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GEOL 331/BSCI 333 Principles of Paleontology

Fall Semester 2018
Archean Fossils & Life's Origins


Banded iron formation

Key Points:
•Whatever definition you use, life requires homeostasis, metabolism, and reproduction.
•LUCA, the last universal common ancestor, was a chemosynthetic autotroph that lived in a hot anoxic environment.
•Life seems to have arisen immediately after end of the Late Heavy Bombardment ~ 3.8 Ga.
•Photosynthesis fundamentally changed Earth environments.
•the earliest microfossils are ~3.77 Ga tubular microfossils from a hydrothermal environment.
•Complex eukaryotic cells seem to have emerged between 2.7 and 2.2 Ga.

"By firm immutable immortal laws Impress'd on Nature by the GREAT FIRST CAUSE,
Say, MUSE! how rose from elemental strife
Organic forms, and kindled into life;
How Love and Sympathy with potent charm
Warm the cold heart, the lifted hand disarm;
Allure with pleasures, and alarm with pains,
And bind Society in golden chains.

(Erasmus Darwin, 1803. 'Production of Life', The Temple of Nature; or, The Origin of Society: A Poem, with Philosophical Notes)

Definitions of Life

There is no single standard definition of life. Different definitions tend to reflect the priorities of their authors. One that Merck is fond of:

"Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation."

These responses may involve:


Isua metaconglomerates
Environmental conditions at time of life origins.

Life is thought to have arisen (for the last time?) shortly after the end of the Late Heavy Bombardment, between 3.9 and 3.8 Ga. The environment in which this happened differed markedly from what we know today.


Requirements for life:

Proteins: These end up being surprisingly easy to form under natural circumstances.

Nucleic acids: Here we have a more complex problem, as the nucleotides of which they are composed are more complex than amino acids, and coaxing them to polymerize is more difficult. Research has focused on the identification of non-living substrates that could serve as a polymerization template:

Membranes:

Energy source: The crucial fact in the foregoing is that the both cell membranes and proteins seem to have originated in environments that are at least intermittently hot.

Synergies: The foregoing research suggests that the major components of life were able to self-assemble independently of one another in the primordial soup, Don't assume from this that the components of life necessarily evolved independently. Black et al., 2013 show that the simple fatty acid decanoic acid binds preferentially to the four RNA nucleotides found in RNA (adenine, guanine, cytosine, and uracil). Moreover, in their bound state, the nucleotides buffer decanoic acid against the disruptive effects of salt water. The result is a natural affinity between fatty acids and RNA nucleotides.


Protein synthesis from Wikipedia
The Ultimate result: Cells in which information encoded in DNA is the template for the synthesis of proteins. The structure of DNA was discovered in 1953, and its role as the physical repository of genes illuminated in the following decades. A brief review of how information encoded as nucleic acid is expressed as proteins goes like this:

Link to simple animation or to a more detailed explication.

But how much of this, and other familiar cellular functions were present in the last common ancestor of life?


Black smoker from NOAA - Ocean Explorer

LUCA (the Last Universal Common Ancestor):

Current thinking maintains that life probably originated in hydrothermal vents (right). These environments were:
  • Interestingly, phylogenetic analyses suggest that among the most primitive organisms are thermophylic prokaryotes known as Archaea. Their special features: The environment in which these conditions are routinely found is near deep sea hydrothermal vents. For some time the most likely locations for the origin of life. Weiss et al., 2016 have attempted to identify genes conserved across all major domains of life (Archaea and Prokaryota) in order to characterize LUCA's environment and ecology. Their conclusions:


    Apex chert microfossils (?) from Schropf, 1993

    Evidence of the Rock Record:



    Cyanobacteria from Plant Science 4 U

    What we definitely know:

    Photosynthesis: Organisms change Earth chemistry

    For a while, organisms got away with chemosynthesis in vent environments (E.G. LUCA), and heterotrophically absorbing the organic materials that were floating around in the ocean. As these started to get scarce, one group, the cyanobacteria, came up with a new method of autotrophically capturing energy from the environment - Photosynthesis,

    6 CO2 + 6 H2O + energy (sunlight)---> C6H12O6+ 6 O2

    Note that oxygen is a product.

    We can't tell from looking at microscopic fossils which were photosynthesizers, but photosynthesis had momentous consequences for life and Earth environments.

    Living cyanobacteria probably provide a good picture of ancient photosynthesizers. Indeed, all other photosynthesizing organisms ultimately rely (directly or indirectly) on cyanobacterial symbionts. Phylogenetic studies of living cyanobacteria suggest a minimum divergence age of roughly 2.8 Ga.

    The Gunflint Chert (1.88 Ga - Michigan) represents a well preserved Paleoproterozoic cyanobacterial flora containing forms that can be directly compared with living ecomorphs.



    Banded iron formation
    Great Oxidation Event: ~ 2.4 Ga oceanic oxygen concentrations begin to rise, indicating that cyanobacteria had become widespread by this point. This accords well with the minimum divergence age of 2.8 Ga.


    Stromatolite in cross-section
    The Age of Slime

    Stromatolites: Beginning about 3.0 Ga, we begin to see abundant fossil stromatolites - laminated bacterial mats. (Not necessarily photosynthesizing.) The earliest are from the 3.43 Ga Strelley Pool Chert of Australia (Allwood et al., 2006) and possibly the 3.7 Ga Isua metasediments (Nutman et al., 2016). These were very common for most of the Proterozoic, but declined during the Neoproterozoic, when, presumably, critters appeared that could eat them.



    Cladogram of the domains of life showing Eubacteria (blue), Archaea (green), Eukaryota (red)

    Eukaryota and the two domains of life:

    Traditionally we distinguish between basic cell types: Woese and Fox, 1977, recognized that some prokaryotic organisms were distinct from bacteria based on their DNA transcription biochemistry and on molecular systematics. These have come to be known as Archaea. It would be simple to say that Eubacteria, Archaea, and Eukaryota are the three domains of life, but no.


    Eukaryote - prokaryote comparison from Wikipedia
    Review the peculiarities of Eukaryota:

    Eukaryote complexity seems to be largely a response to the challenges of the Great Oxidation Event.

    In the rock record:


    A Reasonable but Speculative Time-Line



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