Early Precambrian Eukaryotes
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EARLY EARTH See entries “Origin of Life,” “Salinity History of the Earth’s Ocean,” and “Critical Intervals in Earth History.”
EARLY PRECAMBRIAN EUKARYOTES Joachim Reitner University of Göttingen, Göttingen, Germany
Definition Eukaryotes are probably the result of endosymbiosis, and basically three types exist. The basic ones are the simplest eukaryotic cells with a nucleus only (Giardia) (Graczk, 2005). Secondly, protists have mitochondria, which have been proposed to have an alpha-proteobacterial origin (Sagan (Margulis), 1967). Finally, beside mitochondria, cells of eukaryotic algae contain chloroplasts, former single-celled cyanobacteria achieved via endosymbiosys (Mereschkowsky, 1905; Wallin, 1920; Sagan (Margulis), 1967). The geological record of protists begins long before the Ediacaran vendobionts. Possibly stem group protists were found in ca. 3 Ga-old rocks (Javaux et al., 2010). Within the Late Mesoproterozoic rocks, the divergence of major eukaryotic clades is observed. Beside the fossil record of early eukaryotes, related biomarkers (steranes) were found in up to 2.7 Ga-old rocks, which may support the early origin of eukaryotes before the Great Oxidation Event (Brocks et al., 1999; Brocks et al., 2003a, b). However, the syngeneity of the biomarkers used for the interpretation has recently been questioned (Rasmussen et al., 2008) and the first appearance of eukaryotic biomarkers is still disputed (Brocks
et al., 1999; George et al., 2008; Rasmussen et al., 2008; Waldbauer et al., 2009). The oldest known single-celled eukaryotes are acritarchs, probably ancestors of dinoflagellates. Acritarchs played a significant role in Proterozoic oceans, and the first record comes from the 3.2 Ga-old Moodies Group of South Africa (Javaux et al., 2010). This finding is surprising, since eukaryotic metabolisms normally need oxygen, and the Great Oxidation Event took place about 700 My later, between 2.32 and 2.45 Ga. A similar discrepancy emerged from the first appearance of eukaryotic biomarkers in 2.7 Ga-old rocks (Brocks et al., 1999). Although often controversially debated, biomarker records may give excellent indications to the diversification of major phylogenetic clades. For instance, the distribution of 24-isopropylcholestane revealed a high abundance of (demo-)sponges in the Cryogenian (McCaffrey et al., 1994; Love et al., 2009), thus suggesting sufficient oxygen in shelf regions already 100 My prior to the Cambrian explosion. Furthermore, early Precambrian eukaryotic life is not only indicated by biomarker findings. In some diagenetically early silicified stromatolites, well-preserved eukaryotic cells have been described. The best material is known from the 1.88 Gy-old Gunflint Chert (Ontario, Canada) (Fralick et al., 2002; Awramik and Barghoorn, 1977). Eosphaera tyleri from these cherts is probably an eukaryotic cell (Kazmierczak, 1979). Beside the single-celled eukaryotes, few larger fossils which are most likely multicellular organisms are known, such as Grypania spiralis from the Mesoproterozoic Gaoyuz
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