XVIIIth Intl Conf on Origin of Life 2017 (LPI Contrib. No. 1967) 4071.pdf

July 16-21, 2017 at UC San Diego, CA, USA

The Proterozoic Eon: Life and Environments in Earth’s Middle Age A. H. Knoll Department of Organismic and Evolutionary Biology Harvard University, Cambridge MA 02138, USA * [email protected]

As the Proterozoic Eon began, Earth was already two billion years old. Nonetheless, Protero- zoic rocks provide our first clear chronicle of evolutionary history, documented by well-preserved microfossils, biomarker molecules, isotopic signatures, and sedimentary structures, especially stromatolites. This record informs our interpretations of older rocks [1], while providing a frame- work for understanding the subsequent emergence of a biologically and environmentally familiar world [2]. Early in the Proterozoic Eon, cyanobacteria radiated in oxygenated surface oceans, but recurring anoxia within the photic zone supported regionally extensive primary production by an- oxygenic photosynthetic bacteria. By 1700-1600 million years ago (Ma), eukaryotic cells had gained a foothold in the oceans, adding new morphological and ecological complexity to marine ecosystems [3,4]. Although unambiguous eukaryotic microfossils occur in mid-Proterozoic rocks, protistan diversity remained relatively low until about 800 Ma, when new types of fossils, includ- ing testate and scale-forming taxa, radiated. It is hypothesized that the evolution of eukaryovory – predation by eukaryotic cells on other eukaryotes -- provided an ecological driver for Neoprote- rozoic eukaryotic diversification [5]. Eukaryotes also gained ecological importance in the phyto- plankton, accelerating the transfer of primary production upward into larger size classes and higher trophic levels. The environments within which cyanobacteria, eukaryotes and animals sequentially diversified differed from both those of the Archean Eon and those familiar to us today. In partic- ular, the basal Proterozoic rise of oxygen in the atmosphere and surface oceans permitted innova- tions in energy metabolism and biosynthetic pathways, although persistent subsurface anoxia likely provided a continuing challenge to early aerobic organisms. Following a prolonged interval of low pO2, the atmosphere and oceans began their transition to a more modern redox state near the end of the Eon [2]. Oxygen availability placed significant constraints on Proterozoic evolution, and renewed oxygenation near the end of the eon likely facilitated animal radiation, not least by making carnivory possible [6]. Thus, the ecological theater of Proterozoic environments and the evolutionary play recorded by fossils appear to have influenced each other throughout Earth’s long middle age.

References [1] Knoll AH, Bergmann K and Strauss JV (2016) Philosophical Transactions of the Royal Society, B 371: 20150493, doi.10.1098/rstb.2015.0493. [2] Lyons TW, Reinhard CT and Planavsky NJ (2014) Nature 506:307-315. [3] Butterfield NJ (2015) Palaeontology 58:5–17. [4] Knoll AH (2014) Cold Spring Harbor Perspectives in Biology, doi: 10.1101/cshperspect.a018093. [5] Knoll AH and DJG Lahr (2016) In: K Niklas and S Neumann, eds., The Ori- gins and Consequences of Multicellularity. Vienna Series in Theoretical Biology, MIT Press, pp. 3–16. [6] Sperling EA et al. (2015) Nature 523:451-454.