The eukaryotic cell is strikingly distinct from its much simpler prokaryote relatives, possessing not only a nucleus, but also a complex cytoskeleton, a sophisticated endomembrane system, the capacity for sterol biosynthesis, and mitochondria.  How this complex cell evolved—a process known as eukaryogenesis—has long been a puzzle, in part because the prevailing model holds that the ancestor to all living eukaryotes (LECA, the last eukaryotic common ancestor) appeared before the oldest recognizably eukaryotic fossils, and thus we have no record of the very oldest (stem) lineages. However, there is little empirical support for this model: molecular clock estimates for LECA span more than a billion years, with little basis for discriminating between early and late estimates, and available geological data do not provide unambiguous evidence for crown eukaryotes prior to ca. 1050 Ma. The first biomarker evidence for eukaryotic steranes is even later—ca. 800 Ma—despite the recovery of bacterially-derived hopane biomarkers in rocks dating as far back as ca. 1640 Ma. In my talk I will present two end-member models of early eukaryote evolution—a late LECA model and an early LECA model—and discuss the implications of each, particularly with respect to the influence of Earth surface oxygenation on the rise of complex life. I will also present data on the habitats (redox and water depth) of the earliest eukaryotes, based on new fossils from ca. 1750–1600 Ma strata in northern Australia.

with collaborators Max Lechte², Leigh Anne Riedman¹, Angelo dos Santos², Margaret Whelan², and Galen Halverson²

¹ Department of Earth Science, University of California at Santa Barbara, Santa Barbara, California

² Department of Earth and Planetary Sciences, McGill University, Quebec, Canada

Dr. Susannah Porter received her bachelor’s degree in Mathematics from Yale University in 1995 and her Ph.D. in Biology at Harvard University in 2002. After completing a one-year NASA Astrobiology postdoctoral fellowship at UCLA, she moved to the University of California at Santa Barbara, where she is a Professor and Chair in the Department of Earth Science. Her research focuses on the rise of complex life, from the fossils of early eukaryotes to the first widespread appearance of skeletal animals. She has worked on problems relating to the effects of snowball Earth on life, the preservation of organic-walled microfossils, the earliest evidence for predation, the relationship between carbonate skeletal mineralogy and seawater chemistry, and patterns in skeletal acquisition across all eukaryotes.  She is a Fellow of the Paleontological Society and a recipient of UCSB’s Distinguished Teaching Award.

For zoom information, please get in touch with Rey Garduño (rgarduno@stanford.edu)