1. The hisTory of
Life on earTh
Evidence of the Past

2. Fossils
 Fossils: traces or
imprints of once-living
things
 Dead organism is
covered by layer 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

4. Relative Dating

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.  Scientistsexamine atoms, which over time,
decay by releasing energy
 The time it takes for half a sample of atoms to
decay is its half-life
 Scientists measure the ratio of stable to unstable
atoms to determine the age of a sample of rock

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

10. 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

12.  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

13.  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. Paleozoic Era

15.  Mesozoic Era:
dominated by
dinosaurs and other
reptiles, referred to as
Age of Reptiles
 Firstbirds appeared;
flowering plants
appeared
 By end of era,
dinosaurs and many
other plants and
animal species
became extinct

16.  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

18. Mass Extinctions
Some of the important divisions in
geologic time scale mark times when
rapid changes happened on Earth
During these times, many species
died out completely, or became
extinct
When a species is extinct, is does not
reappear

19. Periods when many species suddenly
become extinct are called mass
extinctions
Most scientists think that the
extinction of the dinosaurs happened
because of extreme changes in the
climate on Earth

20. These changes
could have
resulted from a
giant meteorite
hitting the earth,
or forces within
causing major
earthquakes and
volcanic
eruptions

21. 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

22. Wegener called the single landmass
“Pangaea” which means “all Earth”

23. 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

24.  Each piece of crust is called a tectonic
plate
 Wilson’s theory of how the huge pieces
of crust move is called plate tectonics

25.  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

26. Adaptations to Slow Changes
 When conditions
on Earth change,
organisms may
become extinct
 A rapid change,
such as a
meteorite impact,
may cause mass
extinction

27. 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

28.  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