In 2010, a team of Japanese and British researchers placed a blob of slime mold — Physarum polycephalum, a single cell with no brain, no nervous system, and no neurons — in a model of the Tokyo metropolitan rail network. Food sources were placed at locations corresponding to major stations. Within 26 hours, the slime mold had extended a network of tubes connecting the food sources that was structurally comparable in efficiency and fault tolerance to the actual Tokyo Rail system — a network designed by professional engineers over decades. The slime mold didn’t just find the shortest path between two points. It built a transport network that balanced route efficiency, redundancy, and cost. It solved a multi-objective optimization problem that computer scientists classify as NP-hard. It did this without a single neuron. In 2021, researchers at the Max Planck Institute for Dynamics and Self-Organization discovered how: Physarum stores memories by physically reshaping the architecture of its own body — thickening the tubes where food was found, thinning the tubes where nothing useful existed, and leaving the diameter changes in place as a record of past experience that guides future exploration. The memory is the body. The body is the memory. The organism doesn’t have a brain that stores information about the environment. It turns itself into a map of the environment.
What Physarum remembers
The list of cognitive accomplishments attributed to a brainless, single-celled organism is, at this point, long enough to be unsettling.
Physarum solves mazes. Toshiyuki Nakagaki and colleagues demonstrated in 2000 that when placed in a maze with food at two exits, the slime mold explores the entire maze initially, then prunes its tube network until only the shortest path between the two food sources remains. The pruning is not random — tubes on dead-end branches thin and retract, while tubes on the shortest path thicken with increased cytoplasmic flow. The optimization is hydraulic: cytoplasm flows faster through shorter paths, reinforcing the tubes that carry more flow, in a positive feedback loop that is structurally identical to the ant colony pheromone trail mechanism — except that the ants are a colony of separate organisms communicating through environmental chemicals, and the slime mold is a single cell communicating with itself through fluid dynamics.
Physarum habituates. In 2016, Audrey Dussutour and colleagues at the French National Centre for Scientific Research demonstrated that slime molds can learn to ignore a harmless but aversive substance. When Physarum had to cross a bridge coated with quinine or caffeine to reach food, it initially recoiled and moved slowly. After six to ten exposures, the slime mold crossed without hesitation — it had learned that the bitter substance was harmless. The habituation was substance-specific: a slime mold habituated to caffeine still recoiled from quinine. The habituation persisted for at least two days after the last exposure and then faded — a temporal decay profile that matches habituation in animals with nervous systems, including the sea slug Aplysia, whose habituation was the basis for Eric Kandel’s Nobel Prize-winning work on the molecular mechanisms of memory.
Physarum anticipates. Tetsu Saigusa and colleagues showed in 2008 that when Physarum was exposed to cold, dry conditions at regular intervals, it slowed its movement in anticipation of the next pulse — even after the pulses had stopped. The organism had encoded the timing of a periodic stimulus and was generating a predictive behavioral response. It was expecting something to happen. Without a neuron.
Physarum transfers memory. Dussutour’s team demonstrated in 2016 that when a habituated slime mold was fused with a naive slime mold — which Physarum can do because it’s a single cell that merges with other cells of its species — the resulting fused organism behaved as if habituated. The memory had been transmitted from one cell to another through cytoplasmic fusion. The mechanism is believed to involve signaling molecules — possibly calcium ions or cAMP — that diffuse from the habituated cell into the naive cell and modify its response thresholds. Memory without neurons, transferred without synapses, through a process that looks less like learning and more like infection.
How the body stores information
The Max Planck discovery in 2021, led by Mirna Kramar and Karen Alim, identified the physical mechanism. Physarum is built from a network of interconnected tubes through which cytoplasm flows in rhythmic oscillations. When the organism encounters food, the tubes near the food source soften and dilate — a response mediated by chemical signals that diffuse through the tube network. When the stimulus is removed, the dilated tubes persist. They are wider than they were before the encounter. The width differential encodes the memory: a wider tube means “something useful was here.” When the organism later extends exploratory tendrils, cytoplasm flows preferentially through wider tubes, biasing exploration toward previously rewarding locations. The architecture of the tube network is, literally, a spatial record of the organism’s history.
The elegance of this mechanism is that it is both storage and retrieval in a single structure. Neurons store memories in synaptic weights — the strength of connections between cells. Physarum stores memories in tube diameters — the width of connections between parts of itself. The parallel between synaptic weight and tube diameter is not a metaphor. It is a functional equivalence: both encode past experience as physical changes in a network’s connectivity, and both influence future behavior by altering how signals flow through that network.
The plant cases
Plants lack neurons, brains, and nervous systems. They also exhibit behaviors that meet standard operational definitions of learning and memory — a fact that has generated significant controversy among plant biologists, neuroscientists, and philosophers of mind.
Monica Gagliano at the University of Western Australia demonstrated in 2014 that Mimosa pudica — the “sensitive plant” whose leaves curl when touched — habituates to repeated dropping. Gagliano built a device that dropped potted Mimosa plants from a height of 15 centimeters, 60 times per session. Initially, the plants curled their leaves with every drop. After repeated drops, they stopped responding — they had learned the stimulus was harmless. The habituation was specific: plants that had habituated to being dropped still responded to a new stimulus (shaking). The memory persisted for at least 28 days — longer than many habituation memories in insects. The plant had learned, remembered, and distinguished between stimuli, without a single neuron.
In 2016, Gagliano demonstrated associative learning in pea plants. Seedlings were placed in Y-shaped mazes where one arm contained a fan blowing air. Over training sessions, the fan was paired with a light source. After training, when the light was removed and only the fan remained, the pea plants grew preferentially toward the fan arm — the arm that had been associated with light. The plants had formed an association between two stimuli — wind and light — and used that association to guide behavior in a novel situation. Associative learning, demonstrated in a plant, without neurons, using growth direction as the behavioral output.
The Venus flytrap exhibits a counting mechanism that neuroscientists have described as a short-term memory system. The trap’s trigger hairs must be stimulated twice within approximately 20 seconds for the trap to close — a two-touch threshold that prevents the plant from wasting energy on raindrops or debris. After closure, three to five additional trigger hair stimulations activate the digestive glands. The plant counts mechanical inputs across a time window, and each count triggers a different phase of the predatory sequence. The counting mechanism uses calcium signaling — action-potential-like waves of calcium concentration that propagate through the trap’s cells — to integrate sensory inputs over time. The calcium signal amplitude encodes the count. The plant is using electrical signaling to implement a state machine, which is what neurons do — but without neurons.
What it means for neuroscience
The traditional story of memory goes like this: neurons are the cells that process and store information. Nervous systems are the organ systems that organize neurons into networks. Brains are the centralized structures where the most complex information processing occurs. Memory is what brains do. Everything in that story is true. What the slime mold, plant, and single-cell data reveal is that none of it is necessary. Information storage, pattern recognition, anticipation, habituation, associative learning, and network optimization can all be implemented without neurons — using tube diameters, calcium waves, chemical gradients, and physical restructuring of the organism’s own body.
The Umwelt concept established that every animal lives in a perceptual world defined by its sensory hardware. The memory-without-a-brain literature extends that framework downward: even organisms without sensory organs, without nervous systems, without anything recognizable as a brain, are encoding information about their environments and using that information to modify future behavior. The swarm intelligence post documented computation distributed across thousands of bodies. The brain-body co-evolution post documented the octopus distributing neural processing across eight arms. Physarum distributes memory across a tube network that is simultaneously its circulatory system, its skeleton, and its brain. The organism is all three at once — a transport network that remembers where it’s been and uses that memory to decide where to go.
The mirror neuron system requires neurons. Brain lateralization requires hemispheres. But memory — the ability to encode past experience and use it to modify future behavior — doesn’t require any of those things. It requires a system that can change its physical state in response to experience and use that changed state to influence subsequent behavior. Neurons do this with synaptic weights. Slime molds do this with tube diameters. Plants do this with calcium waves. The fundamental operation is the same. The hardware is completely different. And the fact that evolution discovered this operation in organisms that diverged from the animal lineage over a billion years ago suggests that memory is not an invention of the nervous system. It is a property of life that nervous systems later specialized, refined, and — in certain lineages — made spectacular.
This is the kind of question our Neurozoology course was built to explore — where a single cell with no neurons solves NP-hard optimization problems by reshaping its own body into a map of past experience, a plant that has never had a brain remembers being dropped for 28 days, a Venus flytrap counts to five using calcium waves, and the most disorienting implication of all is that memory — the thing we assumed required a brain — turns out to be older than brains, simpler than neurons, and possibly as fundamental to living systems as metabolism itself.