23.6 Earth's History

Earth’s history is divided into several large units, called eras. Each
era is one major stage in Earth’s history. An era is further divided into
smaller units called periods. Eras and periods help scientists locate
changes and events in Earth’s history.
Some boundaries between eras mark a time when many different
kinds of organisms became extinct within a relatively short time. Such
an event is called a mass extinction. Scientists have developed several
theories to explain what caused mass extinctions. These theories
include asteroid impacts, volcanic activity, disease, and climate change.
For nearly 4 billion years, changes in Earth’s surface, atmosphere, and
oceans have effected the development of living things. Living things, in
turn, have changed Earth. The major divisions of Earth’s history
are Precambrian time and the Paleozoic, Mesozoic, and Cenozoic Eras.
Precambrian Time: 4.6 Billion–542 Million Years Ago
The earliest portion of Earth’s history, known as Precambrian time,
includes the formation of Earth and the early development of life. At
first, Earth’s surface was largely molten and was continually bombarded
by meteorites. By 4 billion years ago, the forces that cause plate
movement were already at work. Soon after, one-celled organisms
appeared in the oceans. Tiny photosynthetic organisms took up carbon
dioxide from the atmosphere and released oxygen. Later in the
Precambrian, simple soft-bodied animals developed. Since soft bodies
don’t usually form fossils, there are few fossils from Precambrian time.
Paleozoic Era: 542–251 Million Years Ago Early in the
Cambrian period (the first period of the Paleozoic Era), a variety of animals
developed in the oceans. Scientists think that some of these animals
were related to the clams and worms found in the oceans today. Many
other types of early animals, such as those shown in Figure 35, became
extinct. But more than 450 million years ago, fishes evolved in the oceans.
Plants and animals, including early reptiles, began to live on land. Dense
forests of mosses and cone-bearing plants covered much of the land.
At times during the Paleozoic, parts of many continents were
flooded by seas. Thick layers of sediment deposited in these seas
formed much of the sedimentary rock found on the continents today.
During the last period of the Paleozoic Era, the Permian period, the
supercontinent Pangaea formed.
When did fishes first appear?
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Figure 35 During the early
Paleozoic Era, life began to evolve
into many different forms. This
scene shows how life might have
looked on the sea floor during the
Cambrian Period.
299 251 200 145 65 23 0
PERMIAN TRIASSIC JURASSIC
Facts and Figures
MESOZOIC
Early History Earth, like the rest of the solar
system, formed about 4.6 billion years ago out
of a cloud of dust and gas. After 50 million
years, Earth was close to its current size and
was so hot it was possibly entirely molten.
The following 4.5 billion years have been a
period of continuous cooling.
The molten state of early Earth allowed
the planet to separate into different layers.
The surface was not stable, so the early crust
would sink back into the molten interior.
CRETACEOUS PALEOGENE NEO-
GENE
CENOZOIC
PERIOD
ERA
Earth’s Surface 735
Eventually, Earth cooled enough to resemble
the planet you see today. Water vapor in the
atmosphere condensed to form rain and
create the oceans. The mantle solidified, and
the continental crust began to form at the
surface. The core cooled enough so that
the inner core began to solidify. The exact
timing of these events is uncertain. However,
radioactive dating shows that Earth had
developed a system of plate tectonics
approximately 4 billion years ago.
Integrate Chemistry
Half of the remaining potassium-40 in a
rock will decay to Ar40 L2
every 1.3 billion
years. This means that if a rock is 3.9
billion years old, or three half-lives of
1
potassium-40, the rock will contain 8
of its original potassium-40. For recent
geologic events like volcanic eruptions,
the isotope carbon-14 is often used to
measure the age. Carbon-14 has a halflife
of 5730 years, so it is useful for
dating geologic and anthropological
objects that are less than a hundred
thousand years old. Ask, If threequarters
of the carbon-14 in a piece
of pottery had decayed into daughter
isotopes, approximately how old is
the pottery? (11,460 years) Why
wouldn’t carbon-14 be useful for
dating rocks? (The ages of rocks are
typically in the tens or hundreds of
millions of years. With a half-life of only
5730 years, only a miniscule amount
of carbon-14 would be left in a rock
that old.)
Logical
Build Reading Literacy
L1
Identify Main Idea/Details Refer
to page 702D in this chapter, which
provides the guidelines for identifying
main idea and details.
Using four index cards, have students
write the name of each geologic era on
a card. While reading A Brief History of
Earth, have students record the main
details of each era on the appropriate
index card. Encourage students to use
bullet points, rather than copying whole
sentences. Have the students share their
cards in small groups and fill in any
missing main details, then align them in
geologic time.
Verbal, Group
Science News provides students
with current information on
Earth’s history.
Answer to . . .
Figure 34 The Triassic, Jurassic, and
Cretaceous periods
Fishes first appeared in
the early Paleozoic Era,
more than 450 million years ago.
Earth’s Surface 735