ABSTRACT
The late Precambrian environment in which animals fi rst emerged differed profoundly from that of the Phanerozoic in one key aspect-the incomplete oxidation of the oceans. Sulfi dic deep water would have encroached into the mixed layer and photic zone, introducing both a toxin and reducing oxygen by the action of sulfi de-oxidizing bacteria. The back-reaction of sulfi de with oxygen in the oceanic mixed layer may have ultimately limited atmospheric oxygen to less than 4% of its present level. Animals would have been restricted to estuarine refugia where input of oxygenated freshwater held sulfi de at bay. This biogeography may have profoundly impacted both the evolution and taphonomy of early animal life. Ultimately, sulfi de and any attendant organic matter pool was swept from the oceans following a several episodes of intense glaciation and oxygen levels rose to Phanerozoic levels. Although there is no compelling evidence for pre-Ediacaran roots of the Metazoa, it is possible that oxygen levels may have permitted some sort of multicellular life immediately following the Lomagundi positive carbon isotope excursion ca. 2200 million years ago; after oxygen fi rst appeared but before the deep oceans became intensely sulfi dic. More work on this unique interval of Earth history is needed.
