Chapter 14 The History of Life

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Chapter 14 The History of Life

• 14.1 Fossil Evidence of Change

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Earths Early History

• Earth formed 4.6 billion years ago from molten
  material.
• Cooled for 500 million years and formed a solid
  crust on the surface. Surface is rich in lighter
  elements, as more dense elements were pulled by
  gravity toward Earths center.
• Atmosphere of Earth consisted of H20, CO2, SO2,
  CO, H2S, HCN, N2 and H2

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Clues in Rocks

• A fossil is any preserved evidence of an
  organism.
• Most organisms decompose before they have a
  chance to become a fossil.
• Those organisms that are most likely to become
  fossilized are those that are covered quickly by
  sediment (no O2) and have hard parts.
• Only one in a million organism becomes a fossil.

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Fossil Formation
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Fossil Formation

• Fossils form in sedimentary rocks sediments
  cover the organism.
• Igneous rocks form from molten rock material
  metamorphic rocks when other rocks are exposed to
  heat and pressure both processes would destroy
  any fossil material in these rock types.

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Relative Dating

• Determine age of fossils by comparing them with
  those in other layers
• Law of Superposition states that younger rock
  layers are deposited on top of older rock layers
• Only true of undisturbed rock layers

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Radiometric Dating

• Dating method that used the decay of radioactive
  isotopes to measure the age of a rock.
• Isotope is a form of an element that has the same
  atomic number but a different atomic mass (mass
  number)
• Half life is the amount of time it takes for ½ of
  the original radioactive isotope to decay or
  change into its decay product

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Radiometric Dating

• Commonly used radioactive elements are
  Uranium-238 (U238), Potassium-40 (K40) and
  Carbon-14 (C14)
• Some radioactive isotopes used for radiometric
  dating are found only is igneous or metamorphic
  rocks (not sedimentary) so isotopes cannot be
  used to date rocks that contain fossils. Relative
  dating of fossils from known igneous or
  metamorphic rocks are used.
• C14 can be used to date fossils directly if they
  are less than 60,000 years old

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Radiometric decay Rate of C14 to N14
Half life 5730 years
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Radiometric decay Rate of U238 to Pb205
Half life 4520 million years (4.510 billion)
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The Geologic Time Scale

• Model that expresses the major geological and
  biological events in Earths history.
• Largest time classification is the eon.
• Eons are divided into eras
• Eras are divided into periods
• Periods are divided into epochs
• See page 397

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Plate Tectonics

• During the Mesozoic era the location of the
  Earths plates underwent a major shift in
  position and are continuing to move today

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Chapter 14 The History of Life

• 14.2 The Origin of Life

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Origin Early Ideas

• Spontaneous generation is the idea that life
  arises from nonlife.
• People believed that it rained frogs, mice
  spontaneous arose out of grain, and from mud came
  insects, fish and worms.

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Francisco Redi 1668

• First controlled experiment to disprove
  spontaneous generation in which flies arose out
  of meat.
• He hypothesized that flies, not meat, gave rise
  to flies

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Other Scientists

• Spallanzani

• Needham

• Organisms exist in air (microbes)
• Believed in biogenesis
• Italian 1700s

• Life force exists that causes life
• Believed in Spontaneous Generation
• English 1700s

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Louis Pasteur 1850s

• Experiment showed that sterile broth remained
  free of microorganisms until exposed to air.
• Flasks still exist today free of microorganisms.
• Theory of spontaneous generation rejected and
  replaced with theory of biogenesis (life from
  life)

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Origin Modern Ideas

• If life can only come from preexisting life
  (theory of biogenesis) then how did the first
  life-form appear?
• Three ideas
• Divine creation
• Extraterrestrial
• Series of chemical events

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Origin Modern Ideas

• Simple organic molecules could have been formed
  from a mixture of gases present in the early
  atmosphere when sparked by lightening.
• Theory proposed by Oparin and Haldren in 1920s
• Called the primordial soup. The oceans as a
  soup of chemicals that could have combined to
  form life

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Origin Modern Ideas

• Simple organic molecules have been formed in the
  laboratory
• Experiments of Miller and Urey in 1953.
• Amino acids, sugars and nucleotides have been
  formed by refining this experiment.

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Origin Modern Ideas

• Another ideas is that the organic reactions that
  preceded lifes emergence began at deep-sea vents
  where sulfur forms the base of a unique food
  chain.

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Origin Modern Ideas

• In order for life to form from nonlife
  (chemicals) three requirements must be met
• Making proteins
• Genetic code
• Molecules to cells

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Making Proteins

• In order for life to exist proteins must be
  formed.
• Stable proteins can be formed on clay particles

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Genetic Code

• A coding system for protein is a requirement for
  life.
• All living things have DNA and RNA.
• Since some RNA sequences have changed little over
  time, scientists consider RNA to be the first
  genetic coding system.
• RNA can also act like an enzyme and could have
  carried out some early life processes
• Thought that RNA replication took place on a clay
  crystal.

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Molecule to Cells

• Cells have a membrane.
• Scientists have tested ways of enclosing
  molecules in membranes, allowing early metabolic
  and replication pathways to develop.
• How cells formed has yet to be explained.

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Cellular Evolution

• The first cells were prokaryotes similar to
  current day archaebacteria.
• Today archaebacteria live in extreme environments
  similar to what may have been experienced on
  early Earth.
• Photosynthesis began by cyanobacteria (not
  archae). Until 1.8 billion years ago no oxygen
  existed in the atmosphere.
• Fossils of these bacteria have been found and
  dated at 3.5 billion years old.

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Cellular Evolution

• Once cyanobacteria produced sufficient oxygen to
  form the ozone layer, conditions were right for
  the evolution of eukaryotic cells.

Ozone layer protects Earth from harmful radiation
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Cellular Evolution

• Eukaryotic cells appeared in the fossil record
  about 1.8 billion years ago. (2 billion years
  after archae)
• The endosymbiont theory suggests that some
  photosynthetic bacteria became engulfed within a
  larger bacteria and became the chloroplast other
  engulfed bacteria became the mitochondria.

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Endosymbiont Theory
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Evidence for the Endosymbiont Theory

• Both mitochondria and chloroplasts contain their
  own circular DNA just like prokaryotes.
• Both mitochondria and chloroplasts contain
  ribosomes that closely resemble archae ribosomes.
• Like prokaryotic cells both mitochondria and
  chloroplasts reproduce by fission, independent
  from the rest of the cell.