Oxygen, animals and aquatic bioturbation: an updated account

The modern biosphere owes its idiosyncratic expression to the activities of oxygen metabolizing organisms, especially animals and eukaryotes (Butterfield, 2011). And with a permanently oxygenated atmosphere established during the ~2.4 Ga Great Oxidation Event (GOE), the stage was set for their early evolutionary debut. Curiously, however, eukaryotic organisms do not appear in the fossil record for another ~800 million years, and animals for another billion years beyond that (Butterfield, 2015a). Since oxygen availability determines the activity of aerobic organisms, there is a longstanding view that, although free oxygen was certainly present through these extended intervals, it remained persistently below levels necessary to support multicellular animals (Nursall, 1959) – or if not animals per se, at least organ-grade bilaterians (Planavsky et al., 2014) – or if not bilaterians per se, at least the large carnivorous bilaterians capable of driving a major evolutionary radiation like the Cambrian explosion (Sperling et al., 2013). Such ‘permissive environment’ causality provides an intuitively satisfying explanation for the delayed arrival of Phanerozoic-style ecosystems, and is supported empirically by geochemical evidence for the expanding oxygenation of mid-late Neoproterozoic oceans (Och & Shields-Zhou, 2012; Cole et al. 2016; Hardisty et al. 2017)