Multicellularity evolved repeatedly when cells acquired adhesion, communication, coordination, and differentiation, enabling larger bodies and specialized functions while creating cellular interdependence.
Multicellularity evolved independently many times, including in fungi and eukaryotic algae and in the ancestors of plants and animals.
Cells had to remain attached, communicate and coordinate different patterns of gene expression. Once cooperation was stable, larger body size and specialized cell types could evolve.
Use the sequence: adhesion -> communication and coordination -> differentiation -> tissues or larger bodies. Repeated origins show multicellularity is not one inherited event shared by all multicellular groups.
A plant leaf combines specialized epidermal, mesophyll and vascular cells, allowing gas exchange, photosynthesis and transport to be divided among cell types.
Multicellularity brings dependence and coordination costs; its syllabus advantages are larger body size and cell specialization, not automatic superiority in every environment.
The HL extension asks how complex cell organization could arise and become useful. Endosymbiosis explains the origin of mitochondria and chloroplasts using bacterial evidence. Differentiation explains how cells with the same genome become specialized through different gene expression and proteomes. Multicellularity explains why adhesion, communication, and differentiation allowed larger bodies and division of labour.