Geologic Time Scale:
 The Earth is over 4.7 billion years old.
 Life started as single-celled organisms and now has
a great variety of multi-celled organisms.
 Scientists study life’s history from fossil records
arranged in a Geologic Time Scale.
Organizing Earth’s History
 The Geologic Time Scale is a timeline that
organizes the events in Earth’s history, like a
calendar.
 Single-celled organisms dominated most of Earth’s
history.
 Plants and fish have only evolved within the past
500 million years.
 Humans evolved about 110,000 years ago.
 If the entire history of Earth were squeezed into a
single day, humans would not evolve until the last
few seconds.
 The scale is divided into eons, era, periods, and
epochs.
 An Eon, billions of years ago, is divided by the
Cambrian boundary when the variety of life
forms exploded
 An Era includes mass extinctions that mark the
boundaries between eras.
 A Period includes blocks of time when a unique
rock series was laid down.
Fossils
 Fossils provide evidence for the geologic time
scale.
 They are imprints or remains of organisms that
were once alive.
Fossil Formation:
 Fossils form when layers of sediment build up over
a dead organism.
 Over time, pressure can cause these layers to turn to
rock and the remains are preserved.
 Fossils form in different ways, depending on the
material the fossil is forming in and the climate.
Fossil
Molds and Casts
Process
Notes
Acids eat away the
If the mold fills
skeleton or shell and with minerals, it
leaves an impression
becomes a cast
(mold) in the rock
Tracks and Trails Imprints are left in
Dinosaur tracks
mud, which later
are fairly
hardens
common
Minerals in plant
Petrifaction
Common with
cells crystallize;
plants
minerals enter
openings or cavities
in shells or bones.
Soft tissue
preservation
Traps
Mummification or
frozen in ice
Whole organisms
locked in tar pits,
asphalt, amber, etc.
Fossils do not last
long
Fossils and Geologic Time Scale
 Most fossils form in sedimentary rock.
 Positions of fossils with in the rock help construct
the geologic time scale because the lowest layer of
rocks are the oldest, meaning the fossils would be
the oldest too.
 Therefore, fossils on the top are from organisms
that lived in more recent times.
Review Questions:
1. Which division in the geologic time scale is
billions of years long?
- Eon
2. During which period in the geologic time scale
was there a great increase in the variety of life
forms on Earth?
- Cambrian Boundary
3. During which epoch in the geologic time scale did
humans evolve?
- Holocene
4. Which type of organism evolved first?
Millions of Years Ago
Event
360-286
First reptiles
410-360
First amphibians
440-410
First insects
505-440
First fish
- first fish
5. How can the Grand Canyon be used to construct the
geologic time scale?
- Fossil algae at the bottom lived more than 2.5 billion
years ago, seed plants in the middle layers lived more
than 65 million years ago.