One of the riddles of modern biology is the slime mold Physarum polycephalum or as we like to call it, Slimer. Slimer has no neurons. It, in fact, has nothing we can scientifically quantify as a brain. Yet, it’s perfectly capable of solving mazes, for example, or demonstrating that transit maps actually are the most effective routes through a city, and it can even make decisions.
But it can’t remember things, right? It’s literally got nothing to remember them with. There’s no way it can do that.
Except, it turns out, that it can. Slimer leaves behind gunk not because it’s gross, but because it creates a weird form of “external memory” that helps it navigate.
It works like this. Slimer is a one-celled organism, and as it moves, it leaves behind crud. This crud is full of proteins and sugars. Being a slime mold, it branches out and, despite having one cell, each branch acts like an independent organism. If it detects food, that branch will send a signal and summon the rest of it.
So how does this process interact with its own slime? Easy: if a branch detects the slime, it’ll just say “Hey, we’ve been here already.”
Obviously it’s not a perfect system; researchers spent a lot of time messing with Slimer, the poor guy. But it does illustrate that there is a form of memory, and may offer clues as to how our own memory evolved.