CHAPTER 19 Earth’s Evolution through Geologic Time
490
C H A P T E R
N I N E T E E N
Earth’ s Evolution through Geologic Time
In Review
The history of Earth began about 13.7 billion years ago when
the first elements were created during the Big Bang. It was from
this material, plus other elements ejected into interstellar space
by now-defunct stars, that Earth, along with the rest of the solar
system, formed. As material collected, high velocity impacts of
chunks of matter called planetesimals and the decay of radioactive
elements caused the temperature of our planet to steadily
increase. Iron and nickel melted and sank to form the metallic
core, while rocky material rose to form the mantle and Earth’ s
initial crust.
Earth’ s primitive atmosphere, which consisted mostly of water
vapor and carbon dioxide, formed by a process called outgassing,
which resembles the steam eruptions of modern volcanoes.
About 3.5 billion years ago, photosynthesizing bacteria began
to release oxygen, first into the oceans and then into the atmosphere. This began the evolution of our modern atmosphere. The
oceans formed early in Earth’ s history as water vapor condensed
to form clouds, and torrential rains filled low-lying areas. The
salinity in seawater came from volcanic outgassing and from
elements weathered and eroded from Earth’ s primitive crust.
The Precambrian, which is divided into the Archean and
Proterozoic eons, spans nearly 90 percent of Earth’ s history, beginning with the formation of Earth about 4.6 billion years ago and
ending approximately 542 million years ago. During this time,
much of Earth’ s stable continental crust was created through a
multistage process. First, partial melting of the mantle generated
magma that rose to form volcanic island arcs and oceanic
plateaus. These thin crustal fragments collided and accreted to
form larger crustal provinces, which in turn assembled into
larger blocks called cratons. Cratons, which form the core of
modern continents, were created mainly during the Precambrian.
Supercontinents are large landmasses that consist of all, or
nearly all, existing continents. Pangaea was the most recent
supercontinent, but other massive continents including an even
larger one, Rodinia, preceded it. The splitting and reassembling
of supercontinents have generated most of Earth’ s major mountain belts. In addition, the movements of these crustal blocks
have profoundly affected Earth’ s climate and caused sea level to
rise and fall.
The time span following the close of the Precambrian, called
the Phanerozoic eon, encompasses 542 million years and is
divided into three eras: Paleozoic, Mesozoic, and Cenozoic. The
Paleozoic era was dominated by continental collisions as the
supercontinent of Pangaea assembled—forming the Caledonian,
Appalachian, and Ural Mountains. Early in the Mesozoic,
much of the land was above sea level. However, by the middle
Mesozoic, seas invaded western North America. As Pangaea
began to break up, the westward-moving North American plate
began to override the Pacific plate, causing crustal deformation
along the entire western margin of North America. Owing to
their different relations with plate boundaries, the eastern and
western margins of the continent experienced contrasting events.
The stable eastern margin was the site of abundant sedimentation
as isostatic adjustment raised the modern Appalachians, causing
streams to erode with renewed vigor and deposit their sediment
along the continental margin. In the West, the Laramide Orogeny
(responsible for building the Rocky Mountains) was coming to
an end, the Basin and Range Province was forming, and
volcanic activity was extensive.
The first known organisms were single-celled bacteria,
prokaryotes, which lack a nucleus. One group of these organisms,
called cyanobacteria, used solar energy to synthesize organic
compounds (sugars). For the first time, organisms had the ability
to produce their own food. Fossil evidence for the existence of
these bacteria includes layered mounds of calcium carbonate
called stromatolites.
The beginning of the Paleozoic is marked by the appearance
of the first life-forms with hard parts such as shells. Therefore,
abundant fossils occur, and a far more detailed record of
Paleozoic events can be constructed. Life in the early Paleozoic
was restricted to the seas and consisted of several invertebrate
groups, including trilobites, cephalopods, sponges, and corals.
During the Paleozoic, organisms diversified dramatically. Insects
and plants moved onto land, and lobe-finned fishes that adapted
to land became the first amphibians. By the Pennsylvanian
period, large tropical swamps, which became the major coal
deposits of today, extended across North America, Europe, and
Siberia. At the close of the Paleozoic, a mass extinction destroyed
70 percent of all vertebrate species on land and 90 percent of
all marine organisms.
The Mesozoic era, literally the era of middle life, is often
called the “Age of Reptiles.” Organisms that survived the extinction at the end of the Paleozoic began to diversify in spectacular
ways. Gymnosperms (cycads, conifers, and ginkgoes) became
the dominant trees of the Mesozoic because they could adapt to
the drier climates. Reptiles became the dominant land animals.
The most awesome of the Mesozoic reptiles were the dinosaurs.
At the close of the Mesozoic, many large reptiles, including
the dinosaurs, became extinct.
The Cenozoic is often called the “Age of Mammals” because
these animals replaced the reptiles as the dominant vertebrate
life-forms on land. Two groups of mammals, the marsupials
and the placentals, evolved and expanded during this era. One
tendency was for some mammal groups to become very large.
However, a wave of late Pleistocene extinctions rapidly
eliminated these animals from the landscape. Some scientists
490
C H A P T E R
N I N E T E E N
Earth’ s Evolution through Geologic Time
In Review
The history of Earth began about 13.7 billion years ago when
the first elements were created during the Big Bang. It was from
this material, plus other elements ejected into interstellar space
by now-defunct stars, that Earth, along with the rest of the solar
system, formed. As material collected, high velocity impacts of
chunks of matter called planetesimals and the decay of radioactive
elements caused the temperature of our planet to steadily
increase. Iron and nickel melted and sank to form the metallic
core, while rocky material rose to form the mantle and Earth’ s
initial crust.
Earth’ s primitive atmosphere, which consisted mostly of water
vapor and carbon dioxide, formed by a process called outgassing,
which resembles the steam eruptions of modern volcanoes.
About 3.5 billion years ago, photosynthesizing bacteria began
to release oxygen, first into the oceans and then into the atmosphere. This began the evolution of our modern atmosphere. The
oceans formed early in Earth’ s history as water vapor condensed
to form clouds, and torrential rains filled low-lying areas. The
salinity in seawater came from volcanic outgassing and from
elements weathered and eroded from Earth’ s primitive crust.
The Precambrian, which is divided into the Archean and
Proterozoic eons, spans nearly 90 percent of Earth’ s history, beginning with the formation of Earth about 4.6 billion years ago and
ending approximately 542 million years ago. During this time,
much of Earth’ s stable continental crust was created through a
multistage process. First, partial melting of the mantle generated
magma that rose to form volcanic island arcs and oceanic
plateaus. These thin crustal fragments collided and accreted to
form larger crustal provinces, which in turn assembled into
larger blocks called cratons. Cratons, which form the core of
modern continents, were created mainly during the Precambrian.
Supercontinents are large landmasses that consist of all, or
nearly all, existing continents. Pangaea was the most recent
supercontinent, but other massive continents including an even
larger one, Rodinia, preceded it. The splitting and reassembling
of supercontinents have generated most of Earth’ s major mountain belts. In addition, the movements of these crustal blocks
have profoundly affected Earth’ s climate and caused sea level to
rise and fall.
The time span following the close of the Precambrian, called
the Phanerozoic eon, encompasses 542 million years and is
divided into three eras: Paleozoic, Mesozoic, and Cenozoic. The
Paleozoic era was dominated by continental collisions as the
supercontinent of Pangaea assembled—forming the Caledonian,
Appalachian, and Ural Mountains. Early in the Mesozoic,
much of the land was above sea level. However, by the middle
Mesozoic, seas invaded western North America. As Pangaea
began to break up, the westward-moving North American plate
began to override the Pacific plate, causing crustal deformation
along the entire western margin of North America. Owing to
their different relations with plate boundaries, the eastern and
western margins of the continent experienced contrasting events.
The stable eastern margin was the site of abundant sedimentation
as isostatic adjustment raised the modern Appalachians, causing
streams to erode with renewed vigor and deposit their sediment
along the continental margin. In the West, the Laramide Orogeny
(responsible for building the Rocky Mountains) was coming to
an end, the Basin and Range Province was forming, and
volcanic activity was extensive.
The first known organisms were single-celled bacteria,
prokaryotes, which lack a nucleus. One group of these organisms,
called cyanobacteria, used solar energy to synthesize organic
compounds (sugars). For the first time, organisms had the ability
to produce their own food. Fossil evidence for the existence of
these bacteria includes layered mounds of calcium carbonate
called stromatolites.
The beginning of the Paleozoic is marked by the appearance
of the first life-forms with hard parts such as shells. Therefore,
abundant fossils occur, and a far more detailed record of
Paleozoic events can be constructed. Life in the early Paleozoic
was restricted to the seas and consisted of several invertebrate
groups, including trilobites, cephalopods, sponges, and corals.
During the Paleozoic, organisms diversified dramatically. Insects
and plants moved onto land, and lobe-finned fishes that adapted
to land became the first amphibians. By the Pennsylvanian
period, large tropical swamps, which became the major coal
deposits of today, extended across North America, Europe, and
Siberia. At the close of the Paleozoic, a mass extinction destroyed
70 percent of all vertebrate species on land and 90 percent of
all marine organisms.
The Mesozoic era, literally the era of middle life, is often
called the “Age of Reptiles.” Organisms that survived the extinction at the end of the Paleozoic began to diversify in spectacular
ways. Gymnosperms (cycads, conifers, and ginkgoes) became
the dominant trees of the Mesozoic because they could adapt to
the drier climates. Reptiles became the dominant land animals.
The most awesome of the Mesozoic reptiles were the dinosaurs.
At the close of the Mesozoic, many large reptiles, including
the dinosaurs, became extinct.
The Cenozoic is often called the “Age of Mammals” because
these animals replaced the reptiles as the dominant vertebrate
life-forms on land. Two groups of mammals, the marsupials
and the placentals, evolved and expanded during this era. One
tendency was for some mammal groups to become very large.
However, a wave of late Pleistocene extinctions rapidly
eliminated these animals from the landscape. Some scientists