EARTH HISTORY:
A Brief Summary
Earth History:
A Brief Summary opens
with a presentation of the origin of Earth, beginning about five billion years
ago with an enormous cloud of interstellar material. Also examined are Earth’s primitive
atmosphere and how its composition changed in response to the evolution of
life. Earth’s history is systematically
detailed with a description of the geologic and biologic events that have
occurred from the Precambrian, through the Paleozoic, Mesozoic, and Cenozoic
Eras.
Learning
Objectives
After reading,
studying, and discussing the chapter, students
should be able to:
1. Describe the origin of Earth and the solar system in general.
2. Discuss how Earth’s atmosphere has evolved and changed through
time.
3. List the principal geologic and biologic events for each era of
geologic time.
Chapter
Summary
The nebular
hypothesis describes the formation of the solar system. The planets and sun began forming about 5
billion years ago from a large cloud of dust and gases composed of hydrogen and
helium, with only a small percentage of all the other heavier elements. As the cloud contracted, it began to rotate
and assume a disk shape. Material that
was gravitationally pulled toward the center became the protosun. Within the rotating disk, small centers,
called protoplanets, swept up more and more of the cloud’s debris. Due to their high temperatures and weak
gravitational fields, the inner planets (Mercury, Venus, Earth, and Mars) were
unable to accumulated and retain many of the lighter components (hydrogen,
helium, ammonia, methane, and water).
However, because of the very cold temperatures existing far from the
sun, the fragments from which the large outer planets (Jupiter, Saturn, Uranus,
and Neptune) formed consisted of huge amounts of hydrogen and other light
materials. These gaseous substances
account for the comparatively large sizes and low densities of the outer
planets.
The decay of
radioactive elements and heat released by colliding particles aided the melting
of Earth’s interior, allowing the denser elements, principally iron and nickel,
to sink to its center. As a result of
this differentiation, Earth’s interior consists of shells or spheres of
materials, each having rather distinct properties.
Earth’s primitive
atmosphere probably consisted of the gases, water vapor, carbon dioxide,
nitrogen, and several trace gases that were released in volcanic emissions, a
process called outgassing. The
first life forms on Earth, probably anaerobic bacteria, did not need
oxygen. As life evolved, plants, through
the process of photosynthesis, used carbon dioxide and water and
released oxygen into the atmosphere.
Once the available iron on Earth was oxidized (combined with oxygen),
substantial quantities of oxygen accumulated in the atmosphere. About 4 billion years into Earth’s existence,
the fossil record reveals abundant ocean-dwelling organisms that require oxygen
to live.
The Precambrian
spans about 87% of Earth’s history, beginning with the formation of Earth about
4.6 billion years ago and ending approximately 570 million years ago with the
diversification of life that marks the start of the Paleozoic Era. It is the
least understood span of Earth’s history because most Precambrian rocks are buried
from view. However, on each continent there is a “core area” of Precambrian
rocks called the shield. The iron ore deposits of Precambrian age
represent the time when oxygen became abundant in the atmosphere and combined
with iron to form iron oxide. The most common middle Precambrian fossils are stromatolites.
Microfossils of bacteria and blue-green algae, both primitive prokaryotes
whose cells lack organized nuclei, have been found in chert, a hard, dense,
chemical sedimentary rock in southern Africa (3.1 billion years of age) and
near Lake Superior (1.7 billion years of age). Eukaryotes, with cells
containing organized nuclei, are among billion-year-old fossils discovered in Australia.
Plant fossils date from the middle Precambrian, but animal fossils came a bit
later, in the late Precambrian. Many of these fossils are trace fossils,
and not of the animals themselves.
The Paleozoic
Era extends from 570 million years ago to about 245 million years ago. The
beginning of the Paleozoic is marked by the appearance of the first life
forms with hard parts such as shells. Therefore, abundant Paleozoic fossils
occur and a far more detailed record of Paleozoic events can be constructed.
During the early Paleozoic (the Cambrian, Ordovician, and Silurian Periods) the
vast southern continent of Gondwanaland existed. Seas inundated and
receded from North America several times,
leaving thick evaporite beds of rock salt and gypsum. Life in the early
Paleozoic was restricted to the seas and consisted of several invertebrate
groups. During the late Paleozoic (the Devonian, Mississippian, Pennsylvanian,
and Permian Periods), ancestral North America
collided with Africa to produce the original
northern Appalachian Mountains, and the
northern continent of Laurasia formed. By the close of the Paleozoic,
all the continents had fused into the super continent of Pangea. During
most of the late Paleozoic, organisms diversified dramatically. Insects and
plants moved onto the land, and amphibians evolved and diversified quickly. 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, altered climatic conditions caused one of the
most dramatic biological declines in all of Earth’s history.
The Mesozoic
Era, often called the “age of dinosaurs,” begins about 245 million
years ago and ends approximately 66 million years ago. Early in the Mesozoic
much of the land was above sea level. However, by the middle Mesozoic, seas
invaded western North America. As Pangea 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
the continent. Organisms that had survived extinction at the end of the
Paleozoic began to diversify in spectacular ways. Gymnosperms (cycads,
conifers, and ginkgoes) quickly became the dominant trees of the Mesozoic
because they could adapt to the drier climates. Reptiles quickly became the
dominant land animals, with one group eventually becoming the birds. The most
awesome of the Mesozoic reptiles were the dinosaurs. At the close of the
Mesozoic, many reptile groups, including the dinosaurs, became extinct.
The Cenozoic
Era, or “era of recent life,” begins approximately 66 million years
ago and continues today. It is the time of the mammals, including
humans. The widespread, less disturbed rock formations of the Cenozoic provide
a rich geologic record. Most of North America
was above sea level throughout the Cenozoic. Due to their different relations
with tectonic 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 Appalachians, causing the streams to erode with renewed
vigor and deposit their sediment along the continental margin. In the West,
building of the Rocky Mountains (the Laramide
orogeny) was coming to an end, the Basin and
Range Province
was forming, and volcanic activity was extensive. The Cenozoic is often called
the “the age of the mammals” because these animals replaced the reptiles
as the dominant land life. Two groups of mammals, the marsupials and the
placentals, evolved and expanded to dominant the 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 believe that early humans hastened their decline by selectively
hunting the larger animals. The Cenozoic could also be called the “age of
flowering plants.” As a source of food, flowering plants strongly
influenced the evolution of both birds and herbivorous (plant-eating) mammals
throughout the Cenozoic Era.
Earth History Outline:
I. Earth's atmosphere
A. Primitive atmosphere formed from volcanic
gases
1. A
process called outgassing
2. Water vapor, carbon dioxide, nitrogen,
and several trace gases
3.
Very little free oxygen
B. Water vapor condenses and forms
primitive oceans as Earth cools
C. Bacteria
evolve
D. Plants evolve and photosynthesis produces
oxygen
E. Oxygen
content in the atmosphere increases
F. By about 4 billion years after Earth
formed, abundant ocean-dwelling organisms that require oxygen existed
II. Earth's history
A.
Precambrian Era
1.
4.6 billion to 570 million years ago
2.
87% of Earth's history”
3.
Only sketchy knowledge
4. Most Precambrian rocks are devoid
of fossils
5.
Precambrian rocks
a. Most are buried from view
b. Each continent has a "core area" of Precambrian rocks called a
shield
c. Extensive iron ore deposits
d. Absent are fossil fuels
6.
Precambrian fossils
a.
Most common are stromatolites
1.
Material deposited by algae
2.
Common about 2 billion years ago
b. Microfossils of bacteria
and algae have been found in chert
1. Southern
Africa (3.1 billion years of age)
2. Lake
Superior area (1.7 billion years of age)
c. Plant fossils date from the
middle Precambrian
d. Animal fossils date from
the late Precambrian
e. Diverse and multicelled
organisms exist by the close of the Precambrian
B.
Paleozoic Era
1. 570 million years ago to about
245 million years ago
2.
First life forms with hard parts
3.
Abundant Paleozoic fossils
4.
Early Paleozoic history
a. Southern continent of
Gondwanaland exists
b.
North America
1.
A barren lowland
2. Seas move inland and
recede several times and shallow marine basins evaporate leaving rock salt and
gypsum deposits
5.
Early Paleozoic life
a.
Restricted to seas
b.
Vertebrates had not yet evolved
c. Life consisted of several
invertebrate groups
1.
Trilobites
2.
Brachiopods
3.
Cephalopods
d. First organisms with hard
parts, such as shells - perhaps for protection
6.
Late Paleozoic history
a.
Supercontinent of Pangaea forms
b. Several mountain belts
formed during the movements of the continents
c. World's climate becomes
very seasonal, causing the extinction of many species
7.
Late Paleozoic life
a.
Organisms diversified dramatically
b.
Land plants
c. Fishes evolve into two
groups of bony fish
1.
Lung fish
2. Lobe-finned fish
which become the amphibians
d.
Insects invade the land
e.
Amphibians diversify rapidly
f.
Extensive coal swamps develop
C. Mesozoic
Era
1. 245 million years ago to about 66
million years ago
2.
Often called the "age of dinosaurs"
3.
Mesozoic history
a. Begins with much of the
world's land above sea level
b.
Seas invade western North America
c. Breakup of Pangaea begins
forming the Atlantic ocean
d. North American plate began
to override the Pacific plate
e. Mountains of western North America began forming
4.
Mesozoic life
a. Survivors of the great
Paleozoic extinction
b. Gymnosperms become the dominant
trees
c. Reptiles (first true
terrestrial animals) readily adapt to the dry Mesozoic climate
d. Reptiles have shell-covered
eggs that can be laid on the land
e.
Dinosaurs dominate
f.
One group of reptiles led to the birds
g. Many reptile groups, along
with many other animal groups, become extinct at the close of the Mesozoic
1. One hypothesis is
that a large asteroid or comet struck Earth
2. Another possibility
is extensive volcanism
D. Cenozoic
Era
1. 66
million years ago to the present
2.
Often called the "age of mammals"
3. Smaller fraction of geologic time
than either the Paleozoic or the Mesozoic
5.
Cenozoic life
a. Mammals replace reptiles as
the dominant land animals
b. Angiosperms (flowering
plants with covered seeds) dominate the plant world
1. Strongly influenced
the evolution of both birds and mammals
2. Food source for both
birds and mammals
c. Two groups of mammals
evolve after the reptilian extinctions at the close of the Mesozoic
1.
Marsupials
2.
Placentals
d. Mammals diversify quite
rapidly and some groups become very large
1. e.g., Hornless
rhinoceros, which stood nearly 16 feet high
2.
Many large animals became extinct
e.
Humans evolve