While many parts and events are missing from the Rise of the First Life on Earth (“abiogenesis”), we have a more complete understanding of the Rise of Multicellular Organisms.

The Orange-colored Slime Mold

The acclaimed science writer Steven Johnson begins his fascinating book Emergence: The Connected Lives of Ants, Brains, Cities, and Computer Software, with a portrait of a beautiful creature which is unfortunately called the Slime Mold (Dictyostelium discoideum). Typically growing on rotting bark and leaves in a forest, this common variety of slime mold looks like a patch of rust-orange lichen. It is actually a simple organism without intelligence or a central nervous system, closely related to the organisms collectively classified as fungus. 

When food supply, temperature, and humidity are favorable, the Slime Mold lives as separated, individual cells. But when conditions are strained, these cells coalesce to form a single, larger organism called a “grex.” Time-lapse photography shows that the grex then crawls very slowly along the forest floor in a unified search for food. When its environment improves, the cells separate and become solitary foragers once again!   At the right time, a grex will also send up “fruiting bodies” for reproduction of a new Slime Mold.

Multicellularity evolved independently in at least 16 different lineages, including animals, plants, and fungi, over 600 million years ago.