DID SNOWBALL EARTH CAUSE MULTICELLULARITY?MODELING SELECTIVE PRESSURES FOR MULTICELLULARITY SPANNING SNOWBALL EARTH GLACIATIONS

Molecular and fossil evidence suggest that complex multicellularity evolved during the late Neoproterozoic era, coincident with Snowball Earth glaciations, where ice sheets covered most of the globe. During this period, environmental conditions—such as sea water temperature and the availability of photosynthetically active light in the oceans—likely changed dramatically. Such changes would have had significant effects on both nutrient availability and optimal phenotypes, creating selective pressures. Recent work identified the increase in viscosity (driven by decreasing water temperatures during glaciation) as a possible driver for an increase in size in heterotrophic eukaryotes, and identifies this increase as a path toward animal multicellularity. Here we explore how changes in light and temperature during Snowball Earth may have impacted organisms and their environments. First, to test whether decreasing temperatures led to a selective pressure for larger organisms—and, perhaps, for multicellularity—we modeled the dependence of nutrient uptake and metabolism on temperature and body size. Second, to test how the icesheet coverage impacted nutrient availability, we solved temporal differential equations that relate the population levels of autotrophs and heterotrophs and the resulting nutrient concentrations they produce. By testing a series of alternate—and commonly debated—hypotheses, we can explore putative selective pressures that led to multicellularity. Understanding the evolution of multicellularity is vital for elucidating the origins of life and the explosion of animal life following the Neoproterozoic.