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GEOL 331/BSCI 333 Principles of Paleontology

Fall Semester 2018
Fossil Protists and Metazoan Origins I


Grypania spiralis: Colonial bacterium, alga, or metazoan?

Key Points:
•The record of eukaryotes of increasing complexity starts ~ 1.6 ga.
•Eukaryotic cells are composites of prokaryote-grade organisms that have colonized one another by endosymbiosis.
•Eukaryote phylogeny is unsettled, but the groups Archaeplastida, Opisthokonta, and Chromalveolata are well-supported.
•Major fossil-forming groups include: Rhodophyta (red algae), Viridiplantae (green plants), Radiolaria, Foraminiferida (foraminiferans), Alveolata (incl. dinoflagellates), Bacillariophyceae (diatoms), Coccolithophyceae (coccolithophorids), choanoflagellates, and Metazoa (animals).
•Each group listed above yields distinct fossils and has its own life-history/reproductive strategy.
•Choanoflagellates appear to be the closest relatives of metazoans.

"The view of evolution as a chronic bloody competition among individuals and species, a popular distortion of Darwin's notion of "survival of the fittest," dissolves before a new view of continual cooperation, strong interaction, and mutual dependence among life forms. Life did not take over the globe by combat, but by networkiing. Life forms multiplied and complexified by co-opting others, not just by killing them." (Lynn Margulis, 1986. Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors)

Micropaleontology:

Includes study of microscopic remains of single-celled organisms. We have touched on the difficult record of early life forms. Starting with the 1.6 Ga Ruyang Group, however, we encounter eukaryotic cell fossils of increasing diversity (Yin, 1997).


Schematic of typical eukaryote from The Gates of Academe

Eukaryota:

The key concept here is the role of endosymbiosis - the coopting of other organisms - in the acquisition of the characters that we associate with the "typical" eukaryotic cell, Including: Thus, depicting their evolution as a strictly branching phylogeny is misleading. What we see instead is reticulation (interweaving) at the largest scales of life-history. (See Palmer et al., 2004)



Endosymbiosis
Organelles are variably distributed in the major groups:

  • Some groups of eukaryotes actually lack some typical organelles, but these seem to be secondary losses:

    Recall that such Eukaryotic abilities as:

    May have been adaptations to the rise of aerobic environments during the Great Oxygenation Event. Another trend we see among eukaryotes is increasing versatility in the management of protein synthesis.



    An operon with: 1.) RNA polymerase, 2.) Repressor protein, 3.)Promoter, 4.) controlling region.
    Operons: We mentioned how information in DNA is transcribed into proteins previously. We didn't discuss what triggers the process. The magic of protein synthesis is that DNA is only transcribed into mRNA when the protein it codes for is needed. E.G.: The bacterium Escherichia coli freely metabolizes glucose, but if glucose is lacking, and lactose is present, it can metabolize lactose by producing an enzyme, beta-galactosidase, that breaks lactose down in to glucose and galactose. But how does it know when to make beta-galactosidase?

    The work of François Jacob and Jacques Monod established the answer (Nobel Prize in 1965):

    The operon: The stretch of contiguous DNA containing the promoter, the controlling region, and the genes coding for beta-galactosidase and other proteins involved in lactose metabolism. Animation.

    Big Point: Somewhere on the genome, the lac repressor protein is, itself, coded for in another operon by a repressor gene whose activity may be regulated by yet other other repressors:

    Eukaryotes, thus, display a degree of metabolic plasticity that prokaryotes lack.


    Eukaryote Phylogeny

    A phylogeny of significant groups from Adl et al., 2005, as summarized by Keeling et al., 2009 appears at right.

    Note: The phylogram to which this cladogram links superimposes the phylogeny of fossil-forming groups on to the stratigraphic record. Typically we could infer minimum divergence ages with some confidence. In the case of Eukaryotes, however, this is perilous as so few yield fossils. El Albani et al. 2014, for instance describe "colonial" organisms with features associated with colonies of eukaryotic cells in the 2.1 Ga Francevillan B Formation of Gabon. These colonies were not organized into bodies in the manner of multicellular fungi, plants, and animals, but were akin to congregations of independent cells like slime molds.

    Hypotheses of the phylogeny or eukaryotes are subject to frequent revision, so we focus on descriptions of major fossil-forming groups, rather than the broad patterns. And yet a few words on large groups:


    Major microfossil-forming groups:

    Odd-balls:


    Acritarch from Digermulen
    Acritarchs: (Proterozoic to... Ordovician? Devonian? Permian? Depends on whom you ask)


    Chitinozoan from Wikipedia
    Chitinozoa (Ordovician - Carboniferous):


    Archaeplastida:


    Chromalveolata:

    Additional reading:


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