Abstract

There is now compelling evidence that the world ocean was perpetually dark under global ice-shelves for 55 million years (Sturtian glaciation) at the start of the Cryogenian Period (717−635 Ma) and again for 5−15 million years (Marinoan glaciation) at the end of that Period. Supporting evidence comes from paleomagnetism, geochronology, sedimentology, geochemistry and numerical simulations of atmosphere−ice−ocean dynamics. The latter suggest that the time lapse from an ice-albedo tipping point at 35% ocean area ice-covered to total darkness was ≤400 years. Yet, there is undeniable fossil evidence for marine phytoplankton including eukaryotic algae before, between and soon after the Cryogenian panglacial chrons. This talk will pose and test the hypothesis that polar−alpine microbial ecosystems, established long before the Cryogenian, simply moved with their ice margins to the equatorial zone of net ablation on Snowball Earth, where their habitat area was vastly enlarged and the cruelty of winter reduced. They thrived and evolved, and when each Snowball chron ended, some returned to the mountaintops while others inhabited a rapidly-warming, meltwater-dominated, nutrient-rich and biologically vacant surface ocean. Accordingly, the extant surface biosphere is derived from a particular subset of pre-Cryogenian life. It will be argued from modern analogs that Tonian polar−alpine habitats had sufficient ecologic, taxonomic and genomic diversity to account for the observed post-Cryogenian biotic radiations. Polar−alpine ancestry and Snowball equatorial intermediaries rationalize otherwise problematic aspects of the molecular phylogenomics of many living marine and terrestrial organisms.