Two of life’s three domains, the Bacteria and Archaea, diversified on the early Archean Earth. The third domain – the Eukarya, or eukaryotes – evolved later, however, at least in part via symbiosis between members of the other great clades. Eukaryotic organisms radiated in Proterozoic oceans characterized by oxygenated surface waters, but, commonly, anoxia at depth. Exceptionally preserved fossils of red algae favor crown group emergence more than 1200 million years ago, but older (up to 1600-1800 million years) microfossils record a moderate diversity of stem- or early crown-group eukaryotes. Major eukaryotic diversification ca. 800 million years ago is documented by increase in the taxonomic richness of complex organic-walled microfossils, including simple coenocytic and multicellular forms, as well as widespread tests comparable to those of extant testate amoebae and diverse scales comparable to organic and siliceous scales formed today by protists in a number of clades. Mid-Neoproterozoic establishment or expansion of eukaryophagy (eukaryotic predation on other large eukaryotic cells) provides a possible mechanism for accelerating eukaryotic diversification long after the origin of the domain. Protists continued to diversify along with animals in the more pervasively oxygenated oceans of the Phanerozoic Eon.
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