The scientific story of slime mold stretches back more than two centuries, but the most remarkable discoveries have come in just the last 25 years. What was once dismissed as an obscure fungal curiosity is now the subject of thousands of research papers and has been sent to the International Space Station. Here is how that journey unfolded.
Early Observations: A Classification Puzzle
The first naturalists to encounter slime molds had no idea what they were looking at. These bright yellow, creeping masses on rotting logs defied the categories available at the time.
In 1654, the German botanist Thomas Panckow included a slime mold in his botanical catalog, describing it as a type of fungus. This misclassification would persist for over two centuries. The organisms looked somewhat like fungi, grew in the same damp habitats, and produced spore-bearing structures.
In 1822, Lewis David de Schweinitz, an American mycologist of German descent, formally described Physarum polycephalum as a species. His description placed it firmly within the fungi, a kingdom where it would remain, at least officially, for a long time.
By the mid-19th century, however, some scientists were beginning to have doubts. The German biologist Heinrich Anton de Bary published extensive studies on slime molds in the 1850s and 1860s, observing their amoeba-like movement and unique life cycle. De Bary coined the term "Mycetozoa" (fungus-animals) to reflect their ambiguous nature. He recognized that these organisms behaved more like animals than fungi, but lacked the tools to properly reclassify them.
A Name That Misled Generations
The term "slime mold" dates to this era. It was a practical label for organisms that looked slimy and moldy. Unfortunately, it embedded a fundamental error: slime molds are not molds, and they are only distantly related to fungi.
The 20th Century: Slime Mold Enters the Lab
By the early 1900s, Physarum polycephalum had become a popular laboratory organism, but not for its behavior. Researchers valued it because its giant single cell made it ideal for studying basic cell biology: nuclear division, cytoplasmic streaming, and cell cycle regulation.
In the 1930s and 1940s, several groups of researchers worked out the complete life cycle of Physarum, from spore to amoeba to plasmodium to fruiting body. This work established the organism as a model system in developmental biology.
Throughout the 1960s and 1970s, Physarum was heavily used in biochemistry research. Its synchronous nuclear division (all nuclei in the plasmodium divide at the same time) made it uniquely useful for studying the mechanics of mitosis. Hundreds of papers were published on its cell biology during this period.
But the most extraordinary properties of slime mold were hiding in plain sight. It would take a paradigm shift in thinking about intelligence and computation to reveal them.
Timeline of Major Discoveries
| Year | Event | Significance |
|---|---|---|
| 1654 | First description by Thomas Panckow | Earliest known record of slime mold in scientific literature |
| 1822 | Schweinitz formally describes P. polycephalum | Species receives its scientific name |
| 1858-1864 | De Bary's studies on myxomycetes | First recognition that slime molds are not true fungi |
| 1960s-1970s | Cell biology and biochemistry research boom | Physarum becomes a standard lab organism |
| 1990s | Molecular studies place myxomycetes among the Amoebozoa | DNA confirms that slime molds sit far from the fungi |
| 2000 | Nakagaki publishes maze experiment | First proof that a brainless organism can solve spatial problems |
| 2008 | Saigusa et al. demonstrate anticipatory behavior | Slime mold can predict periodic events |
| 2010 | Tero et al. Tokyo rail network study | Slime mold designs networks rivaling human engineers |
| 2010 | Adamatzky publishes Physarum Machines | Framework for unconventional biological computing |
| 2010 | Dussutour et al. nutritional decision-making | Slime mold actively balances its diet |
| 2012 | Reid et al. external spatial memory | Slime mold avoids areas covered by its own slime trail |
| 2016 | Dussutour et al. habituation and memory transfer | Learning without a brain, then passed on through fusion |
| 2017 | Audrey Dussutour publishes her book on the "blob" | The nickname takes hold with the French public |
| 2019 | A slime mold goes on show at the Paris Zoological Park | The blob becomes a media star in France |
| 2021 | Physarum sent to the ISS | Thomas Pesquet wakes four slime molds in orbit while 4,500 schools run the same experiment |
| 2022 | CNRS citizen science project "Behind the blob" | About 15,000 volunteers test the effect of rising temperatures |
2000: The Maze That Changed Everything
The turning point in slime mold research came with a simple, elegant experiment. Toshiyuki Nakagaki, a Japanese biophysicist working with Hiroyasu Yamada and Ágota Tóth, filled every corridor of a small plastic maze with pieces of Physarum polycephalum and let them fuse into a single organism. He then placed oat flakes at two exits of the maze.
The slime mold initially spread throughout the maze, sending exploratory tendrils down every corridor. Within hours, it had retracted from the dead ends and reinforced only the shortest path between the two food sources. It had solved the maze.
The paper, published in the journal Nature in September 2000, sent shockwaves through the scientific community. Here was an organism with no brain, no neurons, and no central processing of any kind, finding optimal solutions to spatial problems. The title of the paper was deliberately provocative: "Maze-solving by an amoeboid organism."
Nakagaki's work opened up an entirely new field of research. If slime mold could solve mazes, what else could it do?
Adamatzky and Biological Computing
Andrew Adamatzky, a computer scientist at the University of the West of England, was among the first to see the computational implications of Nakagaki's discovery. If slime mold could find shortest paths, it was essentially performing the same operation as certain graph algorithms used in computer science.
Throughout the 2000s and 2010s, Adamatzky and his collaborators explored the computational capabilities of Physarum with remarkable creativity. They showed that slime mold could:
- Compute shortest paths in weighted graphs
- Approximate Steiner trees (a notoriously difficult optimization problem)
- Implement basic logical gates when combined with other stimuli
- Model motorway networks of multiple countries, often matching or improving on the actual designs
- Solve the Traveling Salesman Problem for small numbers of cities
Adamatzky's 2010 book Physarum Machines: Computers from Slime Mould laid out a comprehensive framework for using living organisms as unconventional computing substrates. The idea was not that slime mold would replace silicon chips, but that studying how it computes could inspire new algorithms and architectures.
Computing Without Electricity
Adamatzky's work demonstrated that computation does not require electronics, transistors, or even neurons. A living network of cytoplasm, responding to chemical gradients and internal oscillations, can perform meaningful calculations. This insight has influenced research in fields ranging from swarm robotics to urban planning.
2010: The Tokyo Rail Experiment
One of the most widely cited slime mold experiments came in 2010, when Atsushi Tero, working with Nakagaki and others, published a study in Science. The team placed oat flakes on a map in positions corresponding to the major cities of the Tokyo metropolitan area. They then let Physarum grow from central Tokyo outward.
The network that the slime mold formed was strikingly similar to the actual Tokyo rail system, a network that had been designed and refined by engineers over more than a century. Measured on cost, transport efficiency and resistance to breakdowns, the slime mold's networks were comparable to the real one, and some of them held up slightly better when a link was cut.
This experiment captured the public imagination like no other. It was covered by media worldwide and became a powerful demonstration that sophisticated optimization does not require sophisticated intelligence in the conventional sense.
Dussutour: Memory Without a Brain
While Nakagaki and Adamatzky explored what slime mold could compute, French biologist Audrey Dussutour at the Centre National de la Recherche Scientifique (CNRS) in Toulouse asked a different question: can slime mold learn?
In a 2016 paper in Proceedings of the Royal Society B, Romain Boisseau, David Vogel and Dussutour showed that Physarum polycephalum can learn through habituation, the simplest form of learning. The slime mold had to cross a bridge laced with quinine or caffeine, two bitter substances it normally avoids, to reach its food. At first it hesitated for hours or refused to cross. Over several days it learned that the substance did it no harm and crossed as fast as it did over a plain bridge.
Later the same year, Vogel and Dussutour went one step further. They trained slime molds to cross salt, then fused them with slime molds that had never met salt. The fused organism crossed without hesitation, as if it had been trained. The learned behavior had been passed on through cell fusion, even when the trained partners were in the minority.
Habituation had long been treated as a hallmark of animals with a nervous system. Finding it in a single cell, and seeing it pass from one cell to another, raised new questions about what memory needs at the most basic level.
Read the full story in our dedicated article on slime mold memory.
2021: Slime Mold in Space
On 12 August 2021, four dry sclerotia of Physarum polycephalum reached the International Space Station in a sealed box designed by CNES with Audrey Dussutour. On 2 September, French astronaut Thomas Pesquet injected a few drops of water into the box, and all four slime molds woke up and grew in weightlessness, watched by a camera that took a picture every ten minutes.
The project was educational first and foremost. Four slime molds are too few to support a scientific paper, and Dussutour said so before the flight. Its real scale was on the ground, where 4,500 French schools were invited to run the same experiment and compare their slime molds with the ones in orbit. The full story is told in our page on slime mold in space.
The following spring, CNRS launched "Behind the blob" (Derrière le blob, la recherche), a citizen science project led by Dussutour in which about 15,000 volunteers raised slime molds at home to see how they respond to varied conditions, temperature in particular.
From Obscurity to Orbit
In just two decades, slime mold went from being a niche laboratory organism studied mainly for its cell biology to a globally recognized research subject, a tool for unconventional computing, and a passenger aboard the International Space Station. Few organisms in the history of science have had such a dramatic rise in status.
What Comes Next?
Current research on Physarum polycephalum spans a remarkable range of fields:
- Computer science: Bio-inspired algorithms based on slime mold network formation are being applied to logistics, telecommunications, and supply chain optimization.
- Robotics: Soft robotics researchers are developing slime mold-inspired actuators that move using oscillating flows rather than motors.
- Cognitive science: Dussutour's work on memory has opened new questions about the minimum biological requirements for learning.
- Urban planning: City planners have used slime mold simulations to evaluate and improve existing transport networks.
The history of slime mold research is, in many ways, a history of changing assumptions. Each decade has brought discoveries that challenged what scientists thought they knew about intelligence, computation, and the boundaries of biological possibility.
To understand the organism itself, start with What Is Slime Mold? or explore the complete fact sheet.