Eukaryotic organisms in Proterozoic oceans

Abstract

The geological record of protists begins well before the Ediacaran and Cambrian diversification of animals, but the antiquity of that history, its reliability as a chronicle of evolution and the causal inferences that can be drawn from it remain subjects of debate. Well-preserved protists are known from a relatively small number of Proterozoic formations, but taphonomic considerations suggest that they capture at least broad aspects of early eukaryotic evolution. A modest diversity of problematic, possibly stem group protists occurs in ca 1800-1300 Myr old rocks. 1300-720 Myr fossils document the divergence of major eukaryotic clades, but only with the Ediacaran-Cambrian radiation of animals did diversity increase within most clades with fossilizable members. While taxonomic placement of many Proterozoic eukaryotes may be arguable, the presence of characters used for that placement is not. Focus on character evolution permits inferences about the innovations in cell biology and development that underpin the taxonomic and morphological diversification of eukaryotic organisms.

Paleoproterozoic sterol biosynthesis and the rise of oxygen

“…In addition, the abundance and diversity of fossil 24-alkylated steranes that is found in Cryogenian and Ediacaran rocks (Briggs and Summons, 2014;Brocks et al, 2016;Grosjean et al, 2009;Kelly et al, 2011;Pawlowska et al, 2013) matches what is known from the record of preserved physical fossils Knoll et al, 2006). Consistent with fossil evidence for an increasing presence and diversification of protists in the midNeoproterozoic (Butterfield, 2015;Knoll, 2014), a recent study of well-preserved sediments from c. 800-750 Ma indicates steranes can be abundant in rocks of this age but that cholestane is dominant and its 24-alkylated counterparts are undetectable, a phenomenon that is not known at any other time in Earth's history (Brocks et al, 2016).…”

“…Yet the majority of Paleoproterozoic and Mesoproterozoic sedimentary rocks described that are known to preserve biomarkers appear to be devoid of eukaryotic steranes, but do contain C 27 -C 35 hopane hydrocarbons of bacterial origin (Blumenberg et al, 2012;Brocks et al, 2005;Flannery and George, 2014;Luo et al, 2015). From a micro-and macrofossil perspective, ample evidence exists for the presence of unambiguously eukaryotic fossil assemblages in the Mesoproterozoic (Butterfield, 2000;Javaux et al, 2001;Javaux et al, 2004;Knoll et al, 2006;Zhu et al, 2016). Accordingly, the failure to detect a concomitant record of eukaryotic steranes suggests that their membrane lipids contained limited amounts of steroids.…”

An archaeal origin of eukaryotes supports only two primary domains of life

“…The earliest fossils identified as eukaryotic appeared by about 1.8 billion years ago 20 ; over this enormous span of time, the accumulation of multiple substitutions in DNA and protein sequences might have erased any signal that would allow the relationship between archaeal and eukaryotic core genes to be established 21 . However, more recent simulations and empirical studies suggest that there are reasons to be cautiously optimistic that this is not the case: functional constraints vary across real DNA and protein sequences so that sites evolve at different rates [22][23][24][25] a, The rooted three-domains tree 14 depicts cellular life divided into three major monophyletic groups or domains: the Bacteria, Archaea and Eukaryota-the latter representing the host lineage, sometimes also called the nuclear or nucleo-cytoplasmic lineage 5 , that acquired the mitochondrial endosymbiont.…”

“…107), and a short amino acid insertion 108 in the broadly conserved elongation factor 1-a that has only been found in TACK Archaea and eukaryotes as indicated by the vertical bar. Accession numbers and additional details are provided in Supplementary Tables 1 and 2. REVIEW RESEARCH years ago 20 . These data are consistent with molecular dating analyses that place the last common ancestor of eukaryotes at between 1.9 and 1.7 billion years ago 76 .…”

Constraining the timing of the Great Oxidation Event within the Rubisco phylogenetic tree

“…This is also presumed to be the case for Earth's more distant past with respect to photosynthetic bacteria, though perhaps to a lesser magnitude (Blank & Sanchez‐Baracaldo, 2010). However, the ability to resolve details about this distant past (specifically, when Rubisco‐mediated carbon uptake evolved or how efficiently ancestral Rubisco proteins functioned under ancient environmental conditions) are limited by the scant traces of geological and paleobiological evidence that survive from that history (Benton, Wills, & Hitchin, 2000; Braakman & Smith, 2012; Knoll, Javaux, Hewitt, & Cohen, 2006). …”