VIEWS OF EARTH’S PAST

4.3 The geologic time scale shows Earth’s past

Big Idea: Rocks, fossils and other types of natural evidence tell Earth’s story

I. Earth is constantly changing

A. James Hutton, Scottish scientist (late 1700s)

1. Found fossils and saw them as evidence of extinct species

2. Noticed different fossilized creatures in different layers of rock

3. 1^st to present a hypothesis about Earth changing over time

4. Theory of uniformitarianism

a. Earth is an always changing place

b. The same forces of change at work today were at work in the past

i. Rivers deposit sediment as they always have

ii. Volcanoes erupt as they always have

B. Gradual vs. sudden changes

1. Gradual: mountains forming, climate, ice on land, continental drift

2. Sudden: volcanic eruption, earthquake, flood, landslide

II. The Geologic Time Scale

A. Divides Earth’s history into intervals of time defined by major events or changes

B. Information from fossils and radioactive dating

C. Oldest evidence of life—3.8 billion years ago

1. Multicellular organisms—1 billion years ago

2. Modern humans—100,000 years ago

D. Imagine Earth’s history shortened into one year

1. Earth forms on January 1^st

2. First life appears in the beginning of March

3. Multicellular organisms appear in mid-October

4. Humans do not show up until ten till midnight on December 31

5. British Geological Survey Geologic Timeline

III. Divisions of geologic time

A. Eon

1. Largest unit of time

2. Earth’s 4.6 billion year history is divided into four Eons

3. The Hadean, Archean, and Proterozoic eons are called Precambrian time and make up about 90% of Earth’s history

a. Consist mostly of tiny, single-celled organisms (microscopic)

b. Soft bodies rarely formed into fossils

4. Phanerozoic eon from end of Precambrian time to the present

a. Many more changes are recorded for this eon

b. Divided into smaller units of time

B. Era

1. Eons divided into eras

2. Most recent eon (Phanerozoic) divided into three eras: Paleozoic, Mesozoic, and Cenozoic

a. Paleozoic era (ancient life)

i. At start, all life lived in the ocean

ii. At end, reptiles, insects, ferns were common

iii. Formation of Pangea occurred

iv. Ends with mass extinction (asteroid vs. event causing ice age)

b. Mesozoic era

i. Spans the next 183 million years

ii. Dinosaurs ruled the Earth

iii. Mammals, birds, and flowering plants first appear

iv. Ends with the extinction of dinosaurs (asteroid theory)

c. Cenozoic era

i. Most recent era, began 65 million years ago, continues today

ii. The “Age of Mammals”

iii. Cenozoic era divided into two periods

C. Period

1. Cenozoic era divided into Tertiary and Quaternary periods

2. Quaternary period

a. From 2 million years ago to the present

b. Mostly a series of ice ages

c. Mammoths, saber-toothed cats, and other giant mammals common early

d. Fossils from first modern humans from this period, 100,000 years ago

e. Now seems the end of this period may be defined by the rise of human civilizations