Chapter 14 The History
of Life
14.1 Fossil Evidence of Change
Earth’s Early History
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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 Earth’s center.
Atmosphere of Earth consisted of H20, CO2,
SO2, CO, H2S, HCN, N2 and H2
Clues in Rocks
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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.
Fossil Formation
Fossil Formation
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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.
Relative Dating
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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
Radiometric Dating
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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
Radiometric Dating
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Commonly used radioactive elements are Uranium238 (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
Radiometric decay Rate of C14 to N14
Half life 5730 years
Radiometric decay Rate of U238 to
Pb205
Half life 4520 million years (4.510 billion)
The Geologic Time Scale
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Model that expresses the major geological and
biological events in Earth’s 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
Plate Tectonics
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During the Mesozoic era the location
of the Earth’s plates underwent a
major shift in position and are
continuing to move today
Chapter 14 The History
of Life
14.2 The Origin of Life
Origin: Early Ideas
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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.
Francisco Redi 1668
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First controlled experiment to disprove spontaneous
generation in which flies arose out of meat.
He hypothesized that flies, not meat, gave rise to flies
Louis Pasteur 1850’s
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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)
Origin: Modern Ideas
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If life can only come from preexisting life
(theory of biogenesis) then how did the first
life-form appear?
Three ideas:
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Divine creation
Extraterrestrial
Series of chemical events
Origin: Modern Ideas
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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
1920’s
Called the “primordial soup”. The oceans as a
“soup” of chemicals that could have combined
to form life
Origin: Modern Ideas
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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.
Origin: Modern Ideas
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Another ideas is that the
organic reactions that
preceded life’s
emergence began at
deep-sea vents where
sulfur forms the base of
a unique food chain.
Origin: Modern Ideas
In order for life to form from nonlife (chemicals)
three requirements must be met:
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Making proteins
Genetic code
Molecules to cells
Making Proteins
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In order for life to exist proteins must be formed.
Stable proteins can be formed on clay particles
Genetic Code
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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.
Molecule to Cells
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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.
Cellular Evolution
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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.
Cellular Evolution
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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
Cellular Evolution
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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.
Endosymbiont Theory
Evidence for the Endosymbiont
Theory
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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.