Environmental conditions at time of life origins.

• Atmospheric composition.
  • Early atmosphere was result of volcanic outgassing of volatiles.
  • Highly reducing atmosphere. Rich in CO₂.
  • Some free oxygen derived from photochemical dissociation by UV in upper atmosphere.
    2 H₂O + uv --> 2 H₂ +O₂

    This would have generated enough oxygen quickly to oxidize chemical building blocks of life near ocean surface.

• Oceanic conditions. Initially strongly acidic, precluding precipitation of carbonate rocks. This was because atmospheric CO₂, dissolved in sea water, forming carbonic acid.

• Land conditions: Here is one spot of optimism. By 3.8 g.a., we have continental conglomerates, showing that streams of water were flowing on surface.

• Energy source
• Proteins: Polymers of amino acids. Structural elements and catalysts.
• Nucleic acids: Regulate synthesis of proteins in proper cells.

Proteins:

• The simple experiment of Miller and Urey in 1953 showed that amino acids are readily synthesized from presumed primordial components of Earth atmosphere.
• S. W. Fox (1959) saw that concentrated solutions of amino acids form proteinoids (short polymers of 18 common amino acids) if heated to 140 deg. C. If phosphoric acid is present, 70 deg C will do.
• When cooled, proteinoids form suspiciously cell-like spheres.
• Fox ultimate found naturally occuring proteinoids in pools associated with Hawaiian volcanoes. It's not a huge stretch to speculate on a similar origin of proper proteins.

• A. G. Cairns-Smith in 1985 observed that RNA nucleotides can bind to the edges of clay minerals like smectite to form RNA-like polymers. (See Genetic Takeover: And the Mineral Origins of Life by A. G. Cairns-Smith).
• RNA World - In 1989, Sidney Altman demonstrated that RNA is capable of acting not only as a template for protein synthesis, but, in limited ways, as a biochemical catalyst. (Particularly interacting with phospholipids like those occurring in cell membranes). Proposed an early stage in the origin of life in which simple "biochemical" processes were carried out entirely by RNA.
• Double-strand DNA is presumably a derivative of RNA.
• Only later did nucleic acids and proteins join forces

Current thinking maintains that life probably originated in hydrothermal vents. These environments were:
• Sheltered from free oxygen, which is toxic
• Rich in thermal energy
• Interestingly, phylogenetic analyses suggest that among the most primitive organisms are thermophylic bacteria known as Archaea. Their special features:
  • Live only at near-boiling temperatures
  • Obtain energy from exotic reactions involving materials readily available in minerals, esp Sulfur.
  • Find oxygen to be toxic.

• The environment in which these conditions are routinely found is near deep sea hydrothermal vents. Currently, these seem like the most likely locations for the origin of life.

Oldest strong evidence of life is 3.7 Ga fractionated carbon in deep sea sediments preserved in Greenland.

Oldest strong body fossils c. 3.4 Ga Fig Tree Cherts.



Oldest stromatolites (see below) c. 3.4 Ga.

Controversial claims:

• Akila, Greenland 3.85 Ga fractionated carbon in "BIF": instead, is a metavolcanic and the graphite a metasomatic product. See paper
• Apex Chert, Australia 3.485 Ga "microfossils": probably bits of organic matter in a hot-springs solution deposit. Shapes grade from reasonable bacterial shapes to wholly inorganic, suggesting supposed biological forms are just part of shape spectrum. See paper.

We can't tell from looking at microscopic fossils which were photosynthesizers, but photosynthesis had momentous consequences for the rock record

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

• BIFs first appear when photosynthesizers start cranking out free oxygen. This oxygen reacts with oxygen sinks like iron to form mineral oxides.

• Their disappearance indicates the saturation of these sinks and the beginning of accumulation of high concentrations of oxygen in atmosphere.

• Terrestrial red bed deposits begin to appear at 1.8 g.a.. This tells us that the oceanic oxygen-sinks had become saturated and free oxygen was now building up in the atmosphere.

• Ozone: As it accumulated, free oxygen in upper atmosphere recombined to form ozone layer (O₃), allowing life to colonize surface waters.

• Oceanic acidity: Of course, by eating up atmospheric CO₂, photosynthesizers caused the acidity of the oceans to diminish, allowing the direct precipitation of carbonate rocks for the first time. Once that was possible, CO₂ concentrations fell very rapidly. as carbon became locked up in rock.

• Respiration: As oxygen was generated, some organisms evolved the ability to use it to release energy from sugars they had eaten. To such aerobic critters, oxygen became a necessity rather than a poison. Today anaerobic organisms are restricted to margial environments.
  C₆H₁₂O₆+ 6 O₂ ---> 6 CO₂ + 6 H₂O + energy

  Look familiar? It's just photosynthesis run backwards. In this case, the energy powers cell activities.

• These form when sediment falls onto a thin film of bacteria. The bacteria bind the sediment, and grow up through it. At any moment, only the top layer is alive.

• When there was nothing around to eat them, stromatolites were very common. Today, they only live in hypersaline environments that exclude other critters, like Shark Bay, Australia

Eukaryotes:

Complex cells, presumed the result of "colonization" of one type of prokaryotic cell by others. Characteristics:

• Typically much larger than prokaryotic cells. >60 microns as opposed to <20.
• DNA contained in nucleus
• Specialized organelles bound by double-layer cell membranes, possessing their own genome. Especially:
  • Chloroplasts: photosynthesizers
  • Mitochondria: aerobic respirers

Oldest biochemical markers of eukaryotes - steranes 2.7 Ga.

Oldest eukaryotic body fossils 2.2 Ga

Acritarchs:

• Common Proterozoic eukaryote body fossils.
• Represent encysted "resting stage" of organism.
• Broadly similar to dinoflagellates.
• Oldest acritarchs 1.7 Ga. Maximum diversity 850 m.a. Marked decline during Snowball earth episode (850-600m.a.) Survivors straggled into the Ordovician.

Oldest metazoan embryos of Doushantuo Formation, 570 Ma (but possibly older trace fossils)

To Syllabus.

Last modified: 22 August 2008