An organism composed of a single cell, lacking any brain or nervous system, appears capable of a sophisticated form of learning.
Habituation, the simplest documented form of learning, involves a progressive decrease in response to a stimulus that is repeated and poses no threat, such as a familiar smell or sound. This phenomenon is widespread among animals and has even been observed in plants. Furthermore, it has been demonstrated in certain protists. These organisms possess complex eukaryotic cells, similar to those found in animals, land plants, and fungi, but are typically single-celled. Examples include the trumpet-shaped Stentor coeruleus and the slime mold Physarum polycephalum.
A significantly more complex type of learning involves the ability to connect disparate stimuli or events, anticipating that one will invariably lead to another. Ivan Pavlov famously illustrated this associative learning with his experiments. By pairing the sound of a bell with the presentation of food to dogs, he observed that the animals would salivate upon hearing the bell alone.
Now, Sam Gershman of Harvard University and his research team have employed similar conditioning techniques to reveal that Stentor also appears to be capable of associative learning.
These remarkable single-celled creatures inhabit ponds and propel themselves using rows of hair-like cilia that run along their bodies. Measuring up to 2 millimeters in length, they are considered giants among unicellular life. One end of the organism features an anchor, known as the holdfast, for attachment to surfaces. The opposite end displays a trumpet-like feeding apparatus.
“When they are attached, their primary activity is filter feeding. If disturbed, they swiftly contract into a spherical shape. During this contracted state, feeding is impossible, making it ecologically advantageous to avoid such responses unless absolutely necessary,” Gershman explained.
He and his colleagues utilized this observed behavior to investigate the extent of Stentor‘s learning capabilities. Initially, they subjected cultures of Stentor cells, typically comprising several dozen individuals within Petri dishes, to strong vibrations at the bottom of the dish. The majority of the organisms responded with rapid contraction. However, as these taps continued at 45-second intervals for a total of 60 impacts, a progressively smaller number of Stentor contracted, indicating habituation to the stimulus.
Subsequently, the Stentor cultures were exposed to a weak tap—a stimulus that generally elicits less contraction—one second before a strong tap. These paired stimuli were repeated every 45 seconds, a duration roughly equivalent to the time required for Stentor to expand again.
Over the course of 10 such trials, the probability of the organisms contracting immediately after the weak tap initially increased before declining. “We observed a distinct peak in the contraction rate on the graph, where it initially rises before falling. If the weak tap were presented in isolation, this pattern would not occur,” Gershman noted.
The researchers interpret this finding as evidence that Stentor has successfully associated the weak tap with the subsequent stronger tap. This observation designates Stentor as the first protist known to exhibit mastery of associative learning. “This raises the fundamental question of whether seemingly simple organisms are capable of cognitive processes that we typically attribute to far more complex, multicellular organisms possessing brains,” Gershman commented.
The findings also suggest an ancient evolutionary origin for associative learning, predating the widespread emergence of multicellular nervous systems by hundreds of millions of years. Further traces of this may still be evident in the way mammalian neurons appear to learn from their inputs, bypassing the need for modifications to the synapses or connections between neurons—which is the conventionally understood mechanism for most learning, he added.
“It is fascinating that a single cell can perform such intricate functions that we previously believed required a brain, neurons, and complex behavioral learning,” stated Shashank Shekhar at Emory University in Atlanta, Georgia. Shekhar’s own research has demonstrated that Stentor can form temporary aggregations to enhance feeding efficiency.
He speculates that other unicellular organisms might also possess the capacity for associative learning. “My intuition suggests that if it exists in one instance, it is likely to be present in many more,” Shekhar remarked.
If an organism is engaged in learning, it implies the capacity for memory storage. The precise mechanisms by which this occurs in Stentor are not yet fully understood. However, Gershman hypothesizes that it involves receptors designed to respond to physical touch by permitting calcium ions to enter the cell. This influx alters the internal electrical voltage, prompting Stentor to contract. He proposes that following repeated exposure to stimuli, certain receptors undergo modifications, effectively acting as molecular switches that prevent contraction.
Reference:
bioRxiv DOI: 10.64898/2026.02.25.708045