Objectives

1
Objectives
The Early Earth

• Describe the evidence used to determine the age
  of Earth.

• Understand why scientists theorize that the early
  Earth was hot.

Vocabulary

• zircon
• asteroid
• meteorite

2
Earths Birth
The Early Earth

• For about the first 4 billion years of Earths
  4.6-billion-year existence, most of the
  life-forms that inhabited Earth were unicellular
  organisms.

• In 1996, the announcement that a meteorite from
  Mars might contain microscopic fossils of
  bacteria rekindled scientific interest in the
  search for life elsewhere in the universe.
• It may be possible to identify clues to the
  possible existence of life on other planets
  through rocks from those planets.

3
Earths Birth
The Early Earth

• There is evidence of lifes beginnings on Earth
  in Precambrian rocks.

• Most of Earths history is contained within the
  4 billion years that make up the Precambrian.

4
How old is Earth?
The Early Earth

• We know that Earth must be at least as old as the
  oldest rocks in the crust.

• The age of the oldest rocks on Earth is between
  3.96 to 3.8 billion years.
• Evidence of 4.1- to 4.2-billion-year-old crust
  exists in the mineral zircon that is contained in
  metamorphosed sedimentary rocks in Australia.
• Zircon is a very stable mineral that commonly
  occurs in small amounts in granite.

5
How old is Earth?
The Early Earth

• Meteorites have been radiometrically dated at
  between 4.5 and 4.7 billion years old.

• The oldest rock samples from the Moon are
  approximately 4.6 billion years old.
• Scientists commonly agree that the age of Earth
  is 4.6 billion years.

6
Earths Heat Sources
The Early Earth

• Earth was most likely extremely hot shortly after
  it formed, and there were three likely sources
  of this heat.

• The first source was radioactivity.

• Radioactive isotopes were more abundant during
  the past.
• One product of radioactive decay is energy, which
  generates heat.

7
Earths Heat Sources
The Early Earth

• The second source of Earths heat was the impact
  of asteroids and meteorites.

• Asteroids are metallic or silica-rich objects
  that are 1 km to 950 km in diameter.

• Meteoroids are small asteroids or fragments of
  asteroids.
• Meteorites are meteoroids that fall to Earth.

• Evidence suggests that collisions, which generate
  a tremendous amount of thermal energy, were much
  more common throughout the early solar system
  than they are today.

8
Earths Heat Sources
The Early Earth

• The third source of Earths heat was
  gravitational contraction.

• As a result of meteor bombardment and subsequent
  accumulation of meteorite material on Earth, the
  size of Earth increased.
• The weight of the material caused gravitational
  contraction of the underlying zones, the energy
  of which was converted to thermal energy.
• The new material also caused a blanketing effect,
  which prevented the newly generated heat from
  escaping.

9
Section Assessment
The Early Earth

• 1. How are meteorites evidence of Earths age?

10
Section Assessment
The Early Earth

• 2. What are were the three likely sources of heat
  on Earth shortly after it formed?

11
Section Assessment
The Early Earth

• 3. Identify whether the following statements are
  true or false.

______ The Precambrian represents about half of
Earths existence. ______ The oldest rocks on
Earth are between 3.96 and 3.8 billion
years. ______ Meteorites are asteroids that fall
to Earth. ______ A meteorite could possibly
provide evidence of life on another planet.
12
End of Section 1
13
Objectives
Formation of the Crust and Continents

• Explain the origin of Earths crust.

• Describe the formation of the Archean and
  Proterozoic continents.

Vocabulary

• differentiation
• Precambrian shield
• Canadian Shield
• microcontinent
• Laurentia

14
Formation of the Crust and Continents
Formation of the Crust and Continents

• Early in the formation of Earth, the planet was
  molten, and numerous elements and minerals were
  mixed throughout the magma.

• Over time, the minerals became concentrated in
  specific zones and Earth became layered.
• As the magma reached the surface and cooled,
  landmasses began to form.

15
Formation of the Crust
Formation of the Crust and Continents

• When Earth formed, iron and nickel, which are
  dense elements, concentrated in its core.

• Lava flowing from the interior of Earth
  concentrated the less-dense minerals near the
  surface of Earth over time.
• The denser minerals, which crystallize at higher
  temperatures, concentrated deeper within Earth
  and formed the rocks that make up Earths mantle.

16
Formation of the Crust
Formation of the Crust and Continents

• Differentiation is the process by which a planet
  becomes internally zoned when heavy materials
  sink toward its center and lighter materials
  accumulate near its surface.

17
Formation of the Crust
Formation of the Crust and Continents
18
Formation of the Crust
Formation of the Crust and Continents

• Earths earliest crust most likely formed as a
  result of the cooling of the uppermost mantle and
  was similar to basalt.

• As sediment-covered slabs of the crust were
  recycled into the mantle at subduction zones, the
  slabs partly melted and generated magmas with
  different mineral compositions.
• These magmas crystallized to form the first
  granitic continental crust, which was rich in
  feldspar, quartz, and mica.

19
Formation of the Crust
Formation of the Crust and Continents

• The formation of the majority of crustal rocks
  was completed by about 2.5 billion years ago.

• As less-dense material has a tendency to float on
  more-dense material, continental crust floats
  on top of the mantle below it.
• Basaltic crust is more dense than granitic crust,
  and therefore, it does not float as high on the
  mantle.

20
Formation of the Crust
Formation of the Crust and Continents
21
The Cores of the Continents
Formation of the Crust and Continents

• A Precambrian shield is a core of Archean and
  Proterozoic rock that forms the core of each
  continent.

• Buried and exposed parts of a shield together
  compose the craton, which is the stable part of a
  continent.
• The Canadian Shield is the name for the
  Precambrian shield in North America because much
  of it is exposed in Canada.

22
The Cores of the Continents
Formation of the Crust and Continents
23
Growth of Continents
Formation of the Crust and Continents

• Microcontinents, which were small pieces of
  continental crust that formed during the Archean,
  began to collide as a result of plate tectonics
  early during the Proterozoic.

• At each of these collision sites, the Archean
  microcontinents were sutured or fused together
  at orogens.
• These orogens are belts of rocks that were
  deformed by the immense energy of the colliding
  continents.

24
Growth of Continents
Formation of the Crust and Continents

• Laurentia, the ancient continent which was
  assembled 1.8 billion years ago, would become
  the core of modern-day North America.

25
Growth of Continents
Formation of the Crust and Continents

• Near the end of the Early Proterozoic, between
  1.8 and 1.6 billion years ago, volcanic island
  arcs collided with the southern margin of
  Laurentia.

• The final phase of Proterozoic growth of
  Laurentia, the Grenville Orogeny, occurred
  between 1.2 billion and 900 million years ago.
• By the end of the Proterozoic, nearly 75 percent
  of present-day North America had formed.

26
Growth of Continents
Formation of the Crust and Continents

• By the end of the Proterozoic, all of the major
  masses of continental lithosphere had formed.

• As the lithospheric plates moved around, they
  periodically collided and sutured together to
  form Rodinia, the first supercontinent.
• Rodinia began to break apart at the end of the
  Proterozoic and continued to do so during the
  Early Phanerozoic.

27
Growth of Continents
Formation of the Crust and Continents
28
Section Assessment
Formation of the Crust and Continents

• 1. Match the following terms with their
  definitions.
• ___ differentiation
• ___ Precambrian shield
• ___ microcontinent
• ___ Laurentia

A. the process by which a planet becomes
internally zoned B. a core of Archean and
Proterozoic rock that forms the core of
continents. C. the ancient continent that formed
the core of modern-day North America D. small
pieces of continental crust that formed during
the Archean
29
Section Assessment
Formation of the Crust and Continents

• 2. What is the Canadian Shield?

30
Section Assessment
Formation of the Crust and Continents

• 3. Why do the rocks of the earliest crust no
  longer exist?

31
End of Section 2
32
Objectives
Formation of the Atmosphere and Oceans

• Describe the formation of Earths atmosphere and
  oceans.

• Identify the origin of oxygen in the atmosphere.
• Explain the evidence that oxygen existed in the
  atmosphere during the Proterozoic.

Vocabulary

• cyanobacteria
• stromatolite
• banded iron formation
• red bed

33
Formation of the Atmosphere and Oceans
Formation of the Atmosphere and Oceans

• Earths early atmosphere was nothing like what it
  is today.

• The oxygen that early forms of algae produced
  through the process of photosynthesis affected
  the development of life on Earth in two very
  important ways.

• It changed the composition of the atmosphere and
  thus made life possible for oxygen-breathing
  animals.
• It produced the ozone layer that filters
  ultraviolet (UV) radiation.

34
The Precambrian Atmosphere
Formation of the Atmosphere and Oceans

• Hydrogen and helium probably dominated Earths
  earliest atmosphere but probably escaped into
  space due to their small masses.

• Gases that have greater masses, such as carbon
  dioxide and nitrogen, cannot escape Earths
  gravity.
• Considerable volcanic activity during the Early
  Precambrian released tremendous amounts of gases
  into the atmosphere through the process of
  outgassing.

35
The Precambrian Atmosphere
Formation of the Atmosphere and Oceans

• The most abundant gases vented from volcanoes are
  water vapor (H2O), carbon dioxide (CO2), nitrogen
  (N2), and carbon monoxide (CO).

• Many geologists hypothesize that outgassing
  formed Earths early atmosphere.
• In addition, the early atmosphere most likely
  contained methane (CH4) and ammonia (NH3).
• Argon (Ar) also began to accumulate during the
  Early Precambrian.

36
Oxygen in the Atmosphere
Formation of the Atmosphere and Oceans

• There was no oxygen in the atmosphere during the
  Precambrian.

The oldest known fossils, which are about 3.5
billion years old, are the remains of tiny,
threadlike chlorophyll-bearing filaments of
cyanobacteria.

• Ancient cyanobacteria used photosynthesis to
  produce the nutrients they needed to survive,
  giving off oxygen as a waste product.

37
Oxygen in the Atmosphere
Formation of the Atmosphere and Oceans

• Oxygen Producers

• The abundance of cyanobacteria increased
  throughout the Archean until they became truly
  abundant during the Proterozoic.
• Stromatolites, which are large mats and mounds of
  billions of cyanobacteria, dominated the shallow
  oceans of the Proterozoic.

38
Oxygen in the Atmosphere
Formation of the Atmosphere and Oceans

• Evidence in the Rocks

• Iron oxides are identified by their red color and
  provide undeniable evidence of free oxygen in the
  atmosphere.
• Evidence indicates that there was little or no
  free oxygen in the atmosphere throughout most of
  the Archean.
• Near the end of the Archean and by the beginning
  of the Proterozoic, photosynthesizing
  stromatolites in shallow marine water increased
  oxygen levels in localized areas, which caused
  banded iron formations to form.

39
Oxygen in the Atmosphere
Formation of the Atmosphere and Oceans

• Evidence in the Rocks

• Banded iron formations are deposits which consist
  of alternating bands of chert and iron oxides.

• Red beds are sedimentary rocks that are younger
  than 1.8 billion years and rusty red in color.

• The presence of red beds in rocks that are
  Proterozoic and younger is strong evidence that
  the atmosphere by this time contained free
  oxygen.

40
Importance of Oxygen
Formation of the Atmosphere and Oceans

• Oxygen is important because most animals require
  it for respiration and it provides protection
  against UV radiation from the Sun.

• Earth is naturally protected from this radiation
  by ozone (O3) molecules that are present in the
  lower part of Earths upper atmosphere.
• Oxygen in Earths atmosphere that was produced
  mainly through photosynthesis also contributes to
  the ozone layer.
• Nearly all the oxygen that is present was
  released into the atmosphere by photosynthesis.

41
Formation of the Oceans
Formation of the Atmosphere and Oceans

• Oceans are thought to have originated largely
  from the same process of outgassing that formed
  the atmosphere.

• As the early atmosphere and the surface of Earth
  cooled, the water vapor condensed to form liquid
  water.
• During the Archean, tremendous amounts of rain
  slowly filled the low-lying, basalt-floored
  basins, thus forming the oceans.

42
Formation of the Oceans
Formation of the Atmosphere and Oceans

• Dissolved minerals made the oceans of the
  Precambrian salty just as they make the oceans
  salty today.

• A recent hypothesis suggests that some of Earths
  water may have come from the bombardment of
  microcomets, or small comets made of frozen gas
  and water.

43
Oxygen in the Atmosphere
Formation of the Atmosphere and Oceans

• Oxygen Causes Change

• The Precambrian began with an oxygen-free
  atmosphere and simple life-forms.
• This oxygen added by cyanobacteria not only
  enabled new life-forms to evolve, but it also
  protected Earths surface from the Suns UV rays.
  
• Oceans formed from abundant water vapor in the
  atmosphere and possibly from outer space.
• Earth was then a hospitable place for new
  life-forms to inhabit.

44
Section Assessment
Formation of the Atmosphere and Oceans

• 1. Match the following terms with their
  definitions.
• ___ cyanobacteria
• ___ stromatolite
• ___ banded iron formation
• ___ red bed

A. sedimentary rocks that are younger than 1.8
billion years and are rusty red in color B. large
mats and mounds of cynobacteria C. deposits that
consist of alternating bands of chert and iron
oxides D. chlorophyll containing bacteria that
may be responsible for the addition of oxygen to
Earths early atmosphere
45
Section Assessment
Formation of the Atmosphere and Oceans

• 2. Why is Earths current atmosphere rich in
  carbon dioxide and nitrogen?

46
Section Assessment
Formation of the Atmosphere and Oceans

• 3. Identify whether the following statements are
  true or false.

______ Stromatolites currently exist on
Earth. ______ Free oxygen is released during
outgassing. ______ Around half of the oxygen that
we breathe today was released into the atmosphere
through photosynthesis. ______ There was little
free oxygen in the atmosphere during the Archean.
47
End of Section 3
48
Objectives
Early Life on Earth

• Describe the experimental evidence of how life
  developed on Earth.

• Distinguish between prokaryotes and eukaryotes.
• Identify when the first multicellular animals
  appeared in geologic time.

Vocabulary

• amino acids
• hydrothermal vent
• prokaryote

• eukaryote
• Varangian Glaciation
• Ediacara fauna

49
Origin of Life on Earth
Early Life on Earth

• Fossil evidence indicates that life existed on
  Earth about 3.5 billion years ago.

• Earth probably could not have supported life
  until about 3.9 billion years ago because
  meteorites were constantly striking its surface.
• This places the origin of life somewhere between
  3.9 and 3.5 billion years ago.

50
Origin of Life on Earth
Early Life on Earth

• Experimental Evidence

• Molecular biologists in the 1920s also suggested
  that an atmosphere containing abundant ammonia
  and methane but lacking free oxygen would be an
  ideal setting for the primordial soup in which
  life may have begun.
• Stanley Miller and Harold Urey set up an
  apparatus that contained a chamber filled with
  hydrogen, methane, and ammonia to simulate the
  early atmosphere.
• Sparks from tungsten electrodes simulated
  lightning in the atmosphere.

51
Origin of Life on Earth
Early Life on Earth

• Experimental Evidence

• Their atmospheric chamber was connected to a
  lower chamber that was designed to catch any
  particles that condensed in the atmospheric
  chamber.

• Only one week after the start of the experiment,
  the lower chamber contained organic molecules
  such as cyanide (CN), formaldehyde (H2CO), and
  four different amino acids.
• Amino acids are the building blocks of proteins,
  the basic substances from which life is built.

52
Origin of Life on Earth
Early Life on Earth

• Experimental Evidence

• Continued experiments showed that 13 of the 20
  amino acids known to occur in living things could
  be formed using the Miller-Urey method.

• Further experiments demonstrated that heat,
  cyanide, and certain clay minerals could cause
  amino acids to join together in chains like
  proteins.
• Miller and Urey demonstrated that however life
  first formed, the basic building blocks of life
  were most likely present on Earth during the
  Archean.

53
Origin of Life on Earth
Early Life on Earth

• The Role of RNA

• The nucleic acids RNA and DNA are the basic
  requirements for reproduction, an essential
  characteristic of life.
• In modern organisms, DNA carries the instructions
  necessary for cells in all living things to
  function.
• RNA ribozymes, unlike DNA, can replicate without
  the aid of enzymes, and may have been the first
  replicating molecules on Earth.
• An RNA-based world may have been intermediate
  between an inorganic world and the DNA-based
  organic world that followed.

54
Origin of Life on Earth
Early Life on Earth

• Hydrothermal Vents and the Beginnings of Life

• Life on Earth may have originated deep in the
  ocean, near active volcanic seafloor rifts.
• Hydrothermal vents are the openings where hot
  water rises and is expelled from the ocean floor.

• All of the energy and nutrients necessary for the
  origin of life are present at these deep-sea
  hydrothermal vents.
• Some scientists hypothesize that during the
  Archean, near hydrothermal vents, amino acids
  joined together on the surfaces of clay minerals
  to form proteins.

55
Proterozoic Life
Early Life on Earth

• The only evidence of life-forms that existed
  before the Proterozoic is the fossilized remains
  of unicellular organisms called prokaryotes.

• A prokaryote is an organism that is composed of a
  single cell, which does not contain a nucleus and
  is the simplest kind of cell.

• A eukaryote is an organism that is composed of a
  cell or cells that contain a nucleus.

56
Proterozoic Life
Early Life on Earth

• The Varangian Glaciation was a widespread
  glaciation event that occurred between 800 and
  700 million years ago that played a critical role
  in the extinction of many members of a group of
  possible eukaryotes, the acritarchs.

• Shortly after the ice retreated toward the poles,
  700 million years ago, multicellular organisms
  first appeared in the fossil record.

57
Ediacara Fossils
Early Life on Earth

• Fossils collectively referred to as the Ediacara
  fauna are the impressions of soft-bodied
  organisms that were discovered in Late
  Proterozoic rocks in the Ediacara Hills of
  southern Australia.

58
Ediacara Fossils
Early Life on Earth

• It is generally agreed that these fossils
  represent animals that were composed of different
  types of eukaryotic cells.

• Scientists are unsure, however, whether the
  Ediacara fauna are relatives of modern animal
  groups or whether they were completely different
  types of organisms.
• The Ediacara fauna seem to provide fossil
  evidence of an ancestral stock of complex
  Proterozoic animals.

59
Ediacara Fossils
Early Life on Earth

• Some scientists consider the similarity in shape
  to animals in other phyla coincidental and that
  the Ediacara fauna represents a virtual dead end.

• Ediacara fossils have been found in all parts of
  the world.
• These organisms seem to have flourished between
  670 and 570 million years ago until an apparent
  mass extinction.

60
Section Assessment
Early Life on Earth

• 1. Match the following terms with their
  definitions.
• ___ prokaryotes
• ___ eukaryotes
• ___ amino acids
• ___ Ediacara fauna

A. organisms that are composed of cells that
contain a nucleus B. an organism that is composed
of a single cell, which does not contain a
nucleus C. fossils of soft-bodied organisms that
were discovered in Late Proterozoic rocks D. the
building blocks of proteins
61
Section Assessment
Early Life on Earth

• 2. When did life most likely develop on Earth?

62
Section Assessment
Early Life on Earth

• 3. Identify whether the following statements are
  true or false.

______ RNA can be easily synthesized under
conditions that likely existed at the surface of
the Archean Earth. ______ Ediacara fauna may
not represent an ancestral stock of any modern
group. ______ Life is currently being synthesized
at hydrothermal vents. ______ Cyanobacteria are
examples of prokaryotes.
63
End of Section 4
64
Section 22.1 Main Ideas
Section 22.1 Study Guide

• Geologists have used radiometric dating to show
  that Earth must be at least 4.2 billion years
  old.

• Because the solar system formed all at the same
  time, Moon rocks and meteorites that are
  approximately 4.6 billion years old suggest that
  Earth is also 4.6 billion years old.
• The early Earth was a very hot place because of
  abundant radioactive isotopes, bombardment by
  meteorites, and gravitational contraction.

65
Section 22.2 Main Ideas
Section 22.2 Study Guide

• Earths early crust formed by the cooling of the
  uppermost mantle. This early crust weathered and
  formed sediments.

• Sediment-covered slabs of this early crust were
  subducted and generated magmas that contained
  granitic minerals.
• During the Archean, microcontinents collided with
  one another throughout the Proterozoic and formed
  the cores of the continents. By the end of the
  Proterozoic, the first supercontinent, Rodinia,
  had formed.

66
Section 22.3 Main Ideas
Section 22.3 Study Guide

• Earths early atmosphere and the oceans formed
  mainly by the process of outgassing.

• Nearly all of the oxygen in the atmosphere is a
  result of photosynthesis.
• Certain minerals oxidize, or rust, in the
  presence of free oxygen. Proterozoic red beds are
  sedimentary rock deposits that contain oxidized
  iron. They are the evidence that there was free
  oxygen in the atmosphere during the Proterozoic.

67
Section 22.4 Main Ideas
Section 22.4 Study Guide

• All the ingredients were present on the early
  Earth to form proteins, the building blocks of
  life. Amino acids, the molecules that make up
  proteins, were likely abundant on the surface of
  the early Earth.

• Prokaryotic cells are generallly small and
  contain no nuclei. Eukaryotic cells contain
  nuclei and are generally larger and more complex
  than prokaryotic cells.
• The first evidence of multicellular animals is
  fossils of 2.1 billion year old eukaryotic algae.

68
Short Answer
Chapter Assessment

• 6. What is the process of differentiation?

69
Short Answer
Chapter Assessment

• 7. When and where did the Ediacara organisms
  flourish?

70
True or False
Chapter Assessment

• 8. Identify whether the following statements are
  true or false.
• ______ During the Varangian Glaciation, the
  glacial ice advanced almost to the equator.
• ______ Asteroids are usually smaller than 1 km
  across.
• ______ Processes which modify a system are
  known as feedback.
• ______ DNA can replicate without the help of
  enzymes.
• ______ By the end of the Proterozoic, all of
  the major masses of continental lithosphere
  had formed.

71
Chapter 22 Images
Image Bank
72
Chapter 22 Images
Image Bank