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The History of Life
Chapter 14
Early Earth
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Was inhospitable!
Very hot due to:
Meteoric impact
 Volcanic eruptions
 Radioactive decay
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Early atmosphere contained:
Mostly water vapor, nitrogen,
and carbon dioxide
 Almost no oxygen.
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Early Earth Timeline
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~4.6 bya: Formation of the Earth
~4.5 bya: Formation of the moon
~4.0 bya: Earth begins to cool
~3.9 bya:
Oldest rocks
 Primordial oceans begin to form
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~3.8 bya: First prokaryotic organisms
~2.5 bya: First stable continents
~2.1 bya: First eukaryotic organisms
Fossils
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Over 99% of the
organisms that have
ever lived on this
Earth are now
extinct!!
We know about these
organisms by
impressions left in
rocks, amber and ice.
Dating Techniques
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Relative Dating
Places items in sequence
 No exact date is assigned
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Numeric Dating
Provides an actual date
 Radioactive Dating: utilizes radioactive isotopes of
elements.
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Geologic Time Scale
Early Theories on the Origin of Life
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Spontaneous Generation
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Idea that nonliving material can produce life
Example: Rotting meat produces flies
Biogenesis
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Idea that living organisms must come from other
living organisms.
Spontaneous Generation
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Italian scientist, Francisco Redi, disproved the
myth, but only for larger organisms.
Biogenesis
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But scientists still believed that maybe
microorganisms arose spontaneously from the
air.
French scientist, Louis Pasteur, finally disproved
spontaneous generation by designing an
experiment where air but no organisms were
allowed to contact a nutrient broth…nothing
grew!
Pasteur’s
Experiment
Modern Theories
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Two developments must have preceded life on
Earth
Formation of organic (carbon-containing) molecules
 Formation of protocells (early cells)
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Formation of Organic Molecules
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Simple organic molecules (carbon containing)
must have formed
These molecules must have organized into more
complex molecules like proteins, carbohydrates,
lipids and nucleic acids.
In 1930, Russian scientist, Alexander Oparin,
hypothesized life began in oceans by chemical
reactions with the atmosphere, lightning, and
Earth’s heat to produce organic molecules.
Primordial Soup Theory
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Developed by Alexander Oparin.
Earth’s ancient atmosphere probably contained
nitrogen, methane, and ammonia gases, but little to no
oxygen.
Energy from sun fueled chemical reactions in the
atmosphere forming organic molecules (Carbon based
molecules) like amino acids
These molecules were then washed into the oceans
making a primordial soup of organic molecules
Eventually these proteins, lipids, and other organic
molecules formed the first cells.
Miller and Urey’s Experiment
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In 1953, American
scientists, Stanley
Miller and Harold
Urey tested Oparin’s
hypothesis.
Their result was the
formation of amino
acids, sugars and
other simple organic
molecules.
This supported
Oparin’s hypothesis.
Formation of Protocells
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Various experiments during the 1950’s showed
how complex organic molecules might have
formed from small organic molecules in pools
of heated water.
American scientist, Sidney Fox, provided a
mechanism to explain the formation of
protocells.
A protocell is a large, ordered structure,
enclosed by a membrane, that carries out some
life activities such as growth and division.
Evolution of the Cell
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First cells were prokaryotes that may have
evolved from protocells.
They were anaerobic organisms, meaning they
lived in the absence of oxygen.
They were also heterotrophs, meaning they
obtained food rather than making it themselves.
First Autotrophs
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Auto means self ; troph means
energy
Were probably similar to today’s
archaebacteria: prokaryotic
chemosynthetic organisms found
in harsh conditions such as deep
sea vents.
Chemosynthesis is the process of
releasing energy from inorganic
compounds such as H2S.
First Photosynthetic Organisms
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Are responsible for the first atmospheric
oxygen.
Were also prokaryotes.
Also responsible for the ozone layer.
The ozone layer provided protection against high
energy radiation such as UV and gamma rays.
 This allowed organisms to move on to land.
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This allowed for more aerobic organisms.
Endosymbiont Theory
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Proposed in the 1960’s by American biologist,
Lynn Margulis.
Explains how eukaryotic cells may have arisen.
States that:
Mitochondria and chloroplasts were actually once
prokaryotic cells. (They both have their own DNA
similar to prokaryotic DNA.)
 A cell ingested these bacteria and the bacteria were
protected while providing energy for the cell.
Eventually the bacteria could no longer live on their
own, and the eukaryotic cell evolved.
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