This document discusses the history of life on Earth through fossils and the geologic time scale. It explains that 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 manageable periods characterized by dominant organisms, including the Precambrian, Paleozoic, Mesozoic, and Cenozoic eras. Mass extinctions periodically wiped out many species, and plate tectonics caused the continents to move over time, changing Earth's climate and requiring adaptation by living things.

1. Evidence of the Past
The History of Life
on Earth

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. 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
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
Fish appeared,
sharks more
abundant, forests
of giant ferns
covered earth

14. Paleozoic Era

15. Mesozoic Era:
dominated by
dinosaurs and
other reptiles,
referred to asAge
of Reptiles
First birds appeared;
flowering plants
appeared
By end of era,
dinosaurs and many
other plants and
animal species

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 “all Earth”
-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

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