Slime moulds show signs of learning, suggesting that the process does not require nerves and may have evolved early in the history of life.
In a simple form of learning called habituation, an organism learns to ignore continuous stimuli over time. Audrey Dussutour and her team at Toulouse University, France, observed single-celled slime moulds (Physarum polycephalum) crossing a bridge in a Petri dish. The bridges were treated with repellent chemicals, either quinine or caffeine, or left untreated. Cells approached and crossed untreated bridges three times faster than cells crossing treated ones. The cells became habituated to treated bridges, crossing them faster after 5 days. However, after 2 days of no chemicals, the organisms' aversion to caffeine or quinine returned. And cells that were habituated to quinine still showed aversion to caffeine, and vice versa, ruling out sensory fatigue or adaptation.
The study suggests that simple learning processes pre-date neuron evolution.
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Single-celled life can learn. Nature 533, 10 (2016). https://doi.org/10.1038/533010d