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Becoming a Fossil (Part 1)
(excerpt from Bill Bryson’s book)
In order to become a fossil, several things must happen. First,
you must die in the right place. Only about 15% of rocks can
preserve fossils, most being sedimentary rocks. Most land
animals don’t die in sediments. They drop in the open and are
eaten or left to rot or weather down to nothing. Therefore, most
fossils form from marine creatures. About 95% of all the fossils
we posses are animals that once lived under water, mostly in
shallow seas. So most likely, the deceased must become buried in
sediment, usually underwater, where it can leave an impression,
like a leaf in wet mud. Another possibility is for the deceased to
be quickly buried underwater and decompose without exposure to
oxygen, allowing the molecules in its bones and hard parts to be
replaced by dissolved minerals, creating a petrified stone version
of itself. Then as the sediments in which the fossil lies are
pressed, folded and pushed about by Earth’s processes, the fossil
must somehow maintain an identifiable shape. Finally, after tens
of millions to hundreds of millions of years of being hidden away,
it must be found and recognized as something worth keeping.
• Provides the best proof of the history of life
showing how extinct species have lead to
today’s species
Formation of Fossils
• Remains of life need to be buried in sediment
under water (sometimes: ice, amber or tar)
• Hard parts (bone& shell) are slowly replaced
by minerals dissolved in water
• Soft parts (skin & feathers) can fossilize if no
O2 is present or if sediment is very fine
• (Demo: Formation of a Fossil Website)
The Story Fossils Tell
What type of rock is it?
What type of fossil is it?
What type of environment did it live in?
Why are these fossils so common even though they are hundreds of
million years old?
• If this fossil was found in Cumberland County, PA, how old is it?
(Use the Geologic Map on next slide)
• What type of rocks are most common in Bucks County?
• What age fossils might you find in Bucks County?
Transitional Species (Common Ancestors)
• Location where two species connect on the “tree of life”
• Scientists search for common ancestors in the fossil record
to show the evolutionary connection between species &
how they change over time.
Transitional Species Example
• Whale Evolution in textbook on pgs 112-113
– (PBS video) -
Becoming a Fossil (Part 2)
(excerpt from Bill Bryson’s book)
It isn’t easy to become part of the fossil record. The fate of
nearly all living things (over 99.9% of them) is to compost
down to nothingness. Even if you make it into the small pool of
organisms, the less than 0.1 % that don’t get devoured, the
chances of being fossilized are very small. Only about one
bone in a billion, it is thought, ever becomes fossilized. If that
is so, it means that the complete fossil legacy of all the
Americans alive today (that’s about 300 million people with
206 bones each) will only be about 50 bones, one quarter of a
complete skeleton. That’s not to say that any of these bones
will actually be found. Fossils are in every sense extremely
rare. Most of what has lived on Earth has left behind no record
at all. It has been estimated that less than 1 species in 10,000
has made it into the fossil record. What we have in the fossil
record is the smallest of samplings of all the life that has existed
on Earth.
Completeness of the Fossil Record?
How do we know how old a fossil is?
TWO Ways to Date Fossils
• Relative age dating
– approximation of dating by
comparing rock layers
• Absolute age dating
– Precision of dating by
measuring radioactive
decay of elements in rock
Relative Age Dating
• Relative age dating follows the Law of Superposition (older rocks
are found under younger rocks)
• Exception to the rule – unconformities (break in the rock record)
• Index Fossil – an organism that lived during a specific period of
time and is abundant.
Unconformities of the Grand Canyon
Absolute Age Dating
• Radioactive Elements: unstable atoms giving off radiation (protons &
neutrons) to become stable.
– Ex: Uranium-238 & Carbon-14
• Radioactive dating: Radioactive decay (going from unstable to stable)
occurs at a constant rate called a half life. Each radioactive element has its own
half life.
• Half life: the amount of time it takes for half the radioactive atoms in a
substance to become stable.
• Examples:
– Uranium-238 has a half-life of 4.5 billion yrs (becomes Lead)
– Carbon-14 has a half-life of 5730 yrs (becomes Nitrogen)
• 100 C14 atoms
50 C14 atoms
? atoms
Example of Radioactive Decay
Absolute Age Dating
II. Comparative Anatomy
• Homologous Structures
• Vestigial Structures
Homologous Structures
• body parts from different organisms that
have the same structures, but different
functions, supporting the idea of a shared
common ancestor
• EX: vertebrate forelimb bones
Homologous OR Not?
Vestigial Structures
• structures that are found in an organism but appear
to serve no function (reduced in size)
• they are remnants of an organism’s evolutionary
– Ex: Whales and snakes have pelvic bones; manatees
“sea cows” have finger nails on their fins
– Humans?
ear muscles
canine teeth
Goose bumps
Tail bone
III. Comparative Embryology
• similarities in the developmental pattern of
organisms exist because of a common ancestor
– vestigial gill slits/pouches
– bony tail
– covered in a fine hair
– Two chambered hearts
Human embryo
Pig embryo
Chicken embryo
Comparative Embryology in Vertebrates
•All vertebrates are
similar in early
stages of
accumulate as
•New development
instructions are
added to old
instructions inherited
from ancestors.
Human Embryology
IV. Comparing Genetics
• An organism’s evolutionary history is held in their DNA
sequence (genetic code)
• If a species changes, their DNA changes
• Genetic testing compares the similarity of DNA between
• The more closely related the species are to each other
the more similarities they share in their DNA
• Ex. Chimpanzees & Humans have over 98% the same DNA
What is DNA?
Cytochrome c
Cytochrome c is a protein organisms need for
respiration. Proteins are made of an amino acid
sequence that is determined from our DNA
sequence. So if the amino acids in the cytochrome
c protein is slightly different between species it
also means their DNA is slightly different.
Virtually every organism uses cytochrome c;
however, each species’ cytochrome c differs
slightly from other species. The differences
among cytochhrome c exist in the amino acid
sequence which were produced by mutations in
the species DNA. These mutations occurred after
the ancestors of the living species diverged.
Therefore, if two species shared common
ancestors until fairly recently, their DNA and
proteins are likely to be more similar.