Learning Without Neurons: A Review of Associative Memory in Protists.

For decades, the capacity for associative learning was considered a hallmark of multicellular organisms possessing complex nervous systems. However, a growing body of evidence in the field of basal cognition is challenging this neuro-centric paradigm, demonstrating that unicellular eukaryotes, or protozoans, exhibit sophisticated behavioral plasticity. This review synthesizes historical and contemporary research into associative learning-specifically classical and operant conditioning-in prominent protozoan models such as Paramecium, Stentor, and the acellular slime mold Physarum polycephalum. We examine the experimental frameworks that have successfully demonstrated anticipatory behavior and stimulus-response pairing, while critically addressing the methodological skepticism that has historically clouded the field. Furthermore, we explore the hypothesized subcellular mechanisms driving these processes, including cytoskeletal restructuring, epigenetic modifications, and biochemical signaling cascades that serve as a non-neural "memory." By reevaluating the cognitive boundaries of the single cell, this paper argues that learning is a fundamental property of biological systems rather than a specialized function of neural tissue. The findings discussed herein have significant implications for our understanding of the evolution of intelligence, the development of synthetic biological systems, and the broadening definition of biological memory.