3 min readHere’s what you’ll learn when you read this story:
- Single-celled organisms aren’t often associated with complex functioning, but a new study suggests that at least some protists exhibit what’s known as associative learning.
- On a petri dish containing the trumpet-shaped microorganism Stentor coeruleus, scientists performed different combinations of weak and strong taps and found that the protist appeared to associate a weak tap with a forthcoming strong tap by curling up in response.
- This finding suggests that evolutionary associative learning predates synaptic modification.
When biologists refer to ‘complex life,’ they almost universally mean multicellular organisms, since having more cells obviously adds a level of intricacy that simpler life-forms lack. But new research shows that we shouldn’t short-change the single-celled organisms that gave rise to all that ‘complex’ life in the first place.
One of the most compelling mysteries of single-celled microorganisms is how exactly they learn—after all, they don’t have any brains. As New Scientist notes, previous studies have shown that non-brainy organisms do have the capacity for basic learning, including habituation—a process where a response declines when exposed to repetitive stimuli. However, associative learning, which connects a response to a specific stimulus event, appeared to be a mental step beyond what protists could achieve. Famously, Russian physiologist Ivan Pavlov demonstrated associative learning in dogs in the 1890s by regularly ringing a bell at mealtimes for his canine subjects. Once the dogs became accustomed to the bell, they’d salivate at every ring, expecting food.
Now, a new study uploaded to the preprint repository bioRxiv suggests that single-celled organisms—specifically, the trumpet-shaped Stentor coeruleus, which is a monstrously large single-celled pond organism known as a ciliate that can stretch to an amazing two millimeters—also display evidence of Pavlovian learning. Two of the key features of S. coeruleus are the holdfast (a branch-like structure that anchors the organism to the pond bed) and the undeniable, trumpet-shaped feeding apparatus. When undisturbed, these protists will feed unimpeded in their characteristic brass-instrument shape. But when bothered, they’ll curl into a ball, disrupting their feeding in the process.
“In a series of Pavlovian conditioning experiments with the ciliate Stentor coeruleus, we show that temporally pairing weak and strong mechanical stimuli results in a transiently enhanced contraction response to the weak stimulus,” the authors wrote in the paper. “Stentor coeruleus appears capable of associative learning, suggesting an ancient evolutionary origin that preceded the emergence of multicellular nervous systems.”
First, scientists performed 60 strong taps—one every 45 seconds (roughly how long it takes for the organism to uncurl)—on the petri dish containing S. coeruleus. At first, the protist curled into a ball as expected, but fewer and fewer contracted as the taps continued (an example of habituation).
Then, the researchers did two more set-ups: A weak-strong tap sequence (separated by one second) and a weak-weak tap sequence. They found that protists in the first group displayed an enhanced reaction to the first weak tap, as if they perceived that it predicted an accompanying strong tap, until their responses eventually decreased over time as predictive habituation took over. The weak-weak group showed no such enhanced reaction, suggesting that S. coeruleus really could learn to associate a weak tap with a forthcoming strong tap.
“Our findings have broad implications for the evolutionary origins of associative learning,” the authors wrote. “If ciliates such as Stentor […] are capable of associative learning, then there must be a cellular mechanism for such learning that does not depend on synaptic modification.”
So, how exactly does S. coeruleus pull this off without any way to store memory? Sam Gershman, a co-author of the paper from Harvard University, told New Scientist that certain receptors respond to touch by letting calcium into the cell, and this receptor (which acts as a kind of switch) eventually stops contraction.
In other words, maybe these “simple” organisms aren’t so simple after all.
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Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.