Learning has been found in every major group of animals in which it has been examined. Although not all animal phyla have been examined with respect to learning and, although no formal phylum-level phylogenetic analysis of learning has been made to date, existing evidence suggests that learning may be ancestral to the animal kingdom. Evidence of learning phenomena in protists affirms this position.
Learning has been ascribed to organisms other than animals and protists, including bacteria, slime molds, and plants. For sure, elements of learning are found across almost all taxonomic groups of living organisms. In unicellular organisms such as phage, networks of cell proteins appear to perform analogously to neural networks, possessing properties that make them capable of a cell-level associative memory. Empirical results suggest that metabolic responses in bacteria are tuned by repeated exposure to particular conditions in a fashion that resembles how behavioral responses are tuned in animals. Evidence of learned behavior in microbes may not be long in coming. Plants, like bacteria, have processes that resemble learning at the level of protein networks within cells, and physiological responses of plant cells consequently change with experience. A kind of memory occurs in plant cells in which information about environmental stress is stored until receipt of a complementary signal required for a developmental change. Although these processes are physiological in scope, the behavioral repertoire of plants is quite rich and learned behavior in plants conceivable.
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