PHYSARUM

In 2000, Toshiyuki Nakagaki grew a mat of Physarum polycephalum across a plastic maze, then set an oat flake at each of two exits. The slime, spread thick through every corridor and dead end, began to withdraw.

Within hours it had abandoned the blind alleys and thinned to a single tube tracing the shortest route between the two foods — the same answer a shortest-path algorithm would return, computed by an organism with no nervous system, no memory cells, no idea it was in a maze at all.

It doesn't search. It floods everywhere at once, then lets the paths that don't pay off starve. Selection, not planning.

In 2010, Atsushi Tero's team laid oat flakes across a scaled map of greater Tokyo — one flake for each surrounding city — put the slime on the spot marked Tokyo, and waited.

In about a day it grew a web of tubes linking every flake. Statistically, the result matched the real rail network engineers spent a century designing: comparable total length, comparable fault tolerance, comparable cost. Cut a tube and the flow re-routes.

The team distilled the behaviour into a rule for adaptive network design — tubes carrying more flow thicken, idle tubes wither — now borrowed for road, pipe and data networks.