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Origins and Evolution of Life

Life arose spontaneously from nonliving matter under the conditions that existed on our primitive planet.
- No direct evidence for the pathway but there is general agreement to an outline of what probably occurred.

Universe is about 10-20 billion years old
The sun and its planets are much younger, about 4.5 billion years old
- Sun formed by the condensation of cosmic gas and dust into a single compact mass
  - The condensation process produced enormous pressures and heat which initiated a thermonuclear reaction which continues today.
Interstellar dust that was not condensed into the forming sun was held in a disk surrounding the newly formed sun by gravitational forces.
This dust eventually condensed into the planets of our solar system.
- Oort Cloud even smaller condensation processes beyond the orbit of Pluto
  - Origins of comet which may have had a significant impact on the evolution of life on earth

Condensation of earth resulted in the stratification of materials.
- Heavy elements Fe and Ni moved to the center, while lighter elements rose to the surface.
- The lightest elements hydrogen, helium and the noble gases formed the first atmosphere.
Gravitational compression, along with radioactive decay resulted in enormous heat, produced a molten core and a mantle (4700 km thick) and a crust (8-65 km thick).
- First atmosphere was contained H2 and He

Gases escaped from the molten core through volcanic activity.
- This atmosphere contained H₂, H₂O, CO, CO₂, N₂, H₂S, HCN, H₂CO.
  - Noticeable absent is O2, making this second atmosphere a "Reducing atmosphere"
Water was present as vapor providing conditions for the development of weather.
- Rains would have formed surface water deposits ("Primordial soups").

Delivery of organic compounds formed during the formation of the solar system by the impact of asteroids and meteors with the planet.

Terrestrial synthesis
- All organisms are composed of similar organic compounds
  - Amino acids, carbohydrates, lipids and nucleic acids

Spark chamber experiments containing the gas composition of the early atmosphere produced many organic compounds and their precursors.

Amino acids, urea, HCN, acetic acid, lactic acid
The amino acids that have been produced in these experiments match those found in modern proteins.

Accumulation of organic monomers was not a sufficient basis for the initiation of life.
- Raw material of living systems
Formation of polymers was the next step.
- Spontaneous linking together of monomers to form polymers: Polypeptides and Nucleic acids.
- Polymerization would have occurred a sites of high concentrations of the monomers
  - Clay or rock surfaces
  - Act as catalysts to promote formation of polymers
- Polymerization required energy input

Proteinoid microspheres aggregates of polypeptides that form when hot aqueous solutions of polypeptides are cooled.
- Swelling in hypotonic solution and shrinking in hypertonic solutions
- Shell between exterior and interior (outer boundary)
- Internal organization
- Growth and budding (similar to yeast)
- Aggregation into clusters (similar to bacteria)
Liposomes aggregation of lipids into a droplet (spherical lipid bilayer)
- Formation of membrane like structure (shell between interior and exterior environments)
Coacervates are an aggregation of several types of organic molecules
- Coacervates have a tendancy to incorporate hydrophilic substances
  - Coacervates tend to increase in size
  - Develop internal structure
  - Develop surface active process

Since the aggregates have a boundary between their interior and the external environment the chemical reactions that occur in the interior depend only on the local internal conditions.
Any catalytic activity depends on the microenvironment and availability of reactants.
Selective and regulatory influences on the internal chemistry are established

Naked genetic systems may have been incorporated into nonliving aggregates.
- Progenotes
First cells arose about 3 to 5 billion years ago
- Anaerobic heterotrophs: dependent on the abiotic synthesis of organic molecules
- Competition would have reduced availability of resourses
Natural selection would have eliminated the inefficient cells
- Favored mutations that would allow for the conversion of available resources into the necessary compounds.
- Strong selection for organisms that could use alternative nutrients
Cells must have had some mechanism to handle the release of chemical energy resulting from the breakdown of organic compounds
- Since all organisms today use ATP as the principle energy currency it is likely that ATP was probably an early evolutionary development.

Primitive heterotrophs probably developed increasingly more complex biochemical pathways due to competition for diminishing resources.

Chemotrophy is the use of energy released from the splitting of the covalent bonds of molecular hydrogen and new bonds are formed

This energy could be used to produce organic compounds no longer available in the soup.

Photoautotrophs used cyclic photophosphorylation is the use of light energy to synthesis ATP.
Noncyclic photophosphoryaltion arose about 2.5 billion years ago
- Use of light energy to synthesis ATP and carbohydrates (from water and carbon dioxide)
- By-products of this process (O2) changed the evolutionary pressures on cells
  - Oxygen is toxic to biological systems
  - Ozone layer developed screening the U.V.

Oldest fossils date to 3.6 billion years ago and are probably cyanobacterium responsible for the oxygen revolution
Eucaryotic organisms about 2.1 billion years ago
- Origins of organelles by endosymbiosis
- Two-thirds of the earth's history passed before the eucaryotes arose