The History of Life
Chapter 17
Fossils

Preserved traces and remains of ancient life.
The Fossil Record

Paleontologists study ancient life through fossils.


They make inferences about past life and group similar organisms
together to create the fossil record.
Fossils show that more than 99% of all living species that have ever
existed have become extinct.
How Fossils Form

Is the fossil record complete?


Many more organisms die without leaving traces than those that do.
Most fossils form in sedimentary rock.
Figure 17-2 Formation of a Fossil
Section 17-1
Water carries small rock
particles to lakes and seas.
Dead organisms are buried by
layers of sediment, which forms
new rock.
The preserved remains may
later be discovered and
studied.
Interpreting Fossil Evidence

Relative Dating –

Estimates a fossil’s age by
comparing to other fossils
present in the same layer.


Index fossils identify a particular
era
Absolute (Radioactive) Dating
–

A more exact method of
determining a fossil’s age by
determining the percent of a
radioactive element left in the
sample.
Relative Dating
Compare/Contrast Table
Section 17-1
Comparing Relative and Absolute Dating of Fossils
Relative Dating
Absolute Dating
Can determine
Age of fossil with respect to
another rock or fossil (that is,
older or younger)
Age of a fossil in years
Is performed by
Comparing depth of a fossil’s
source stratum to the position of
a reference fossil or rock
Determining the relative
amounts of a radioactive isotope
and nonradioactive isotope in a
specimen
Imprecision and limitations of age
data
Difficulty of radioassay
laboratory methods
Drawbacks
Geologic Time Scale



Represents evolutionary time.
Precambrian time - before complex life
Eras – after Precambrian time.



Paleozoic – fish and amphibiams
Mesozoic – age of dinosaurs
Cenozoic – age of mammals
Geologic Time Scale with Key Events
Section 17-3
Era
Cenozoic
Mesozoic
Paleozoic
Precambrian
Time
(millions of
Period
Time years ago)
Quaternary
Tertiary
Cretaceous
Jurassic
Triassic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
1.8–present
65–1.8
145–65
208–145
245–208
290–245
363–290
410–363
440–410
505–440
544–505
650–544
Key Events
Glaciations; mammals increased; humans
Mammals diversified; grasses
Aquatic reptiles diversified; flowering plants; mass extinction
Dinosaurs diversified; birds
Dinosaurs; small mammals; cone-bearing plants
Reptiles diversified; seed plants; mass extinction
Reptiles; winged insects diversified; coal swamps
Fishes diversified; land vertebrates (primitive amphibians)
Land plants; land animals (arthropods)
Aquatic arthropods; mollusks; vertebrates (jawless fishes)
Marine invertebrates diversified; most animal phyla evolved
Anaerobic, then photosynthetic prokaryotes; eukaryotes, then
multicellular life
Earth’s Early History

Ancient earth was hostile.




Poisonous atmosphere ; no oxygen
Oceans were a “hot thin soup”
UV radiation, lightening, volcanos…
So how did life begin?
Figure 17-8 Miller-Urey Experiment
Section
17-2
The
first
organic molecules…
Mixture of gases
simulating
atmospheres of
early Earth
Stanley Miller
recreated early
earth’s atmosphere
Made simple
organic molecules
(amino acids…)
Spark simulating
lightning storms
Condensation
chamber
Water
vapor
Cold
water
cools
chamber,
causing
droplets
to form
Liquid containing
amino acids and
other organic
compounds
Coacervates
Proteinoid
microspheres may
have formed in
shallow pools as
precursors to cells.
Evolution of RNA and DNA
Stromatolites


Ancient prokaryotes added oxygen to the atmosphere.
The rise of oxygen caused drove some life forms to
extinction while others evolved.
Figure
17-12 Endosymbiotic
Theory
Endosymbiotic
Theory
– origin of eukaryotic
cells
Chloroplast
Aerobic
bacteria
Ancient Prokaryotes
Nuclear
envelope
evolving
Plants and
plantlike
protists
Photosynthetic
bacteria
Mitochondrion
Primitive Photosynthetic
Eukaryote
Ancient Anaerobic
Prokaryote
Primitive Aerobic
Eukaryote
Animals, fungi, and
non-plantlike protists
This theory proposes that eukaryotic cells arose from symbiotic relationships
between bacteria and cells.
Sexual Reproduction and Multicellularity
This ancient jellyfish was
an early multicellular
animal.
With the advent of sexual
reproduction the rate of
evolution took off.
Evolution of Life
Early Earth was hot; atmosphere contained poisonous gases.
Earth cooled and oceans condensed.
Simple organic molecules may have formed in the oceans..
Small sequences of RNA may have formed and replicated.
First prokaryotes may have formed when RNA or DNA was enclosed in microspheres.
Later prokaryotes were photosynthetic and produced oxygen.
An oxygenated atmosphere capped by the ozone layer protected Earth.
First eukaryotes may have been communities of prokaryotes.
Multicellular eukaryotes evolved.
Sexual reproduction increased genetic variability, hastening evolution.
Heterotroph Hypothesis

The first cells were…




Heterotrophs  autotrophs
Anaerobic  aerobic
Unicellular multicellular
Asexual  sexual
Patterns of Evolution

Extinction

The last member of a species dies….(failure of a species to
adapt)
Patterns of Evolution

Adaptive radiation

An ancestral form evolves into diverse forms through natural
selection
Patterns of Evolution

Convergent evolution

Unrelated species evolve similar adaptations due to similar
environments
Patterns of Evolution

Coevolution Evolution of two different species
in response to each other: symbiosis
Patterns of Evolution

Gradualism –

A slow and steady rate of evolution
Patterns of Evolution

Punctuated equilibrium –

Periods of rapid evolution followed by long stretches of
stability
Flowchart
Section 17-4
Species
that are
Unrelated
form
Related
in
under
under
in
in
Inter-relationshiops
Similar
environments
Intense
environmental
pressure
Small
populations
Different
environments
can undergo
can undergo
can undergo
can undergo
can undergo
Coevolution
Convergent
evolution
Extinction
Punctuated
equilibrium
Adaptive
radiation