The Oldest Fossils & Prokaryote Fossil History


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 from Panoramio.com
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: Don't assume that the components of life given above evolved independently. Black et al., 2013 show that the simple fatty acid decanoic acid binds preferentially to the four RNA nucleotides (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.


Black smoker from NOAA - Ocean Explorer

Location of 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 from Schropf, 1993

    Concrete evidence:

    Controversial claims:



    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, 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 autographically 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 the rock record

    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.

    Great Oxidation Event: Between 2.4 and 2.3 Ga atmospheric oxygen concentrations rise to roughly 1%. Indicates that cyanobacteria had become widespread by this point.

    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
    Banded Iron formations (BIFs): Late Archean - Early Proterozoic (~3.0 - 1.8 Ga) Cherts with alternation of gray and rust red bands of hematite (Fe2O3).



    Stromatolite in cross-section
    The Age of Slime

    Stromatolites: Beginning about 3.0 Ga, we begin to see abundant fossil stromatolites - laminated bacterial mats. 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 showed up.



    Eukaryote - prokaryote comparison from Wikipedia

    Eukaryota:

    Complex cells characterized by:

    Evolution:


    Acritarchs (right): The first common eukaryote fossils.


    A Reasonable but Speculative Time-Line


    Additional reading:

    To Next Lecture.
    To Previous Lecture.
    To Syllabus.