COMPLEX MULTICELLULARITY AS AN EVOLUTIONARY RESPONSE TO VISCOUS SNOWBALL EARTH OCEANS

Because of the cold homogenous environmental conditions of the Snowball Earth oceans, there were no warm areas for life to find refuge. Yet there is surprisingly little extinction associated these glaciations. Life, therefore, must have adapted to the extreme conditions of Snowball Earth. For the small unicellular organisms that lived prior to Snowball glaciation, the most important change that glaciation made to seawater may have been an increase in viscosity caused by cold temperatures and high salinities. Water viscosity plays a fundamental role in the ecology of protists and microbes, directly influencing how efficiently they feed and move. Pre-Snowball oceans were likely stratified with hot tropical surface waters and cold polar and deep water. This wide variety of sea temperatures would yield a wide variety of viscosities. Given the presence of equatorial sea ice, the temperatures of the Snowball oceans would have been uniformly low, near -4 °C and consequently viscosity would have been uniformly high. Complex multicellularity, with cellular differentiation, spatial organization, and large body sizes arose at least six times independently coincident with the Snowball Earth glaciations. Here, I propose that multicellularity is an evolutionary response to high viscosity seawater. Multicellularity acts as a means to escape the effects of high viscosity on feeding and motility allowing organisms to keep their ancestral Reynolds numbers and therefore maintain efficient feeding and motility. Multicellular organisms evolving from unicellular organisms living in hot tropical seawater prior to glaciation could maintain constant Reynolds number even in cold viscous seawater by forming multicellular clusters of over 512 cells. Clusters of this size are significantly larger than the simple multicellular clusters that arose prior to the Cryogenian, and once low viscosities of seawater return after glaciation, this large body size provides access to novel ecological opportunities associated with using and manipulating flowing water.