EUKARYOTES WERE LIKELY AEROBIC AND ESTABLISHED IN PROTEROZOIC ECOSYSTEMS

The collective evidence available for the early evolutionary history of eukaryotes is not always aligned. Most critically, the body fossil record and molecular clock predictions suggest a Paleoproterozoic or Mesoproterozoic origin of eukaryotes, yet there is a lack of eukaryotic biomarkers until the Neoproterozoic. Some have argued that the Proterozoic biomarker record shows a late appearance of steranes because eukaryotic organisms were a minor part of global ecosystems relative to bacteria, and thus their signal was swamped by bacterial biomarkers. Others have argued that early stem-group eukaryotes may have lived only or mainly in anoxic environments, thus leaving behind no sterol record, and that crown-group eukaryotes thus evolved relatively late. We argue that combined evidence from fossils, geochemical records, and phylogenetic analyses are best interpreted to suggest that eukaryotes were aerobic, diverse, and well-established in early to mid Proterozoic ecosystems. In addition, modern understandings of microbial ecosystems suggest that even low concentrations of eukaryotic biomass can have a large impact on marine communities. We must closely examine and integrate both biological and geological evidence and examine points of agreement and contention to move towards a comprehensive understanding of the history of the eukaryotic lineage. Indeed, it is in these apparent disagreements that we gain new insights into the true origin and early evolutionary history of this vastly important and diverse group. This work follows on Andy Knoll’s seminal research on early eukaryotic ecosystems and his ongoing efforts to more fully integrate biological and geological perspectives on Earth-life co-evolution.