Lesson 1 Geologic History and the Evolution of Life

Name Date Class

LESSON 1

Geologic History and the Evolution of Life

Directions: Write the correct term in the boxes to the right of each definition. Then unscramble the letters from
the shaded boxes to spell a ninth term.

1.dying out

2.connects continents that previously
had been separated by water

3.occurs when a physical barrier
separates a population from the rest
of its species

4.a subdivision of an eon

5.the longest unit of geologic time

6.a series of marks or points at known
intervals

7.event during which many species die

8.a subdivision of a period.

9.When they are unscrambled, the letters in the shaded boxes spell
, which is a subdivision of an era.

Geologic Time9

Name Date Class

LESSON 1

Geologic History and the Evolution of Life

Directions: On the line before each definition, write the letter of the term that matches it correctly. Each term
is used only once.

1.a subdivision of periods on the geologic time
scale

2.the time line of Earth’s past

3.is nearly 90 percent of Earth’s history

4.the largest unit of geologic time

5.Mass extinctions are caused by sudden changes
in this.

6.the disappearance of many species within a short
time period

7.Organisms are dependent on this for survival.

8.provides evidence of meteorite impact

9.blocks sunlight resulting in environmental
changes

10.the change in species over time as organisms
adapt to their environments

11.connects two continents that were once separated

12.separation of species due to a physical barrier

13.the earliest life-forms

14.the sudden evolution of new, complex life-forms

A.Cambrian explosion

B.environment

C.eon

D.epoch

E.evolution

F.fossil record

G.geographic isolation

H.geologic time scale

I.iridium

J.land bridge

K.mass extinction

L.Precambrian time

M.single-celled
organisms

N.volcanic activity

Geologic Time13

Name Date Class

LESSON 1

Geologic History and the Evolution of Life

Key Concept How is evolution affected by environmental change?

Directions: On the line before each statement, write T if the statement is true or F if the statement is false.

1.The earliest of all fossil evidence is in rocks that are about 1.5 billion years old.

2.The earliest life-forms were single-celled.

3.Some Precambrian life-forms were multicellular.

4.Scientists discovered that multicelluar organisms existed about 2 bya.

5.Scientists hypothesize that large, Precambrian, soft-bodied multicellular
organisms still exist.

6.The Cambrian explosion occurred about 700 mya.

7.The Cambrian explosion is significant because this is when the first soft-tissued
organisms evolved.

8.Organisms from the Cambrian period left behind more fossil record evidence
than organisms from Precambrian time.

9.Organisms from the Cambrian period had anatomy that was much different
from present-day animals.

10.Scientists hypothesize that some Cambrian period organisms are distant
ancestors of present-day organisms.

11.Fossil records from the Archean eon show that single-celled organisms existed
at this time.

Geologic Time19

Name Date Class

LESSON 1

The Biggest Event in the History of Earth

When Earth was a scant 50 million
years old, an infant planet still forming
and still largely molten, it collided with an
object about the size that Mars is today.
Fragments of the object and much of
Earth’s crust were flung into space. Quickly,
this material assumed an orbit around
Earth. Eventually, this material coalesced,
or combined, to become our Moon. This is
the best accepted theory of how the Moon
formed, but there are several others.

The Fission Theory

The fission theory proposes that Earth
was once spinning very rapidly—fast
enough to throw some of its crust into
orbit, which then coalesced into the Moon.
However, lunar soil has been exposed to
more intense heat, or baking, than Earth
soil. Lunar soil is not as hydrated as Earth
soil, meaning there are fewer molecules of
water inside lunar rock. If the Moon were
made entirely from Earth’s crust, the rocks
would not be different in this way.

The Capture Theory

The capture theory proposes that the
Moon formed somewhere else and was
simply captured by Earth’s gravity as it
drifted by. Scientists are reluctant to
believe that the Moon just drifted by at the
right time, at the right angle, and at the
right speed for Earth’s gravitational field

to capture it. This theory also does not
explain why Moon rocks have experienced
more baking than Earth rocks.

The Condensation Theory

The condensation theory proposes
that the Moon and Earth coalesced at the
same time from the material in the solar
nebula that formed the solar system. If this
were true, the Moon would have the same
composition as Earth. But it doesn’t. This
theory also fails to explain the Moon rocks’

extra baking.

The Giant Impactor Theory

An impactor about the size of Mars
struck Earth at an oblique angle. This
glancing blow, rather than a direct hit,
vaporized the impactor and removed some
of Earth’s molten mantle. The cataclysmic
collision created such heat that the ejected
material flew into space like molten
fireworks. This extreme heat explains the
extra baking of Moon rocks. The glancing
blow also gave Earth its 24-hour days. The
Moon now orbits Earth in the same
direction as Earth’s rotation.

This impactor material and Earth’s
mantle were put in orbit around Earth, and
this material eventually formed the Moon.
The giant impactor theory is the best fit
with what is known today.

Applying Critical-Thinking Skills

Directions: Respond to each statement.

1.Develop an idea about how Earth’s history might have been different if the Moon had
not been present.

2.Describe what might have happened to early Earth if, instead of an oblique hit, it had
been delivered a direct hit from a Mars-sized object while Earth was still mostly molten.

20Geologic Time

Name Date Class

LESSON 1

Time-Travel Trips

Ninety percent of Earth’s history had already happened before the beginning of the
Paleozoic era. That early time is the Precambrian era, from the formation of Earth 4.6 bya to
542 mya. This era is divided into three eons—the Hadean, the Archean, and the Proterozoic.
Each of these eons experienced distinguishing events, especially the Archean and the
Proterozoic.

Plan a Travel Brochure

Suppose you are a travel agent who specializes in time travel. One of the trips is a tour
of Precambrian time, and visitors get to briefly visit and observe Earth during each of the
Precambrian eons. Your tour promises visitors that they will see at least one thing that
distinguishes each eon.

Design and illustrate a folding, three-page brochure that promotes your time tour of the
Precambrian. Write a brief statement that describes what the visitors will see on the tour,
and illustrate each eon in color. Include your notes on the panels below.