The History of Life
The Record of Life
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You may have seen thrilling
movies or read books that describe
travel in time. The characters
often encountering strange
organisms.
 Did the differences you saw
surprise you?
 After all, everything changes over
time.
 It is difficult to imagine what Earth
might have been like that long ago!
Early History of Earth
Early Earth was inhospitable!
It was probably very hot, volcanoes might have erupted
frequently spewing gases and lava
• These gases helped form Earth’s early atmosphere which
contained little oxygen but abundant carbon dioxide and
nitrogen
About 3.9 billion years ago, earth might have cooled enough
for water in the atmosphere to condense
• This would have led to millions of years of rainstorms,
enough to fill Earth’s oceans
 It is in the oceans that scientist propose that the first
organisms appeared between 3.9 and 3.5 billion years ago
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History in Rocks
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Scientist cannot be sure that earth formed in this
way. There is no direct evidence of the earliest
years of Earth’s history.
 The physical processes of Earth constantly
destroy and reform rock.
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The oldest rocks that have been found on earth
formed only about 3.9 billion years ago
Rocks are an important source of information
about the diversity of life that once existed on
the planet
Fossils – Clues to the Past
 The
millions of species that live today are
probably only a small fraction of all the
species that ever existed.
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About 99% of species are extinct – they no
longer live on Earth
– evidence of an organism that lived
long ago.
 Fossil
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Scientists use fossils to learn about ancient
species.
Fossils Cont.
 A person
who studies fossils is a
Paleontologist
 Fossils can form in many different ways.
 Fossils are used to understand
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Events that happened long ago
Kinds of organisms that lived in the past
Behavior of organisms that lived in the past
Ancient climate and geography
Could ferns have lived in
Antarctica?
Scientist have discovered fossil remains of
ferns in the rocks of Antarctica. These
fern fossils are related to ferns that grow in
temperate climates on Earth today.
How did they get there?
Pangaea link
How fossils form
 Fossils

occur in sedimentary rocks
What is a sedimentary rock?
 Fossils
form when organisms are buried in
small particles of mud, sand or clay soon
after they die.

Over time, the particles compress and harden
into sedimentary rock
 Fossils
can still form today in the bottom of
lakes, streams and oceans.
The Fossilization Process
 Describe
how the movements of earth
might expose a fossil.
 Why do you think that fossils are rare in
igneous and metamorphic rock?
 Follow
this link to see the process
Fossils in different layers of
sedimentary rock vary in age

Ways of dating fossils

Relative dating – The deeper the layer of rock, the
older the fossil is. Layers closer to the top are
younger (Geologic law)
• Relative dating only gives you an estimate of age, not the
actual age of a fossil

Radiometric dating – Uses radioactive isotopes in
rocks or fossils (radioactive isotopes break down over
time giving off radiation)
• Decay rate of the isotopes can measured – half life
• Scientist use potassium-40 to date older samples (1.3 billion
years) and carbon-14 to date fossils less than 50,000 years
old
The Geologic Time Scale

By examining layers of sedimentary rock and
dating the fossils that are found in the layers,
scientists have been able to put together a
chronology (calendar) of the Earth’s history.
 Draw the geologic time scale on pg 383 and
384. Be sure to include major life forms, major
events, the period, era and time in years on your
time scale. Make your time line to scale.
How do we know how old fossils
are?
Dating – dating of fossils based
on depth
 Relative
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Layers at the surface are younger than those
at deeper layers
Just an estimate, not really accurate
 Radiometric dating – uses radioactive
isotopes in rocks or fossils
 Radioactive isotopes are atoms with unstable
nuclei that decay over time giving off radiation
 Radioactive isotopes form a new element
after they decay
 All radioactive isotopes have a characteristic
decay rate which scientist use
 Half life – time it takes for half of the
radioactive isotope to decay into a new
element – used to measure decay rate
 Example

When a rock forms it contains radioactive
isotopes that decay to half of its original
amount in 1 million years. Today, if a rock
contained equal amounts of the radioactive
isotope and the new substance, the rocks
would be 1 million years old.
 Scientists
use potassium-40, a radioactive
isotope that decays to argon-40, to date
rocks containing very old fossils.

Chemists have determined that potassium-40
decays to half its original amount in 1.3 billion
years
 Scientist
use carbon-14 to date fossils less
than 50,000 years old
History of Life - Review
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What was early earth like?
How do we think oceans were formed? When?
When and where do we think life began?
How do we know about organisms from the past?
How do fossils develop?
How can we determine the age of a fossil?
What four eras is the geologic time scale divided into? What types
of organisms mark each era?
The continents were at one time joined together in a landmass
known as Pangaea. How long ago was that? Refer to your geologic
time scale to understand when that occurred.
Summarize the trend of how organisms developed during Earth’s
history.
The Origin
of Life
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Francesco Redi’s experiment
(1668)
How do the results differ in the two
jars?
What might you conclude from
these results?
Louis Pasteur’s experiment –
finally disproved spontaneous
generation (mid 1800’s)
Which came first, the chicken or
the egg?
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Biologists have accepted the concepts of
biogenesis for more than 100 years
Biogenesis does not answer the question of how
life began on Earth
No one will ever know for certain how life began
on earth. We can only make hypotheses about
conditions on early Earth.
How does chemistry become life??
 Evolution
of complex molecules
Primordial Soup
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Step 1: simple organic molecules must
have formed (molecules that contain
carbon)
Step 2: Simple organic molecules must
have become organized into complex
organic molecules (proteins, carbohydrates,
and nucleic acids)
1930’s Oparin (Russian) – hypothesized
that life began in the oceans

Energy from sun and lightning triggered
chemical reactions to produce small
organic molecules from substances in
the atmosphere
1953 Miller and Urey (American) –
replicated conditions in the lab and
produced amino acids, sugars, and other
molecules
The next step…

How did we get from simple organic
compounds to complex organic compounds?
 In the presence of heat, without oxygen,
amino acids link to from proteins. ATP is
produced in a similar way.
 Sidney Fox found that if you continued to heat
the complex organic compounds protocells
developed.
The Evolution of Cells
 The
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first true cells…
Prokaryotes that evolved from a protocell
Anaerobic (no oxygen present in atmosphere)
Food – organic molecules abundant in oceans
Autotrophs evolved to inhabit harsh conditions
(ie. Archaebacteria)
Endosymbiont Theory
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Eukaryotes probably evolved from prokaryote cells
Theory proposes that eukaryotes evolved through a symbiotic
relationship between prokaryotes
 Evidence:
• Chloroplast and cyanobacteria – resemble eachother
• mitochondria and bacteria – some look similar
• Chloroplast and mitochondria contain DNA that is similar to the DNA
in prokaryotes
• Today, some prokaryotes live in close association with eukaryotes
Life Today
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Could new life originate on Earth today?
 List the specific evidence that supports each
conclusion.
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Life comes from existing life
Life probably originated on Earth through the reaction
of chemicals in Earth’s atmosphere and their further
reaction on Earth’s surface
Cells probably evolved as the chemicals on early
Earth became more organized.