Chapter 17 – The History of Life
Section 17-1:
The Fossil Record
Paleontologists are scientists who study fossils
Fossils can provide insight into where
an organism lived, food, predators,
and physical structure  all of this
information = the fossil record
The fossil record shows how species
have changed over time.
The fossil record shows that fossils occur
in a certain order and that life has
changed
More than 99% of all species that
have lived on Earth are now extinct
For a fossil to form, the remains of the
organism or some trace has to be preserved
Most fossils form in
sedimentary rock
The quality of fossil preservation varies; sometimes there are
just imprints of soft parts, sometimes hard parts get replaced
with long-lasting mineral compounds, and sometimes
organisms get buried so quickly they are almost perfectly
preserved
Once a fossil is discovered, there
are several ways a paleontologist
can interpret them
Many times, they must reconstruct an
extinct species so they have to look for
similarities & differences between the
fossil and living organisms
A fossil’s age is also important and can
be determined using 2 techniques;
relative dating and radioactive dating
Relative dating = when the age of
a fossil is determined by
comparing its placement with that
of fossils in other layers of rock
With sedimentary rock, older
layers are on the bottom and
newer layers are on the top
Index fossils can also be used to
compare relative age  they are
easily recognized, only lived
during a certain period of time,
and were widely distributed
Radioactive dating = the age of the fossil
is calculated based on the amount of
remaining radioactive isotopes it contains
Some elements in rocks are
radioactive and take a certain
amount of time to break down
A half-life = the amount of time
required for half of the
radioactive atoms to decay
Carbon-14 is a common
isotope used to date fossils
Paleontologists use a geological time scale to represent
evolutionary time
The scale was first developed by
studying rock layers and index fossils
When scientists found major changes in
fossil animals and plants they used
those times as markers between
segments of geologic time
Later, radioactive dating was used to put
specific ages to the segments of time and
they found that there was not a standard
length of time for the segments
Geologic time begins with
Precambrian time, it is
about 88% of Earth’s
History
After Precambrian time, the basic divisions
of the geologic scale are eras and periods
There are currently 3 eras after
Precambrian time; Paleozoic Era,
Mesozoic Era, and the Cenozoic Era
The eras are broken up into smaller
time frames called periods
Section 17-2:
Earth’s Early History
Geologic evidence shows that
Earth is about 4.6 billion
years old and was not created
by one single event
Early Earth was much different
than the current Earth we know
Earth’s early atmosphere probably contained at
least 5 compounds that are no longer part of the
atmosphere
Earth was also very hot, it was
not until about 3.8 billion
years ago that Earth was cool
enough for oceans to form
1953 – Miller and Urey tried to figure out
how the first beings formed
They used the suggested gasses of
the early atmosphere and electric
sparks for the lightning
They were able to produce organic
compounds including amino acids
Scientists now know that their
experiments were not accurate,
but similar experiments have
produced organic compounds
Another unanswered question is which
appeared first, RNA or DNA?
Under the right conditions, RNA
can help DNA duplicate
Some RNA can even grow and
duplicate itself
One hypothesis is that an RNA
based form of life could have led
to the DNA control we know now
Earth’s first atmosphere
contained very little oxygen
Over time, photosynthetic
bacteria became common in
shallow seas and started to
produce oxygen
The oxygen combined with
iron in the oceans and caused
the oceans to rust
The iron fell out and formed
bands of iron in Earth’s crust
The rise of oxygen in the atmosphere
drove some life forms to extinction,
while other life forms evolved new
ways to use oxygen for respiration
The oxygen also started to accumulate
in the atmosphere and replace several
other compounds
About 2 billion years ago,
prokaryotic cells began evolving into
the ancestor of all eukaryotic cells
The endosymbiotic theory says that eukaryotic
cells formed from a symbiosis among several
different prokaryotic organisms
Basically, smaller prokaryotes
lived inside larger prokaryotes and
eventually formed organelles
Prokaryotes that used ATP evolved
into mitochondria
Other prokaryotes that carried out
photosynthesis became chloroplasts
Sometime after eukaryotic cells arose,
they began to reproduce sexually
This sped up the evolution process
by diversifying organisms  they
no longer had the exact same DNA
as the “parent”
Section 17-3:
Evolution of Multicellular Life
The fossil record indicates that
major changes occurred in Earth’s
climate, geography, and life forms
Precambrian time: simple anaerobic forms of life appeared
Photosynthetic forms then
started to add oxygen to the
atmosphere
Aerobic forms of life appeared
and eukaryotes appeared
Life existed only in the sea
Paleozoic Era: lasted about 300 million years fossil evidence shows that early in the Paleozoic
Era, there was a diversity of marine life.
Cambrian Period = Diversification during this period is
termed the “Cambrian Explosion”
Many of the first known
representatives of the
animal phyla evolved
Ex. Invertebrates,
brachiopods, and trilobites
Shells and outer skeletons
appeared
Ordovician/Silurian Periods – ancestors of modern octopi
and squids appeared
Some arthropods were the
first animals to live on land
Some of the first vertebrates
and land plants appeared
Devonian Period – Also called the “Age of the Fishes”
Insects started to appear on land
Sharks appeared late in the period
Vertebrates began to invade the land – some of
the first four-legged vertebrates evolved into the
first amphibians
Carboniferous/Permian Periods – life was expanding over
Earth’s continents
Reptiles evolved from amphibians
Winged insects appeared
(dragonflies, cockroaches)
Plants formed vast swampy areas
where eventually the remains
formed thick deposits of sediment
that changed into coal over time
The mass extinction at the end of the Paleozoic affected both
plants and animals on land and in seas  as much as 95% of
the complex life in the oceans disappeared
Mesozoic Era – lasted about 180 millions years and included
increasing dominance of dinosaurs and the appearance of
flowering plants
Triassic Period – those that survived the mass extinction
became the main forms of life
Important organisms were
fishes, insects, reptiles, and
cone-bearing plants
Sometimes the Mesozoic is called the
Age of the Reptiles
Some of the first dinosaurs appeared
Mammals also appeared late in the
period, but were very small
Jurassic Period – dinosaurs became the dominant animals on
land
Dinosaurs “ruled” the Earth
for about 150 millions years
One of the first birds
appeared during this time
Cretaceous Period – reptiles were still the dominant
vertebrates
Flying reptiles became extinct
during this period
There were many reptiles in
the sea with the fishes
New plant life came about;
leafy trees, shrubs, and small
flowering plants
Another mass extinction occurred at the end of the Era –
more than half of all plant and animal groups were wiped
out, including all of the dinosaurs
Cenozoic Era – started about 65 million years ago to present
- mammals evolved adaptations that allowed them to live in
various environments – on land, in water, and even in the air
– also termed the Age of the Mammals
Tertiary Period – the climate was warm and mild
Whales and dolphins evolved
Grasses evolved
Some mammals and birds
became very large
Quaternary Period – Earth’s climate was changing – Earth
cooled and went through a series of ice ages  About
20,000 years ago, Earth’s climate began to warm back up
Many of the animals we are
familiar with became common
The fossil record suggests the
early human ancestors appeared
about 4.5 million years ago – but
they did not look entirely human
Homo sapiens may have
appeared as early as 200,000
years ago in Africa
Section 17-4:
Patterns of Evolution
Macroevolution refers to large-scale
evolutionary patterns and processes that
occur over long periods of time.
6 important topics in macroevolution are
extinction, adaptive radiation, convergent
evolution, coevolution, punctuated equilibrium,
and change in developmental genes.
Extinction – it usually happens because species
compete for resources and environments change
Under the changing environment
of a mass extinction, extinction
isn’t related to ordinary natural
Mass extinctions usually
selection
resulted in a burst of
evolution afterward that
led to the production of
many new species.
Adaptive Radiation – when a single species or small group of
species has evolved, through natural selection and other
processes, into diverse forms that live in different ways
Ex. Darwin’s finches
On a larger scale, examples
include dinosaurs, which resulted
from adaptive radiation of reptiles
Convergent evolution – the process by which
unrelated organisms come to resemble one another
Different types of organisms start off
with different “raw material” for
natural selection, but share the same
environmental demands like moving
though air, moving through water, or
eating similar foods
Natural selection could mold body structure to fit the
environment; ie. arms and legs into wings or flippers
Ex. Many aquatic animals
have streamlined bodies for
swimming through water and
have similar looking parts
that do not share a common
evolutionary history  these
are analogous structures
Coevolution – the process by which two species evolve
in response to changes in each other over time
Punctuated equilibrium – some species have not evolved
much through time, they are in a state of equilibrium
Sometimes something happens to upset
the equilibrium and a change occurs
Many new species are produced by
periods of rapid change when the
equilibrium is upset
The rapid change can be caused by small populations
becoming separated from the main part or even when a
small population migrates to a new environment
Punctuated equilibrium describes
the pattern of long, stable periods
interrupted by brief periods of
more rapid change
Developmental genes and body plans – changes in genes for
growth can produce differentiation during embryological
development and can produce transformations in body shape
and size
Small changes in control genes can produce large changes in
adult animals