A thousand amoebae can become one crawling thing

When Dictyostelium runs out of bacteria, the separate cells signal, stream together, and assemble into a slug that migrates as a single body. Then some of them die to build a stalk.

Dictyostelium discoideum is a soil amoeba. While food lasts, each cell hunts alone and divides alone. When the bacteria are gone, starving cells release pulses of cyclic AMP, and neighbours relay the pulses, so the population streams inward along a chemical rhythm. Tens of thousands of cells adhere into a mound, then a finger, then a slug a millimetre or two long that moves toward light and heat. The cells inside it are not fused. They are coordinated. At the end of the migration the slug stands up. The front cells become a stalk of dead cellulose, and the rest become spores on top of it.

The stalk is the part to sit with. Those cells do not get to leave. The spores do. A society of equals becomes a body with a disposable tissue, and it does this without a sperm, an egg, or a nervous system — whenever the picnic ends. The same species is used to study that decision because the decision is visible in a dish, over a few hours, from cells you can separate with a pipette and watch come back together.