The document discusses the history of life on Earth through fossils and the geologic time scale. It describes how fossils provide evidence of past life and how scientists use relative and absolute dating to determine the age of fossils. The geologic time scale divides Earth's history into eras characterized by dominant organisms, including the Precambrian, Paleozoic, Mesozoic and Cenozoic eras. Mass extinctions periodically wiped out many species, possibly due to meteor impacts or volcanic activity. Plate tectonics caused the continents to move over time.

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