Experimental Snowball Earth Viscosity Drives the Evolution of Motile Multicellularity

During the 70-million-year span of the Cryogenian Snowball Earth glaciations, low ocean temperatures beneath global sea ice increased water viscosity up to fourfold. In the absence of adaptation, unicellular organisms living in this viscous environment were limited in their ability to move and acquire nutrients. We experimentally test the hypothesis that multicellularity evolved in order to overcome this viscosity-induced metabolic deficit. In the presence of Snowball Earth viscosities, we find that populations of unicellular green algae evolve motile multicellular phenotypes in addition to other phenotypes that optimize different combinations of size and speed. As the Snowball Earth subsided and warm seas returned, the novelty of motile multicellularity permitted these organisms to take physical control over their local environment for the first time. This innovation may underpin the evolution of dominant multicellular lineages on Earth today. Significance statementBeginning 720-million years ago, two global glaciations -- together known as the Snowball Earth -- covered the planet with a thick layer of ice for a total of 70-million years. Several groups of complex multicellular organisms independently radiated at this time, including animals, green algae, and red algae. All of these clades include lineages with large bodies made of thousands of cells, multiple cell types, and spatial organization. At first glance, it seems that life merely survived despite the Snowball Earth glaciations. We find experimental evidence that the Snowball Earth glaciations were instead an evolutionary trigger for the diversification of complex multicellular groups.