Researchers have demonstrated that the single-celled protist Stentor coeruleus can engage in associative learning, similar to Pavlov's experiments with dogs. This finding suggests such cognitive abilities may predate the evolution of brains by hundreds of millions of years. The study highlights unexpected complexity in simple organisms.
The trumpet-shaped protist Stentor coeruleus, which lacks a brain or neurons, inhabits ponds and measures up to 2 millimeters in length. It swims using hair-like cilia and attaches to surfaces via a holdfast at one end, while feeding through a trumpet-like apparatus at the other. When attached, these organisms filter feed, but they contract into a sphere if disturbed, halting feeding in the process.
Sam Gershman at Harvard University and his colleagues investigated Stentor coeruleus's learning potential through conditioning experiments. Initially, they applied strong taps to the bottom of Petri dishes containing cultures of dozens of these cells every 45 seconds for a total of 60 taps. The organisms contracted rapidly at first but habituated over time, with fewer contractions as the taps continued, demonstrating the basic learning form of habituation.
In a more advanced test, the researchers paired a weak tap—typically eliciting fewer contractions—with a strong tap occurring 1 second later. This sequence repeated every 45 seconds across 10 trials, aligning with the time needed for Stentor to unfurl. The contraction rate after the weak tap initially rose before declining, a pattern not observed with the weak tap alone. "We saw this bump in the graph where the contraction rate initially goes up before going down," Gershman explained.
This response indicates associative learning, where the weak tap becomes linked to the stronger one, marking the first such demonstration in a protist. "It raises the question of whether apparently simple organisms are capable of aspects of cognition that we generally associate with much more complex, multicellular organisms with brains," Gershman noted. The discovery points to an ancient origin for this learning type, predating multicellular nervous systems by hundreds of millions of years.
Shashank Shekhar at Emory University, who has studied Stentor aggregation for efficient feeding, called the results fascinating. "It’s fascinating that a single cell can do such complex things that we thought required a brain, that required neurons, that required behavioural learning," he said. Shekhar suspects other unicellular organisms may possess similar abilities. The mechanism likely involves touch-sensitive receptors allowing calcium influx, altering cell voltage and triggering contraction, with repeated stimuli modifying these receptors as a molecular switch for memory storage.
The findings appear in a preprint on bioRxiv (DOI: 10.64898/2026.02.25.708045).
