A more remote possibility is that planarian worms themselves have somehow changed over the last six decades—falling victim to pollution or genetic drift. Gershman finds this scenario unlikely. “What are the chances that a bunch of researchers just happened to do these studies at the particular time when this phenomenon happened?” he asked, incredulous. “They just got extremely lucky, in the millions of years of planarian evolution? And then our luck ran out?”
No matter the reason, in 2026, despite their nervous system and simple brains, planarians don’t learn. From an evolutionary perspective, this might actually make sense. “The reason that we learn associations, to some degree, is so that we can predict danger and avoid it,” Snyder said. But planarians have a different relationship to danger. Their regenerative physiology, so key to McConnell’s experiments, protects them from blunt trauma. Bitten in half, they simply grow back. What use is memory to such a creature? “That’s a whole other philosophical conundrum,” she said.
It’s a great time to be wrestling with such conundrums. Planarian learning may be a dead end, but memory transfer experiments with other organisms are back in scientific vogue—and those experiments appear to be working. In 2018, the neuroscientist David Glanzman of the University of California, Los Angeles, performed a memory transplant on the sea slug Aplysia californica, a darling model organism for memory research owing to its relatively simple nervous system and gigantic neurons. After training the slugs to respond to a shock to their tails, Glanzman was able to transfer the sensitization from one slug to another via a direct injection of genetic material. This suggested that some aspect of the memory was stored in RNA, which was McConnell’s contention.
Then, in 2021, the Princeton University geneticist Coleen Murphy found that Caenorhabditis elegans worms—microscopic roundworms with 302 neurons to their name—could learn to avoid a pathogenic bacterium by eating, or even just swimming around in, pureed worms who had learned the hard way. Murphy’s group identified a retrotransposon, a jumping segment of genetic material, called Cer1, that appears to “carry a memory” between individuals, she said. A few years later, a group at the Indian Institute of Science published a paper suggesting that trained C. elegans worms release extracellular vesicles—small lipid particles containing genetic information—that can impart their training to their naïve counterparts.
None of these researchers are half as flashy as McConnell was, but their work indicates that he may have been right about worm memory after all. He just bet on the wrong kind of worm—and doubled down on it, despite inconsistent evidence. In the end, he lost his reputation, but his sheer gusto sparked the curiosity of another unconventional scientist. Fortunately, this one’s following the evidence.
At Harvard, Gershman is shifting his focus from the inscrutable planarian to the more legible C. elegans. It may not regenerate when it’s cut in half, but C. elegans is a long-standing model organism in neuroscience—and it’s been consistently shown to learn. With new experiments underway, Gershman is cautiously optimistic. “I just hope we’re not going down another rabbit hole,” he told me. A wormhole, on the other hand—that’s for certain.
Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.