• Energy source
• Proteins: Polymers of amino acids. Structural elements and catalysts.
• Nucleic acids: Regulate synthesis of proteins in proper cells.
• Semipermeable membranes: In which to package and isolate the components of life.

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

• In living cells, these are made of highly impermeable phospholipid bilayers. (Indeed, protein channels regulate transport across the membrane.) Jack Szostak of Harvard has shown that fatty acids that would have been common in the Archean oceans can form vesicles that are permeable to nucleotide monomers and amino acids, but not to polymers of these.
• Osmotic pressure from nucleic acid polymers causes larger vesicles to "steal" fatty acids from smaller ones that they encounter. Mechanical forces cause larger vesicles to fission.
• The result, the beginning of natural selection, in which fatty acid vesicles that grow faster dominate.

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.

Hydrothermal vent

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

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

Bitter Springs Chert microfossil (850 m.a.)

For a while, organisms got away with eating 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 capturing energy from the environment - Photosynthesis,

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

Cyanobacteria

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

• The presence of hopanes in 2.7 g.a. rock from West Australia seemed to clinch the presence of cyanobacteria at that time. More recent work by Fischer has called this into question. At this point, the oldest unambiguous cyanobacterial body fossils are roughly 2.4 g.a.

• 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.

Banded iron specimen

Stromatolites: Beginning about 3.0 g.a., we begin to see fossil stromatolites - laminated bacterial mats. Reports of older stromatolites and body fossils are problematic (see Brasier et. al, 2006) These were very common for most of the Proterozoic, but declined during the Neoproterozoic, when, presumably, critters appeared that could eat them.

• 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

Sectioned stromatolite

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