For over a billion years, life on Earth consisted entirely of single-celled organisms. Then, around 2.1 billion years ago, something remarkable occurred: individual cells began living together, cooperating and specializing. The earliest known multicellular fossils, the Francevillian biota from Gabon, Africa, show organized structures up to 12 centimeters across -- far larger than any single cell.
The transition to multicellularity likely happened independently multiple times across different lineages. The key innovation was cellular adhesion -- the ability of cells to stick together and communicate. Once cells could signal each other and coordinate behavior, the door opened to differentiation: individual cells taking on specialized roles such as feeding, defense, or reproduction, rather than every cell performing every function.
This revolution was made possible by an earlier event: endosymbiosis. Around 2 billion years ago, a larger cell engulfed a smaller bacterium capable of using oxygen for energy. Instead of being digested, the smaller cell became a permanent resident -- the ancestor of mitochondria. This partnership gave eukaryotic cells vastly more energy, fueling the complexity needed for multicellular life. A similar event later produced chloroplasts in plant cells.
Multicellularity unlocked an entirely new design space for evolution. Organisms could grow larger, develop tissues and organs, and exploit ecological niches impossible for single cells. This innovation paved the way for every plant, fungus, and animal that would follow -- including, eventually, us. It remains one of the most consequential transitions in the history of life.