The environmental context of early animal evolution

Animals originated and evolved during one of the most unique times in Earth history - the Neoproterozoic Era. A large dataset of >10,000 Neoproterozoic-Paleozoic shale samples compiled by the Sedimentary Geochemistry and Paleoenvironments Project is interrogated here to better understand the landscape early animals inhabited. Using a space-for-time translation, animal ecosystems along modern natural gradients of oxygen and primary productivity are then used to conceptualize Neoproterozoic ecosystems. Analyses of redox-sensitive trace metals demonstrate that animals evolved in a relatively low-oxygen ocean, although perhaps not considerably less oxygenated than the Paleozoic. Anoxic water columns were generally ferruginous (iron-rich) rather than euxinic (sulfide-rich, as in the modern ocean), and sulfide stress was likely limited. Habitats suitable for chemosymbiotic lifestyles based on sulfide oxidation were also likely rare. Analyses of sedimentary total organic carbon suggest that early animals lived in an ocean with lower primary productivity compared to the preceding Mesoproterozoic or following Paleozoic. Combined with an inability to inhabit productive regions in this low-O2 ocean - where aerobic respiration would quickly draw down oxygen to lethal levels - Neoproterozoic animal communities would have likely been more food limited than generally appreciated, leading to important impacts on ecosystem structure and organismal behavior.

Speaker: Erik Sperling, Stanford