The evolution of multicellularity

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Not one event but a repeated one, and that repetition is the most important fact about it. Simple multicellularity — cells that stay attached after division — has arisen independently dozens of times across bacteria, algae, fungi, plants and animals, which tells us the step is easy. Complex multicellularity, meaning three-dimensional bodies with differentiated cell types, regulated development and cells that cannot survive alone, has arisen far fewer times: animals, fungi, land plants, red algae, brown algae and a small number of others. Every one of those is a eukaryote. The hard part is not sticking together but the evolution of an enforceable division of labour, because a cell that abandons its own reproduction to build somebody else's body is vulnerable to cheating, and the standard solution is a single-cell bottleneck each generation that keeps the whole body genetically identical. The oldest taxonomically resolved case is the red alga Bangiomorpha, which also shows differentiated holdfast cells and spores; a far older and much-argued candidate is the 2.1 billion-year-old Francevillian biota of Gabon.

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