Imagine a flat surface, perhaps a tabletop, upon which some tiny, perhaps one-celled, critters live. These critters need both water and sugar to live, and this tabletop upon which they find themselves is basically a desert. The wind blows and occasionally deposits a few molecules of water or sugar within reach. This just barely enables them to survive and reproduce themselves.
Now suppose that onto this tabletop fate places a drop of water at some spot, and a crumb of sugar at another spot a centimeter from the water. Suppose that this distance, a centimeter, is much further than any one of these critters can travel in its entire lifetime, but suppose that the critters do have ability to pick up raw materials, carry them for small distances, and then drop them again.
This environmental feature, the pair of reserves of water and sugar, looks like a niche ready to be exploited. If the critters can learn appropriate rules of behavior, millions of them can start to live in a filament of trade between the water and sugar.
The critters who would make up this chain of trade would need to follow some simple rules. Such rules might be:
- If you see water on the left, carry it to the right and set it down.
- If you see sugar on the right, carry it to the left and set it down.
- If you get thirsty or hungry, help yourself to what you need from the materials that pass through your possession.
- The rules are not arbitrary. The rules work because they help the critters exploit an environmental feature which is bigger than any of the critters, and which none of the critters can change. So in a sense the environment in which the critters live determined the rules, more than the critters themselves.
- The perhaps-surprising fact that millions of critters can live successfully by following only a few simple rules derives from the simplicity of the environmental feature. The rules are simple because the feature (a distance separates the two essential resources) is simple.