Think about a house party. People enter at random, gradually form into little clumps and eddies of approximately equal size, and may eventually group themselves into exclusive pairs. It's a phenomenon that occurs often in nature and can be so weird that it seems to border on the mystical. Microdroplets of water arrange themselves into six-pointed snowflakes. Millions of living cells, each groping toward its own tiny goal, collectively form the mathematically magnificent shape of the Nautilus shell.
Ants build and maintain a colony.
They're not dictated by some external authority, as is the symmetrical stomp of a drill team. Artificial life behaves just this way. In a classic case each digital critter is given just three rules on how it should move relative to its neighbors. When let loose their collective motion eerily resembles the flocking behavior of birds. They had evolved.