History of Life on Earth

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History of Life on Earth

• How long has life been on Earth?
• What are the relationships between organisms as
  time moved forward?

BioH - Ch 19
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Early Earth
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Early Earth Atmosphere

• Hydrogen Cyanide
• Carbon Dioxide
• Carbon Monoxide
• Nitrogen
• Hydrogen Sulfide
• Water

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Experimental Evidence
The spontaneous formation of organic molecules
was first demonstrated experimentally in the
1950s, when Stanley Miller (then a graduate
student) showed that the discharge of electric
sparks into a mixture of H2, CH4, and NH3, in the
presence of water, led to the formation of a
variety of organic molecules, including several
amino acids.
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Millers Experiment

• At the end of one week, Miller observed that as
  much as 10-15 of the carbon was now in the form
  of organic compounds.
• Two percent of the carbon had formed some of the
  amino acids which are used to make proteins.
• Perhaps most importantly, Miller's experiment
  showed that organic compounds such as amino
  acids, which are essential to cellular life,
  could be made under the conditions that
  scientists believed to be present on the early
  earth.

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Agents of Metabolism
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Origin of Prokaryotic cells

• The next step in evolution was the formation of
  macromolecules.
• The monomeric building blocks of macromolecules
  have been demonstrated to polymerize
  spontaneously under plausible prebiotic
  conditions. Heating dry mixtures of amino acids,
  for example, results in their polymerization to
  form polypeptides.
• But the critical characteristic of the
  macromolecule from which life evolved must have
  been the ability to replicate itself. Only a
  macromolecule capable of directing the synthesis
  of new copies of itself would have been capable
  of reproduction and further evolution.

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Next Steps

• Only nucleic acids are capable of
  self-replicating through complimentary
  base-pairing.
• These self-replicating units may have been
  enclosed in a double layer of phospholipids.
• This would be considered the first cell.

• Because cells originated in a sea of organic
  molecules, they were able to obtain food and
  energy directly from their environment.
• But such a situation is self-limiting, so cells
  needed to evolve their own mechanisms for
  generating energy and synthesizing the molecules
  necessary for their replication.

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Evolution of Metabolism

• The generation and controlled utilization of
  metabolic energy is central to all cell
  activities, and the principal pathways of energy
  metabolism are highly conserved in present-day
  cells.
• All cells use adenosine 5'-triphosphate (ATP) as
  their source of metabolic energy to drive the
  synthesis of cell constituents and carry out
  other energy-requiring activities, such as
  movement (e.g., muscle contraction).

• Generation of metabolic energy, using glycolysis
  - the anaerobic breakdown of glucose to lactic
  acid to produce ATP.
• Photosynthesis utilizes energy from sunlight to
  drive the synthesis of glucose from CO2 and H2O,
  with the release of O2 as a by-product.
• The O2 released by photosynthesis is used in
  oxidative metabolism, in which glucose is broken
  down to CO2 and H2O, releasing much more energy
  than is obtained from glycolysis.

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Endosymbiotic Theory
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Common view of evolution of life forms
See text Pages 306-307
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Where does this lead?