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Section 1
The History of Life
Fossil Evidence of Change
Land Environments
 Earth formed about 4.6 billion years ago.
 Gravity pulled the densest elements to the
center of the planet.
 The planet began to cool after about 500
million years.
 A solid crust formed on the surface.
 Water vapor condensed and formed the
oceans.
 Most scientists believe life began in the
oceans
Section 1
The History of Life
Fossil Evidence of Change
Atmosphere
 The gases that likely made up the atmosphere
are those that were expelled by volcanoes.
 Water vapor, Carbon dioxide, Sulfur dioxide,
Carbon monoxide, Hydrogen sulfide,
Hydrogen cyanide, Nitrogen, and Hydrogen
 NO Oxygen (O2)
Section 1
The History of Life
Fossil Evidence of Change
Radiometric Dating
 Uses the decay of
radioactive isotopes
to measure the age of
a rock
 Radioactive dating is
the method used to
determine the age of
the earth and of
fossils.
Section 1
The History of Life
Fossil Evidence of Change
Clues in Rocks
 A fossil is any preserved evidence of an
organism.
 Examples of fossils include bones, shells,
and impressions of dead organisms left in
rocks.
 Most organisms decompose before they have
a chance to become fossilized.
Section 1
The History of Life
Fossil Evidence of Change
Fossil Formation
 Nearly all fossils are formed in sedimentary
rock.
 The sediments build up until they cover the
organism’s remains.
 Minerals replace the organic matter or fill the
empty pore spaces of the organism.
Section 1
The History of Life
Fossil Evidence of Change
Dating fossils
 Relative dating is a
method used to
determine the age of
rocks by comparing
them with those in
other layers.
Section 1
The History of Life
Section 1
The History of Life
Fossil Evidence of Change
 Plate tectonics describes the movement of several
large plates that make up the surface of Earth.
 These plates, some of which contain continents,
move atop a partially molten layer of rock
underneath them.
Section 1
The History of Life
Section 2
The History of Life
The Origin of Life
Origins: Early Ideas
 Spontaneous generation is the idea that life arises
from nonlife.
 Francesco Redi, an Italian scientist, disproved the idea
that flies arose spontaneously from rotting meat.
Section 2
The History of Life
The Origin of Life
 The theory of biogenesis states that only living
organisms can produce other living organisms.
 Louis Pasteur designed an experiment to show that
biogenesis was true even for microorganisms.
Section 2
The History of Life
The Origin of Life
Simple Organic Molecule Formation
 Life may have began at the surface of the
ocean where UV light from the Sun and
lightning were the primary energy sources.
 Many scientists believe life began at the depths
of the ocean where geothermal vents provided
the energy for organic molecule formation.
Section 2
The History of Life
The Origin of Life
The Primordial Soup Model
 In the 1920’s Oparin and Haldane both
suggested that the early Earth’s oceans
contained large amounts of organic
molecules.
 This hypothesis became known as the
primordial soup model.
 In 1953, the primordial soup model was tested
by Miller and Urey.
Section 2
The History of Life
The Origin of Life
 Stanley Miller and
Harold Urey were the
first to show that simple
organic molecules
could be made from
inorganic compounds.
 Later, scientists found
that hydrogen cyanide
could be formed from
even simpler molecules
in simulated early Earth
environments.
Section 2
The History of Life
The Origin of Life
The Bubble Model
 In 1986, the geophysicist Louis Lerman
suggested that the key processes that formed
the chemicals needed for life took place within
bubbles on the ocean’s surface.
Section 2
The History of Life
The Origin of Life
The Bubble Model
Section 2
The History of Life
Section 2
The History of Life
The Origin of Life
Genetic Code
 Some RNA sequences appear to have
changed very little through time.
 Many biologists consider RNA to have been
life’s first coding system.
 Other researchers have proposed that clay
crystals could have provided an initial
template for RNA replication.
Section 2
The History of Life
The Origin of Life
Cellular Evolution
 Scientists hypothesize that the first cells were
prokaryotes.
 Many scientists think that modern prokaryotes
called Archaea are the closest relatives of
Earth’s first cells.
Section 2
The History of Life
The Origin of Life
Photosynthesizing Prokaryotes
 Fossil evidence of cyanobacteria has been
found in rocks as old as 3.5 billion years.
 Cyanobacteria eventually produced enough
oxygen to support the formation of the ozone
layer.
 Ozone blocks the sun’s UV rays
 With oxygen and the ozone layer in the
atmosphere, life could migrate onto land.
Section 2
The History of Life
The Origin of Life
The Endosymbiotic Theory
 The ancestors of eukaryotic cells lived in
association with prokaryotic cells.
 It has been suggested that a large prokaryote
engulfed a smaller one, which continued to live
and provide energy to the larger prokaryote
 The relationship between the cells became
mutually beneficial, and the prokaryotic symbionts
became organelles in eukaryotic cells.
 This theory explains the origin of chloroplasts and
mitochondria.
Section 2
The History of Life
The Origin of Life
Section 2
The History of Life