Imagine you are walking through a forest and rolling over a log with your foot. On its underside sprawls something wet and yellow – much like something you might have sneezed at…if that something was banana yellow and fanned out into elegant fractal branches.
What you would see is the plasmodium form of Polycephalic Physarum, the multi-headed slime mold. Like other slime molds found in nature, it performs an important ecological role, helping to break down organic matter to recycle it into the food web.
This weird little organism has no brain or nervous system; its mottled, bright yellow body is just a cell. This species of slime mold has thrived, more or less unchanged, for a billion years in its moist, decaying habitats.
And, over the past decade, it’s changed the way we think about cognition and problem solving.
“I think it’s the same kind of revolution that happened when people realized that plants could communicate with each other,” said biologist Audrey Dussutour of the National Center for Scientific Research.
“Even these tiny microbes can learn. It gives you a little humility.”
P. polycephalic – adorably dubbed “The Blob” by Dussutour – isn’t exactly uncommon. It can be found in dark, moist, and cool environments like leaf litter on a forest floor. It’s also really special; although we call it a “mold”, it is not actually a fungus. It is also not an animal or a plant, but a member of the Protista kingdom – a sort of catch-all group for anything that cannot be clearly classified into the other three kingdoms.
It begins its life as several individual cells, each with a single nucleus. Then they fuse together to form the plasmodium, the vegetative life stage in which the organism feeds and grows.
In this form, fanning out in veins to forage for food and explore its surroundings, it is still a single cell, but containing millions or even billions of nuclei swimming in the cytoplasmic fluid confined within the bright yellow membrane.
Like all organisms, P. polycephalic must be able to make decisions about its environment. He needs to search for food and avoid danger. It needs to find the ideal conditions for its reproductive cycle. And this is where our little yellow friend gets really interesting. P. polycephalic does not have a central nervous system. He doesn’t even have specialized fabrics.
Yet he can solve complex puzzles, like mazes, and memorize new substances. The kind of tasks we used to think only animals could do.
“We’re talking about brainless cognition, obviously, but also without any neurons. So the underlying mechanisms, the whole architectural framework of how it processes information is totally different from how your brain works,” said said biologist Chris Reid. from Macquarie University in Australia told ScienceAlert in 2021.
“By providing it with the same problem-solving challenges that we have traditionally given to animals with brains, we can begin to see how this fundamentally different system could achieve the same result. This is where it becomes clear that for many of them things – which we’ve always thought required a brain or some sort of higher information processing system – that aren’t always necessary.”
P. polycephalic is well known to science. Decades ago, it was, as physicist Hans-Günther Döbereiner of the University of Bremen in Germany explained, the “workhorse of cell biology”. It was easy to clone, store and study.
However, as our tools for genetic analysis have evolved, organisms such as mice or cell lines such as HeLa have taken over, and P. polycephalic fell by the wayside.
In 2000, biologist Toshiyuki Nakagaki from RIKEN in Japan brought the little beast out of retirement – and not for cell biology. His article, published in Naturewas titled “Maze Solving by an Amoeboid Organism” – and that’s exactly what P. polycephalic did.
Nakagaki and his team had placed a piece of plasmodium at one end of a maze, a food reward (oats, because P. polycephalic loves oat bacteria) to another, and watched what happened.
The results were stunning. This strange little acellular organism managed to find the fastest way through every maze thrown at it.
“It sparked a wave of research into what other kinds of tougher scenarios we can test slime mold with,” Reid said.
“Virtually all of these were surprising in some way, and surprised researchers in how slime mold actually worked. It also revealed some limitations. revelation as to how this simple creature can accomplish tasks which have always been assigned and considered the domain of higher organisms.”
Filled with surprises
Nakagaki recreated the Tokyo Metro, with station nodes marked with oats; P. polycephalic recreated it almost exactly – except the slime version was more damage resistant, in which if one link was broken, the rest of the network could continue.
Another team of researchers found that the protist could effectively solve the traveling salesman problem, an exponentially complex mathematical task that programmers routinely use to test algorithms.
Earlier this year, a team of researchers discovered that P. polycephalic can “remember” where it has previously found food based on the structure of the veins in that area. This follows previous research by Dussutour and colleagues, which found that slime mold blobs can learn and remember substances they dislike, and communicate that information to other slime mold blobs. once merged.
“I’m always amazed at how, in a way, they are complex because they always surprise you in an experience, they would never do exactly what you choose to do,” Dussutour said.
In one case, his team was testing a growth medium used for mammalian cells and wanted to see if the slime would like it.
“This hated this. He started building this strange three-dimensional structure so he could take the lead and escape. And I’m like, ‘Oh my God, this organism’.”
A processing network
Although technically a single-celled organism, P. polycephalic is considered as a network, exhibiting a collective behavior. Each part of the slime mold works independently and shares information with its neighboring sections, without centralized processing.
“I guess the analogy would be neurons in a brain,” Reid said. “You have this brain which is made up of a lot of neurons – it’s the same with slime mold.”
– Audrey Dussutour (@Docteur_Drey) April 3, 2021
This brain analogy is truly intriguing, and it wouldn’t be the first time P. polycephalic was compared to a neural network. The topology and structure of brain networks and slime mold patches are very similar, and both systems exhibit oscillations.
It’s not entirely clear how information is propagated and shared in slime mold, but we do know that P. polycephalicIts veins constrict to act as a peristaltic pump, pushing cytoplasmic fluid from section to section. And the oscillations of this fluid seem to coincide with encounters with external stimuli.
“These oscillations are thought to convey information, process information, by the way they interact and actually produce behavior at the same time,” Döbereiner told ScienceAlert.
“If you have a network of Physarum go to a certain food, it changes its oscillation pattern when it encounters sugar: it begins to oscillate faster. Due to these faster oscillations, the whole organism begins to change its pattern of oscillation and begins to flow in the direction where the food was found.”
He and his colleagues published a paper in 2021 demonstrating that these oscillations are extraordinarily similar to oscillations observed in a brain, only a hydrodynamic system rather than electrical signals.
“What is relevant is not so much what oscillates and how the information is transported”, he explains, “but that it oscillates and that a topology is relevant – is a neuron is connected to 100 neurons or just two; is a neuron connected just to its neighbors or is it connected to another neuron far away.”
As exciting as her escapades may seem, any researcher working with her will tell you that P. polycephalic is not, in itself, a brain. It is not capable of higher level processing or abstract reasoning, as far as we can tell.
As intriguing as the notion may sound, it’s also not likely to evolve into something like a brain. The organism has had a billion years to do so and shows no signs of heading in that direction (although if any sci-fi writers like the idea, feel free to run with it).
In terms of overall biology, slime mold is extremely simple. And by that very fact, it changes the way we understand problem solving.
Just like other organisms, it needs food, it needs to navigate its environment, and it needs a safe place to grow and reproduce. These issues can be complex, yet P. polycephalic can solve them with its extremely limited cognitive architecture. It does this in its own simple way and with its own limitations, Reid said, “but that in itself is one of the beautiful things about the system.”
In a sense, this leaves us with an organism—a moist, slimy, moisture-loving blob—whose cognition is fundamentally different from ours. And, just like the Tokyo subway, it can teach us new ways to solve our own problems.
“It really teaches us about the nature of intelligence, challenges some views, and fundamentally expands the concept,” Reid said.
“It forces us to question those long-held anthropocentric beliefs that we are unique and capable of so much more than other creatures.”
A version of this article was first published in June 2021.
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