEVIN: What does it mean for a chunk of cytoplasm, that you can pick up and hold in your hand and analyze molecularly, electrically, and so on, to really store an experience or a memory or an inference from past experiences.

NARRATOR: So, they tested Physarum’s veins for signs of bioelectrical signals, and found that it produced electrical waves that matched the flow of its protoplasm within.

MICHAEL LEVIN: What we’re doing is attempting to read the mind of the Physarum.

NARRATOR: As it turns out, reading Physarum’s mind is all about body language.

MICHAEL LEVIN: One of the important things about Physarum, its body structure and behavior are simultaneous, in other words, the behavior of the slime mold is executed by altering its body structure.

NARRATOR: Meaning, if Physarum was communicating internally using bioelectric signals, those signals would change along with Physarum’s shape.

Michael’s colleague Andrew Adamatzky attempted to communicate with Physarum by sending it electrical signals. He found that different signals would cause Physarum to grow in different ways. Some signals sent between two points made Physarum reconfigure its body as it would between two oat flakes. Other signals repelled Physarum, causing it to shift its body away from the source, much in the way it does with salt.

Andrew and Michael believe that this shows Physarum is using bioelectricity to process information, as it adapts to the world around it, and that Physarum’s simple design was a step along the way towards the evolution of multicellular organisms with specialized cells, like neurons, and more complex brain circuitry.

MICHAEL LEVIN: I think evolution discovered, very early on, that electricity and the circuits that you can make out of very simple electrical components are an extremely powerful and robust system for processing information, making decisions, storing memories.

NARRATOR: Things it now appears that Physarum was doing long before the arrival of creatures with brains.

AUDREY DUSSUTOUR: We started to wonder whether a blob that had learned something could transfer its learning to another blob.

NARRATOR: In other words, could Physarum communicate a memory?

Because of what she had learned about Physarum’s shape-shifting nature,Audrey knew she could cut it into pieces and it would reform.

AUDREY DUSSUTOUR: If you take a blob and cut it in half, you always end up with two separate blobs. So, how does that work?

NARRATOR: It works because of Physarum’s odd, soupy anatomy. Most single celled organisms have a single nucleus. When they grow by dividing, each new cell still has a single nucleus. If you were to cut that in half you’d have two halves of a dead cell. But when Physarum grows, its nuclei divide, but they all remain within the same cell membrane.

And that’s why, when you divide one Physarum in half, you will have two complete living Physarum. And these Physarum can also join together with any other Physarum that they encounter.

AUDREY DUSSUTOUR: If you then take those two blobs and put them side by side, they’ll combine. For blobs, one plus one equals one.

The fusion occurs when the two membranes stick together. They open up, and then the networks of veins connect to create a single, autonomous blob.

NARRATOR: So, Audrey brought together thousands of Physarum that she had accustomed to salt, with others that weren’t—so-called “naïve” ones.

AUDREY DUSSUTOUR: If we allowed two blobs to fuse together for three hours, the information passed from one to the other. The naïve blob now liked salt.It looked as though the memory was passed through the system of veins.

NARRATOR: Slime mold demonstrated the ability to communicate the information it had learned. And over time, Audrey began to notice that Physarum from different places showed different behaviors.

To Audrey, this is slime mold personality.

AUDREY DUSSUTOUR: We realized that, as there were lots of differences between our blobs.

NARRATOR: Depending on where they came from, her Physarum seemed to have distinct characteristics. The Japanese Physarum is fast, the Australian, careful and well-mannered.

AUDREY DUSSUTOUR: Its behavior is much more consistent from one day to another, and it does not escape.

NARRATOR: The American is problematic.

AUDREY DUSSUTOUR: When the American blob arrived, we gave it organic oat flakes. And it completely refused. It even got out of its box, because it preferred the well-known American brand.

NARRATOR: She put them to the test in a 250-millimeter dash.

SOUND OF A STARTER’S PISTOL

NARRATOR: And out of the blocks, Japan is certainly showing speed, with America close behind.

AUDREY DUSSUTOUR: The Japanese blob is extremely fast and often makes bad decisions.

NARRATOR: Australia has not yet started.

AUDREY DUSSUTOUR: The Australian blob takes a little more time to make decisions.

NARRATOR: Japan, now sending pseudopods in the wrong direction, has lost its lead to the U.S., but the U.S. appears to be slowing down, possibly attempting an escape. And Japan takes back the lead. Australia still not sure whether to start. America, now moving forward again. And the winner: Japan!

The discoveries of these pioneers studying cognition in brainless organisms have scientists rethinking thinking. Organisms like slime mold provide a window into evolution’s early experiments in problem solving.

MICHAEL LEVIN: We’re just starting to scratch the surface of understanding the cognitive capabilities of these really radically different biological architectures.

TANYA LATTY: We can think of them as, “this is how intelligence started way back, the early origins of decision making.” We can also learn a little bit about some of the rules that underlie problem-solving across lots of different types of systems.

NARRATOR: And more importantly, for us at least, early solutions to problem-solving, like these, would lead to the specialization of cells.

TANYA LATTY: They’re an example of, sort of, the most basic form of multicellularity. They don’t have organs or anything like that, so they’re the very beginning of what we would expect to see in multicellular life.

MICHAEL LEVIN: Neurons did not appear from scratch during evolution but actually are the result of the specialization of much more ancient cells. All of the machinery that we find in neurons, in synapses, all of these things were present even in our unicellular ancestor.

NARRATOR: Leading to the development of more complex life, and, eventually, to creatures like us.

Credits

PRODUCED, DIRECTED AND EDITED FOR NOVA BY

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Craig Sechler

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Inspired by the bookTout ce que vous avez toujours voulu savoir sur le Blob sans jamais oser le demander by Audrey Dussutour © Editions Equateurs, 2017

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A NOVA Production by Semicolon Pictures LLC for WGBH Boston in association with Hauteville Productions for ARTE France and CNRS Images

Production support for Secret Mind of Slime provided by Centre national du cinéma et de l'image animée

Original Secret Mind of Slime production funding provided by La Région Occitanie / Pyrénées – Méditerranée and  Procirep-Angoa

© 2020 WGBH Educational Foundation

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The Blob: A Genius without a Brain: © Hauteville Productions, ARTE France and CNRS

This program was produced by WGBH, which is solely responsible for its content. Some funders of NOVA also fund basic science research. Experts featured in this film may have received support from funders of this program.

Original NOVA funding for this program was provided by Draper, the David H. Koch Fund for Science, the NOVA Science Trust and the Corporation for Public Broadcasting.

Image credit: (Slime mold)

© Hauteville Productions, ARTE France and CNRS Images

Participants

František Baluška, Hans-Günther Döbereiner, Audrey Dussutour, Tanya Latty, Jonghyun Lee, Michael Levin, Stefano Mancuso, Toshiyuki Nakagaki, Daniel Schenz