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The History of Life &
The Theory of Evolution
J. Quigley
2008
Early History of Earth
Early earth was inhospitable: very hot with
volcanoes spewing lava and gases
 The gases from the volcanoes helped create the
ancient atmosphere which probably had little
oxygen, a lot of water vapor, carbon dioxide and
nitrogen.

Early Earth
Around 3.9 billion years ago Earth might
have cooled enough for water in the
atmosphere to condense.
 The condensing water might have led to
millions of years of rainstorms and
lightning creating the oceans
 Some scientists believe between 3.9 and
3.5 billion years ago, the first organisms
appeared in the ocean

History in Rocks:
Scientists can’t be certain
how earth formed, but rocks help create hypotheses
Fossils are clues to
the past.
 A fossil is evidence of
an organism that lived
long ago.
 Fossils can form many
different ways

Types of fossils
Trace Fossils- a
marking left by an
animal like a
footprint, trail or
burrow.
 Cast Fossils- when
minerals in rocks fill
the space left by an
organism, they make
a replica of the
organism

Types of fossils
Imprint Fossils- thin
objects such as a leaf fall
into sediment, they leave
an imprint when the
sediment hardens into
rock.
 Mold Fossils- when an
organism is buried in the
sediment and then decays
leaving an empty space

Types of fossils
Petrified Fossils- Minerals
sometimes penetrate and
replace the hard parts of an
organism, producing copies
of them
 Amber Preserved and
Frozen Fossils- At times an
entire organism is trapped
in ice or tree sap that
hardens into amber

Paleontologists- Detectives of the
past
Studying fossils is like solving a mystery.
 Paleontologists study ancient life and use
fossils to try to understand the events that
happened long ago.
 Fossils can explain where the organism
lived, what it ate, size, and even
sometimes behaviors.
 Fossils can also help explain ancient
climate and geography

Sedimentary Rock

For a fossil to form,
organisms usually
have to be buried in a
small particles of
mud, sand or clay
soon after they die.
These particles
compress over time
and harden into
sedimentary rock
(Look at page 381)
Dating Fossils: (not “dating” but
how old are they) ha-ha!

Relative DatingFossils that are found
deeper in the ground
are older than ones
found closer to the
surface. This is the
principal of geological
law. Layering of rocks
tells scientists which
fossils are older than
others

Radiometric DatingScientists use
isotopes in the fossils
to determine the
exact age of the
fossil. Isotopes
“decay” at specific a
speed, so they can
look at the isotopes
and determine the
age of something.
A Trip Through Time
The geologic time scale is divided into four
eras: Precambrian, Paleozoic, Mesozoic
and Cenozoic.
 Each era is divided into smaller amounts
of time called periods
 Time scale is distinguished by the
organisms that lived during the time
period.

Geologic Time Scale
Geologic Time Scale
The fossil record indicates that there were
several occurrences of mass extinction
that fall between the time divisions
 Mass extinction is an event where entire
groups of organisms disappear from the
fossil record almost at once
 The geologic time scale begins with the
formation of earth about 4.6 billion years
ago

Pangaea


About 245 million
years ago the
continents where
joined in a landmass
called Pangaea
About 66 million years
ago most of the
continents had moved
away from each other
and look like they do
now
The Origin of Life
Modern Ideas
Early Ideas
Biologists have accepted
Spontaneous
the concept of
generation: the idea
BIOGENESIS for more
that non-living
material can produce than 100 years…but
biogenesis doesn’t answer
life.
the question “How did life
Cells only can come
begin on earth?”…no one
from other cells, so
will ever know for sure.
this idea was
disproved.
(biogenesis means cells
come from other cells)
Modern Ideas
Scientists believe two things needed to
happen before life could begin
1. Organic molecules (contain carbon) must
have formed
2. The organic molecules formed
carbohydrates, fats, proteins and nucleic
acids
The next step proposed by scientists was
the molecules would make complex organic
compounds called protocells.

Protocells: a large
ordered structure,
enclosed by a
membrane that
carries out some of
life’s activities such as
growth and division
True Cells



The 1st life forms may have been prokaryotes
that evolved from a protocell.
Because the atmosphere didn’t have much
oxygen, it is believed that the 1st organisms
were anaerobic (don’t need oxygen)
The prokaryotes probably ate the complex
molecules that were floating in the ocean
(heterotrophs: do not make their own food)
Archeabacteria
These first cells are thought to be Archeabacteria,
these prokaryotes still survive today in the
Earth’s harshest environments
(volcanoes, deep sea vents, hot springs, salt beds)
First True Cells: Prokaryotes
Photosynthesizing (make nutrients using
energy from the sun) prokaryotes were
probably the next organism to evolve.
 Photosynthesis produces oxygen
 Because of the extra oxygen, organisms
started to become aerobic (using oxygen)

Eukaryotes: Endosymbiont Theory
Prokaryote ingests some aerobic bacteria.
Aerobic bacteria is protected and prokaryote
gets energy
2. Over time the aerobes become mitochondria,
no longer able to live on their own.
3. This is an animal cell
1.
Endosymbiont Theory
Some primitive prokaryotes ingested
cyanobacteria, which contain photosynthetic
pigments
2. The cyanobacteria become chloroplasts and no
longer can live on their own
3. This is a plant cell
1.