2. Fossils
Fossils: traces or
imprints of once-living
things
A dead organism is
covered by layers of
sediment, which
presses together to
form sedimentary rock
3. The Ages of Fossils
Sedimentary rock has layers, with the
oldest layers usually on the bottom and
newest on the top
Layers where the fossils are found tells
a scientist the relative age of fossil
Relative dating: Estimating age of
fossil by its position in the rock layers
5. Absolute Dating
When scientists want to
determine the age of a
fossil more precisely,
they use absolute
dating to get an exact
age
Absolute dating:
method of measuring
age of object in years
6. Scientists examine atoms, which over time,
decay by releasing energy
The time it takes for half a sample of atoms to
decay is its half-life
7. The Geologic Time Scale
Scientists use a type of calendar to
divide the Earth’s long history
Calendar is divided into very long units
of time since the Earth formed so long
ago
Geologic Time Scale: the standard
method used to divide the Earth’s long
natural history into manageable parts
8.
9. Divisions in Geologic Time Scale
Divided into eras, which are characterized
by the type of organisms that dominated
the Earth at that time
Precambrian Time: from formation of
Earth 4.6 billion years ago to about 543
million years ago
Volcanic eruptions, meteorites, intense
radiation from the sun
10.
11. Early atmosphere had
no oxygen—first
organisms were
prokaryotes (no
nucleus)
Cyanobacteria
appeared produced
own food and
released oxygen
Ozone layer forms in
upper atmosphere
and absorbs radiation
from the sun
12. Paleozoic Era:
rocks rich in fossils
of animals such as
sponges, corals,
clams, squids, and
trilobites
Fishes appeared,
sharks more
abundant, forests of
giant ferns covered
earth
14. Mesozoic Era:
dominated by
dinosaurs and other
reptiles, referred to as
Age of Reptiles
First birds appeared;
flowering plants
appeared
By end of era,
dinosaurs and many
other plants and
animal species
became extinct
15. Cenozoic Era:
sometimes called
“Age of Mammals”
Mammals
included
mastodons,
saber-toothed
cats, camels,
and giant
ground sloths
Included some
periods known
as ice ages
16. Mass Extinctions
Many species died out completely, or
became extinct at the end of each
era
When a species is extinct, it does
not reappear
Periods when many species suddenly
become extinct are called mass
extinctions
17. Most scientists think that the
extinction of the dinosaurs
happened because of extreme
changes in the climate on
Earth
18. These changes
could have
resulted from a
giant meteorite
hitting the earth,
or forces within
causing major
earthquakes
and volcanic
eruptions
19. The Changing Earth
Pangaea
-German scientist
Alfred Wegener
noticed that the
continents of Earth
look like pieces of a
puzzle
-proposed that long
ago the continents
formed one
landmass
surrounded by
gigantic ocean
20. Wegener called the single landmass
“Pangaea” which means “all Earth”
21. Do the Continents Move?
In mid-1960’s, J.
Tuzo Wilson came
up with idea that
continents were
not moving by
themselves
Wilson thought that
huge pieces of
Earth’s crust were
pushed by forces
within the planet
22. Each piece of crust is called a tectonic
plate
Wilson’s theory of how the huge
pieces of crust move is called plate
tectonics
23. According to Wilson, outer crust of Earth
is broken into seven large plates and
several smaller ones
Motion of the plates causes continents to
move
24. Slow changes, such as moving
continents, allow time for adaptation
Everywhere on Earth, living things
are well adapted to location where
they live
Yet in that location, there is evidence
that organisms that lived there in the
past were very different
Adaptations to Slow Changes
25. Example: Animals
currently living in
Antarctica are able to
survive very cold
temperatures. But
under the frozen
surface are the
remains of tropical
forests
Conditions have
changed many times
during Earth’s history
