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The History of Life
Chapter 12
Fossils and Ancient Life
 A fossil is the preserved remains or evidence of an
ancient organism

Scientists who study fossils are paleontologists
 The fossil record is the grouping of similar organisms
from oldest to most recent

It can provide evidence about the history of life on Earth
and show how different groups of organisms have
changed over time
Extant v/s Extinct
 More than 99 percent of all species that have ever
lived on Earth have become extinct

The term extinct is used to describe a species that no
longer has a living representative

The term extant is used to describe a species that has
living representatives
The Formation of Fossils
 A fossil can be as large and complete as an
entire, perfectly preserved animal, or as small
as a tiny fragment of a jawbone or leaf

For a fossil to form, either the remains of
the organism or some trace of its presence
must be preserved
 The formation of any fossil depends on a
precise combination of conditions

Because of this, the fossil record provides
an incomplete record of the history of life –
for every organism that leaves a fossil,
many more die without leaving a trace
Figure 17-2 Formation of a Fossil
The Formation of Fossils
Section 17-1
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Relative Dating of Fossils
 Sedimentary strata
reveal the relative
ages of fossils:


In relative dating, the
order of rock strata is
used to determine the
relative age of fossils.
Older fossils are found
in deeper layers of
strata, and younger
fossils are found in
superficial layers of
strata.
Radioactive Dating of Fossils
 Radioactive dating is the use of half-lives to
determine the age of a sample


Radioactive elements decay, or break down,
into non-radioactive elements at a steady rate
called a half-life
A half life is the length of time required for half
of the radioactive atoms in a sample to decay
 In radioactive dating, scientists calculate the
age of a sample based on the amount of
remaining radioactive isotopes it contains
Formation of Earth
 Earth’s early atmosphere probably contained
hydrogen cyanide, carbon dioxide, carbon
monoxide, nitrogen, hydrogen sulfide, and
water


About 4 billion years ago, Earth cooled
enough to allow the first solid rocks to form on
its surface
About 3.8 billion years ago, Earth’s surface
cooled enough to allow water to remain liquid
 Could organic molecules evolve under these
conditions?
The First Organic Molecules
Section 17-2
http://bcs.whfreeman.com/thelifewire/content/chp03/0301s.swf
Miller and Urey’s experiments
suggested how mixtures of the
organic compounds necessary
for life could have arisen from
simpler compounds present on a
primitive earth.
Mixture of gases
simulating
atmospheres of
early Earth
Spark simulating
lightning storms
Condensation
chamber
Water
vapor
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Miller and Urey produced
amino acids, which are needed
to make proteins, by passing
sparks through a mixture of
hydrogen, methane, ammonia,
and water.
Cold
water
cools
chamber,
causing
droplets
to form
Liquid containing
amino acids and
other organic
compounds
Origin of Eukaryotic Cells
http://www.sumanasinc.com/webcontent/animations/content/organelles.html
 The endosymbiotic theory, championed by Lynn
Margulis, proposes that eukaryotic cells arose from living
communities formed by prokaryotic organisms
Chloroplast
Aerobic
bacteria
Ancient Prokaryotes
Nuclear
envelope
evolving
Plants and
plantlike
protists
Photosynthetic
bacteria
Mitochondrion
Primitive Photosynthetic
Eukaryote
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Ancient Anaerobic
Prokaryote
Primitive Aerobic
Eukaryote
Animals, fungi, and
non-plantlike protists
Patterns of Macroevolution


Macroevolution refers to the large scale
evolutionary changes that take place over long
periods of time
Six important patterns of macroevolution are:
1.
2.
3.
4.
5.
6.
Mass extinctions
Adaptive radiation
Convergent evolution
Coevolution
Divergent evolution
Punctuated equilibrium
Mass Extinctions
 Extinctions occur all the time
More than 99% of all
species that ever lived are
extinct today
 Usually, extinctions occur at a
constant rate
 Several times, however,
huge numbers of species
have disappeared in
mass extinctions
 Paleontologists think that
most mass extinctions in the
past were caused by multiple
factors
 Asteroids
 Volcanic activity
 Changing position of
continents
 Changing sea levels

Adaptive Radiation
 Adaptive radiation is the RAPID evolution of a group of
organisms following some large-scale disturbance.
 Adaptive radiation typically occurs when a few
organisms make their way to new, often distant
areas or when environmental changes cause
numerous extinctions, opening up ecological niches
for the survivors.
 Fossil evidence indicates that mammals underwent a
dramatic adaptive radiation after the mass extinctions of
dinosaurs 65 million years ago.
Adaptive Radiation in Darwin’s
Finches
Convergent Evolution
 Convergent evolution describes 2 unrelated
species that share similar traits.

These similarities are not due to common ancestry, but rather
a result of similar environmental factors (creates analogous
structures).
Coevolution
 Coevolution describes
the evolution of one
species in response to
new adaptations that
appear in another
species of which the
first shares close
interaction.
Divergent Evolution
 Divergent evolution is the
accumulation of differences
between groups which can lead
to the formation of new species.
 It is usually a result of diffusion of
the same species to different and
isolated environments which
blocks the gene flow among the
distinct populations.
 These barriers to gene flow allow
differentiated fixation of
characteristics through genetic
drift and natural selection.
Patterns of Evolution
Punctuated Equilibrium
 Punctuated equilibrium is a pattern of evolution in
which long stable periods (gradualism) are
interrupted by brief periods of more rapid change

Typically occurs when new niches become available
following a mass extinction
Gradualism v/s Punctuated Equilibrium
Species descended from a common
ancestor gradually diverge more and
more in morphology as they acquire
unique adaptations.
A new species changes most as it
buds from a parent species, and
then changes little for the rest of its
existence.
Patterns of Macroevolution
Section 17-4
Species
that are
Unrelated
form
Related
in
under
under
in
in
Interrelationshiops
Similar
environments
Intense
environmental
pressure
Small
populations
Different
environments
can undergo
can undergo
can undergo
can undergo
can undergo
Coevolution
Convergent
evolution
Extinction
Punctuated
equilibrium
Adaptive
radiation
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