Somehow, this single-celled organism [Physarum polycephalum] had memorised the pattern of events it was faced with and changed its behaviour to anticipate a future event. That's something we humans have trouble enough with, let alone a single-celled organism without a neuron to call its own.-- Slime mold to DARPA: Justin Mullins on the future of artificial intelligence.
... [Max] Di Ventra speculates that the viscosities of the sol and gel components of the slime mould make for a mechanical analogue of memristance. When the external temperature rises, the gel component starts to break down and become less viscous, creating new pathways through which the sol can flow and speeding up the cell's movement. A lowered temperature reverses that process, but how the initial state is regained depends on where the pathways were formed, and therefore on the cell's internal history.
In true memristive fashion, [Leon] Chua had anticipated the idea that memristors might have something to say about how biological organisms learn. While completing his first paper on memristors, he became fascinated by synapses - the gaps between nerve cells in higher organisms across which nerve impulses must pass. In particular, he noticed their complex electrical response to the ebb and flow of potassium and sodium ions across the membranes of each cell, which allow the synapses to alter their response according to the frequency and strength of signals. It looked maddeningly similar to the response a memristor would produce. "I realised then that synapses were memristors," he says. "The ion channel was the missing circuit element I was looking for, and it already existed in nature."
In The Social Amoeboe: The Biology of Cellular Slime Molds, John Tyler Bonner concludes:
We can see the beginning of an era of enlightenment for slime molds...the day may come where we may hail Alan Turing, along with his other claims to fame as the Robert MacArthur of developmental biology...[but] we still have a long -- and interesting way to go. And the reason we all started working on cellular slime molds is that they were supposed to be so simple!