Animals, fungi, and algae with complex multicellular bodies all evolved independently from unicellular ancestors. The early history of these major eukaryotic multicellular clades, if not their origins, co-occur with an extreme phase of global glaciations known as the Snowball Earth. Here, I propose that the long-term loss of low viscosity environments due to several rounds global glaciation drove the multiple origins of complex multicellularity in eukaryotes and the subsequent radiation of complex multicellular groups. Under this scenario, life adapts to Snowball Earth oceans by evolving large size which acts to compensate for high viscosity seawater and maintain fluid flow at sufficient levels. Warm, low viscosity seawater returned with the melting of the Snowball glaciers and with it, by virtue of large multicellular bodies, new ways of life were unveiled.

Body sizes compiled from taxonomic and paleontological literature.