Transcript Historical Evidence for Evolution #2

Historical Evidence for
Evolution
How can we tell that evolution HAS
happened?
The Fossil Record
• For centuries, scientists have believed that
fossils were remains of organisms that had
lived in ancient times
• Many scientists saw a “progression of
forms” in the fossil record, suggesting that
ancient organisms may have changed over
long periods of time
• How do fossils form?
Fossilization
A sea-dwelling Ammonite (or other
organism) dies, and sinks to bottom of
ocean. Scavengers feed on the fleshy
parts until only the calcium-rich shell
remains.
Sediments (silt and sand) begin to
settle and cover up the shell.
Over many thousands of years,
sedimentation covers the shell to a
depth of several feet.
“Permineralization”: as the shell
decays, water infused with minerals
begins to pass through it, replacing the
chemicals in the shell with rock-like
minerals (Calcite, Iron or Silica).
Over millions of years, the shell becomes
completely replaced with these minerals. The
original shape of the shell remains, but it is now
made of “rock”.
“Erosion”: Movements of the earth crust, over
many thousands or millions or years, results
in the ocean floor being raised above sealevel, so that the sedimentary rocks become
exposed to forces of erosion.
“Exposure”: Having lain hidden for millions
of years, the fossil may eventually become
exposed, either by erosion, or a
paleontologist’s rock-hammer!
What Does The Fossil Record Tell
Us About Evolution?
• Sedimentary rocks can be dated by studying the
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decay of radioactive isotopes they contain.
In some cases, successive layers of rocks of
different ages are so well-preserved that
“families” of similar fossils can be traced through
the layers, over several million years of history
This apparent transition suggests a pattern of
development, and allows scientists to infer
changes from one “form” to another and
construct a Phylogenetic Tree…
Trilobite Evolution
Phylogenetic
trees show the
evolutionary
history of related
organisms, based
on their inferred
divergence from
ancestral species
The History Of Life
• The fossil record has
•
allowed scientists to
locate the “first
appearance” of different
organisms within rocks of
a specific age
The pattern of change &
progression suggests that
recent organisms might
be modified forms of
ancient organisms that
previously existed
The Geological Timescale
You will now pretend that you
are paleontologists, and will be
given some paper fossils to
classify according to their age.
After you have arranged them
according to their age, you will
then infer from the data HOW
the fossils have changed over
time, and WHEN these specific
changes occurred.
Is The Evidence Strong?
• What do you think?
• What else might you want to know, in
order to be more certain that organisms
had changed form, and become other
organisms?
• Next, we will look at some additional
forms of evolutionary evidence…
The number of individual facts supporting
the theory is in the hundreds of millions.
• DNA similarities and differences.
• Anatomical similarities and differences.
• Behavioral similarities and differences.
• Physiological similarities and differences.
• Embryological development similarities
and differences.