Experimental snowball earth viscosity drives the evolution of motile multicellularity.

During the 70-million-year Cryogenian Snowball Earth glaciations, low ocean temperatures beneath global sea ice increased water viscosity over 3-fold. Unicellular eukaryotes in this environment would have faced severe constraints on movement and nutrient uptake, while nascent multicellular organisms, with larger size and collective mobility, could have gained an adaptive advantage. We experimentally tested whether viscosity-induced metabolic constraints could have selected for multicellular phenotypes, potentially driving the origin or early diversification of multicellularity. Under Snowball Earth-like viscosities, populations of unicellular green algae evolved motile multicellular forms, along with other phenotypes that optimize size and speed. As oceans warmed and viscosities decreased, the diversity of multicellular phenotypes may have allowed early multicellular organisms to exert novel physical control over their local environments. This pivotal period in Earth's history could have shaped the emergence and eventual dominance of certain multicellular lineages, highlighting a potential ecological driver of one of life's major evolutionary transitions.