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Formation of Earth
 Geologic evidence shows that Earth
is about 4.6 billion years old
 the
early atmosphere was composed
of some of the following gases:
water vapor, hydrogen, methane,
and ammonia – What’s missing?
Oxygen gas
 3.8
bya thunderstorms produced
enough rain to create oceans
 organic
molecules cannot
spontaneously form today due to
the presence of oxygen
 Stanley
Miller and Harold Urey
(1950) tested a theory that organic
molecules could form under early
Earth conditions

The experiment
produced amino
acids, sugars,
and nucleotides.
Spontaneous generation
◦ Scientists believed that life could
be produced from nonliving
matter
(dirty rags  mice)
1.
2.
Francesco Redi (1668) designed a
controlled experiment to disprove
spontaneous generation
Louis Pasteur (mid
1800’s) designed a
controlled experiment
to show that
microorganisms cannot
spontaneously arise
◦ Put broth in 2 flasks
with S-shaped necks.
3.
4.
Scientists now believe in
biogenesis, living organisms come
only from other living organisms
 The
first cells were
probably heterotrophic,
anaerobic, & prokaryotic
 Next to come along were
the chemosynthetic
bacteria
 Photosynthesizing
bacteria evolved next,
producing oxygen
 after
oxygen builds up in
the atmosphere, aerobic
bacteria evolved
 oxygen in the
atmosphere was
converted to ozone by
lightning giving the
Earth the ozone layer
 the
ozone layer protects from UV
radiation and allowed more complex
organisms to evolve
 eukaryotic cells may have come
from bacteria living inside other
bacteria (endosymbiont theory)
Fossil Record
 fossils are any evidence of a dead
organism
 99% of all life forms that have ever
existed on earth are now extinct
 fossils are usually found in
sedimentary rock
Fossil Age
 relative dating is used only to
describe whether something is
younger or older than some point of
reference
Fossil Age
 radiometric dating uses the halflife of certain elements to
determine the absolute age of the
fossil
 the
geologic time scale is a
calendar based on the appearance
of different types of organisms
that have appeared during Earth’s
history
◦ is divided into 4 eras
◦ a mass extinction occurred at the end
of each era
4. Cenozoic Era
3. Mesozoic Era
2. Paleozoic Era
1. Precambrian Era