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The History of the Earth
Part I
The Fossil Record
The petrified sap of ancient
trees, the peat bogs, tar
pits, the polar glaciers and
ancient rocks all contain
preserved organisms.
These are known as fossils!
The Fossil Record
The study of these fossils is
called Paleontology!
Paleontologists collect
fossils and infer what past
life forms were like.
 Their body structures
 What they ate
 What ate them
 The environment in
which they lived
They also classify these
fossils.
The Fossil Record
Paleontologists group similar organisms together and
arrange them in the order in which they lived—from oldest
to most recent.
Together, all this information about past life is called the
fossil record.
The fossil record provides evidence about the history of life
on Earth. It also shows how different groups of organisms,
including species, have changed over time.
The Fossil Record
The fossil record indicates
that more than 99% of all
species that have ever lived
on Earth have become
extinct.
Extinction occurs when the
entire species dies out!
The Fossil Record
Most fossils form in sedimentary
rock.
Sedimentary rock is formed when
exposure to rain, heat, wind, and
cold breaks down existing rock
into small particles of sand, silt
and clay.
These particles are carried by
streams and rivers into lakes or
seas, where they eventually settle
to the bottom.
As layers of sediment build up
over time, dead organisms may
also sink to the bottom and
become buried.
The Fossil Record
As layers of sediment build up
over time, dead organisms
may also sink to the bottom
and become buried.
If conditions are right, the
remains may be kept intact
and free from decay.
The weight of layers of
sediment gradually
compresses the lower layers
and, along with chemical
activity, turns them into rock.
Interpreting Fossil Evidence
Paleontologists determine the
age of fossils through two
ways: relative dating and
radioactive dating.
In relative dating, the age of
a fossil is determined by
comparing its placement with
that of fossils in other layers
of rock.
The oldest fossils would be on
the bottom, meanwhile the
youngest would be on top.
Relative Dating
Scientists also use index
fossils to compare the relative
age of fossils.
To be used as an index fossil,
a species must be easily
recognized and must have
existed for a short period but
have had a wide geographic
range.
Relative dating allows
paleontologists to estimate a
fossil’s age compared with
that of other fossils.
Radioactive Dating
Radioactive dating, on the
other hand, allows scientists
to use radioactive decay to
assign absolute ages to
rocks.
Some elements found in
rock are radioactive, and
radioactive elements decay,
or break down, into
nonradioactive elements at
a steady rate, which is
measured in a unit called
half-life.
Radioactive Dating
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
isotopes it contains.
Carbon-14 is the most used
element to date fossils.
Radioactive Dating
Organisms take this in when
they breathe while they are
alive. After the organism dies,
the Carbon-14 begins to
decay to the nonradioactive
Carbon-12.
By comparing the amounts of
Carbon-14 and Carbon 12
within a fossil, scientists can
determine when the organism
lived!
The more C-12, the older the
organism is!
Geologic Time Scale
Paleontologists have divided the
earth’s existence into a Geologic
Time Scale, divided into different
sections, each breaking down into
smaller units.
Scientists created the Geologic
Time Scale by studying rock
layers and index fossils
worldwide.
They placed Earth’s rocks in order
according to relative age.
The major changes in fossilized
animals and plants were used to
divide the scale into sections.
Geologic Time Scale
The Precambrian period is
the oldest period on the
Geologic Time Scale.
Although few multicellular
fossils exist in this time,
this time covers up about
88% of the Earth’s history.
After Precambrian Time, the
basic divisions of the
geologic time scale are eras
and periods.
Eras & Periods
Scientists divide the time
between the Precambrian and
the present into three eras:
the Paleozoic, the Mesozoic,
and the Cenozoic.
The Mesozoic is the “Age of
Dinosaurs,” however
mammals began to evolve in
this era.
The Cenozoic is called the
“Age of Mammals.”
Eras are then divided into
Periods, which are shorter.
Earth’s Early History
Earth is around 4.6 billion
years old.
Its atmosphere probably
contained hydrogen cyanide,
carbon dioxide, carbon
monoxide, nitrogen,
hydrogen sulfide, and water.
About 4 billion years ago,
earth cooled enough for the
first solid rocks to form on its
surface, and about 3.8 billion
years ago, the Earth cooled
enough for water to remain a
liquid.
Earth’s Early History
Because there was no oxygen
to destroy the early
compounds, and because
there was no life to eat the
first protobionts, life was able
to form out of basic
elements—abiogenesis!
The Miller-Urey Experiment
helped to develop this theory.
Although scientists now claim
that the compounds found in
Early Earth were different,
these new tests have
corroborated the idea of
abiogenesis.
Earth’s Early History
Cells similar to modern
bacteria appeared 200-300
million years after the Earth
cooled enough to carry liquid
water.
These were known as
protobionts or proteinoid
microspheres.
They are believed to have
been the vessels which
provided a safe environment
to RNA.
Free Oxygen
Microscopic fossils of singlecelled prokaryotic bacteria
have been found in rocks
more than 3.5 billion years
old.
These had formed in the
absence of oxygen—they
were anaerobic.
Over time, however,
photosynthetic bacteria took
over—around 2.2 billion years
ago, during the Precambrian
time.
These organisms began to
steadily churn out oxygen, a
byproduct of photosynthesis.
Free Oxygen
Oxygen combined with the
iron in the oceans, rusting
the waters!
The iron oxide fell to the
bottom of the sea floor,
forming great bands of iron,
that are the source of most
of the iron mined today!
Without iron, the oceans
changed color from brown
to blue-green.
Free Oxygen
As oxygen began to
accumulate in the
atmosphere, the ozone layer
formed, and the skies turned
the blue color which we know.
However, this oxygen drove
many of the first life forms to
extinction—it was poisonous
to them!
Other life-forms evolved new,
more efficient metabolic
pathways that used oxygen
for respiration.
Origin of Eukaryotic Life
About 2 billion years ago,
prokaryotes—single celled
organisms without internal
membranes, evolved into
eukaryotes.
Eukaryotes are multicellular
and contain internal
membranes, such as a
nucleus.
The Endosymbiotic Theory
Other prokaryotic cells
entered this first ancestral
eukaryotic cell.
Instead of being digested or
hurting it as a parasite, the
prokaryotic cells became part
of the eukaryotic cell.
The Endosymbiotic Theory
states that eukaryotic cells
formed from a symbiosis
among several different
prokaryotic organisms.
The Endosymbiotic Theory
The Endosymbiotic Theory
proposes that eukaryotic cells
arose from living communities
formed by prokaryotic organisms.
This is one reason why
Chloroplasts and Mitochondria
have two membranes!
They also contain their very own
DNA and ribosomes, which are
similar to that of bacteria.
They also reproduce by binary
fission when the containing cell
goes through mitosis.
Sexual Reproduction and
Multicellularity
After eukaryotes evolved,
they began to reproduce
sexually.
This allowed for evolution to
take place at far greater
speeds than before—due to
increase in diversity!
A few hundred million years
later, these life-forms
became multicellular!