Origin and Early Evolution of the Eukaryotes: Perspectives from the Fossil Record

Abstract

The emergence of a complex, eukaryotic cell is one of the major steps in the evolution of life on Earth. Eukaryotic organisms include a range of macroscopic life such as animals, plants, fungi, as well as a plethora of microscopic single-celled or colonial protists. The first evidence for eukaryotic life appears in the geologic record around 1650 million years ago (Ma), as organic-walled microfossils—cellular vesicles often preserved as carbonaceous compressions in siliciclastic rocks. Early eukaryotes were predominantly single-celled and minute for about a billion years, until the onset of macroscopic multicellularity in algae and animals in the Ediacaran Period (635–538 Ma). Here I review the earliest evidence of eukaryotic life, including a range of Proterozoic organic-walled microfossils and problematica. These fossils contain a suite of morphological and geochemical characters that offer clues about their palaeobiology. Complex microfossil morphology like spines can be considered a proxy for the appearance of a cytoskeleton, very early in eukaryote history, in the late Paleoproterozoic. Multidisciplinary studies on fossil features such as cell morphology, cell wall ultrastructure and its chemical composition, ancient forms of multicellularity, as well as understanding the environments these microorganisms inhabited, enable the use of the fossil record to inform the timing and mode of eukaryogenesis.