Transcript Document

The Evidence for Evolution
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The Fossil Record
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Three events of fossilization:
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organism buried in sediment
calcium in material must mineralize
surrounding sediment hardens
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The Fossil Record
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Dating fossils
– radioactive isotopes have decayed more in
older rocks
 steady decay rate allows measurement
History
– arraying fossils according to age provides
evidence of successive evolutionary change
 one of strongest lines of evidence for
evolution
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History of Life
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How are fossils formed?
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unaltered preservation
– like insects or plant parts trapped in amber, a
hardened form of tree sap
permineralization=petrification
– in which rock-like minerals seep in slowly and
replace the original organic tissues with silica,
calcite or pyrite, forming a rock-like fossil
replacement
– An organism's hard parts dissolve and are
replaced by other minerals, like calcite, silica,
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pyrite, or iron
Fossil Formation
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Rivers bring sediment to
the ocean. Sedimentary
rocks containing fossils
form on the ocean floor.
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Over time, additional
strata are added,
containing fossils from
each time period.
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As sea levels change and
the seafloor is pushed
upward, sedimentary rocks
are exposed. Erosion by
rivers reveals strata;
deeper strata contain older
fossils.
Younger stratum with
more recent fossils
Older stratum with
older fossils
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The Fossil Record
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Gaps in the fossil record
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Gaps are expected given the low likelihood
of fossil preservation and recovery.
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On a finer scale, evolutionary changes
within some types of animals is known in
exceptional detail.
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What is a fossil?
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A fossil is remains of ancient life
Fossil is derived from the Latin term fossilis meaning any object extracted from the
ground including minerals and archeological
artifacts
In the 18th century paleontology (the study of
ancient life) was created
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Why are fossils important?
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It enabled scientists to see occurrence of
extinction in different species
It gave Darwin evidence that earth is older than
previously believed and that very slow changes
over a long period of time can add up to
substantial changes in organisms
Can be used to correlate and match up rock
units from different places giving relative ages
It shows evidence of continental drift – the belief
that the continents were once one large
landmass
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Plummer et al 2003
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Fossilization
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Hard parts of organisms are more likely to be
preserve than soft parts.
Soft parts are likely to decay or be consumed by
other organisms
Because of this soft bodied creatures such as
jellyfish may not be fossilized either
Buried organisms are more likely to be fossilized
because it minimizes the decay, consumption, and
destruction of the remains
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Altered remains
Carbonization of a leaf
Unaltered Parts
Impression - Cast and
Mold
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Permineralization –
petrified wood
Trace fossil
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Radiometric Dating and the Half Life Concept
One way to Determine Age of Fossils
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A half life is the time required for 50% or half
of radioactive atoms to decay
C14 is an organic version of carbon found
only in living critters
Therefore, by keeping track of its decay, we
can use its concentration to date fossils.
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Carbon-14 – used to date fossils
Carbon-14 (14C) a rare isotope of carbon, that has 6
protons and 8 neutrons
14C
decays to 14N at a constant rate
Every 5,600 years half the 14C in a sample will emit a
beta particle (electron) and decay to 14N
Thus 5,600 years is called the half life of 14C
Because of 14C’s short half life, it is not useful for
dating million year old fossils, it is only accurate to
about 50,000 years
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Half-lives
256 14C atoms
at time 0
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Half-lives
128 14C and
128 14N atoms
after 5,600
years or
1 half-life
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Half-lives
64 14C and
192 14N atoms
after 11,200
years or
2 half-lives
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Half-lives
32 14C and
224 14N atoms
after 16,800
years or
3 half-lives
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Half-lives
16 14C and
240 14N atoms
after 22,400
years or
4 half-lives
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Half-lives
8 14C and
248 14N atoms
after 28,000
years or
5 half-lives
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Half-lives
4 14C and
252 14N atoms
after 33,600
years or
6 half-lives
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Half-lives
2 14C and
254 14N atoms
after 39,200
years or
7 half-lives
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Carbon-14
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14C
is used to date organic samples like
wood, hair, shells (CaCO3) and other
plant and animal products
Atmospheric 14C is incorporated into
organic molecules by plants during
photosynthesis
Animals that eat the plants get 14C from
the plants they eat
The current ratio of 14C to 12C in the
atmosphere is immeasurably small
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Carbon-14
Sometime in the Ancient Past
Plant absorbs both
C12 and C14 in the
ratio they exist in
the atmosphere
CO2 fixation
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Carbon-14
A Plant Grows Absorbing CO2
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Carbon-14
The Plant Dies
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Carbon-14
It Is Buried
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Carbon-14
Over Time 14C Decays to 14N
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Carbon-14
Over Time 14C Decays to 14N
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Other Isotopic Dating Methods
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14C dating
is not useful for dating
geological strata so other methods have
been developed using isotopes with
much longer half lives
Examples include:
Isotope
Product Half life
Potassium-40
Argon-40
1.4 x 109
Uranium-238
Lead-206
4.5 x 109
Uranium-235
Lead-207
0.7 x 109
Rubidium-87 Strontium-87
48.6 x 109
Thorium-232
14.0 x 109
Lead-208
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Evidence of Evolution
in Body Forms
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Cheirolepis. A fish.
Middle Devonian.
Panderichthys.
Amphibian like
fish. Early late
Devonian.
Acanthostega gunneri.
A water based
amphibian.
Ichthyostega stensioei.
An amphibian.
Late Devonian.
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Fins
to
Limbs
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Pectoral fins from three Late Devonian lobefins
(Eustenopteron, Panderichthys and Sauripteris).
Pectoral limbs from a Late Devonian tetrapod
(Acanthostega) and two Carboniferous tetrapods
(Proterogyrinus and Limnoscelis).
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Modern Walking Fish
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Mudskippers appeared about 22 mya.
Store oxygen in their gills.
Can walk, leap and even climb mangroves!
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Human vs. Chimp Skulls
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Note the enlarged incisors and
canines in chimp.
Frontal lobes in genus
Homo, gradually gets
larger.
Brow ridge gradually
retreats.
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Walking Upright
In apes, the foramen magnum is closer to the
back of the skull
As homonids adapt to walking upright, the
Foramen magnum gradually moves further
back beneath the skull.
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Pelvis, Knee
and Foot
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Feet of early homonids retain some features
suitable for climbing.
Pelvis is clearly bipedal.
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Ontogeny
Recapitulate
s Evolution
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Ontogeny Recapitulates Evolution
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Evidence in Vestigial Structures
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Evidence in Development - Gills
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Evidence in Vestigial Structures
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Evidence in Vestigial Structures
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Evidence in Vestigial Structures
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Evidence in DNA Sequences
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The Evolution of Horses
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Modern members of Equidae (horses, zebras,
donkeys) are descendents of a lineage originating
55 mya.
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The earliest known members were
 small size
 multiple toes
 small, simple teeth
Evolutionary change has not occurred at a
constant rate.
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Evolutionary Changes in Horse Body Size
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The Anatomical Record
Homologous Structures
– structures with different appearances
and functions that all derived from a
common ancestor
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The Molecular Record
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Distantly related organisms are expected to
accumulate a greater number of evolutionary
differences than closely related species.
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