Transcript Early Evolution

Chapter 22
The Origin and History of Life
Early Evolution
• The universe began with the Big Bang
about 13.7 bya (billion years ago)
• Our solar system began about 4.6 bya
• The Earth is 4.55 billion years old
• By 4 bya the Earth had cooled enough for
outer layers to solidify and oceans to form
• By 4-3.5 bya life emerged
bya
2
Proposed Stages of Life’s Origins
4. Polymers enclosed in
membranes acquired cellular
properties
3. Polymers became enclosed
in membranes 
protobionts
2. Nucleotides polymerized to
form DNA and RNA, amino
acids polymerized to form
proteins
1. Nucleotides and amino acids
were produced prior to the
existence of cells
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Stage 1: Origin of organic molecules
• Conditions on primitive Earth may have
been more conducive to spontaneous
formation of organic molecules
• Prebiotic or abiotic synthesis
– Little free oxygen gas
– Dilute solution called “prebiotic soup”
• Several hypotheses on where and how
organic molecules originated
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Electrical discharge
Miller-Urey
experiment
Electrodes
Reducing atmosphere
thought to be like early earth
To vacuum
Gases
H 2O
H2
CH4
NH3
Cold water
Condenser
Precipitating
droplets Products included amino
acids, purines, pyrimidines
Boiling water
Trap
Sample containing
organic molecules
such as amino acids
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• Reducing atmosphere hypothesis
– Based on geological data
– Atmosphere rich in water vapor, H2, CH4, NH3
(and little if any O2)
– Miller and Urey Chamber simulates atmosphere
and bolts of lightning
• Formed precursors, amino acids, sugars and
nitrogenous bases
• First attempt to apply scientific experiments to
understand origin of life
– Since 1950s, ideas about early Earth
atmosphere have changed, may have been a
neutral environment
• Still similar results for abiotic synthesis
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• Extraterrestrial hypothesis
– Meteorites brought organic carbon to Earth
• Including amino acids and nucleic acid bases
– Opponents argue that most of this would be destroyed
in the intense heating and collision
•Deep-sea vent hypothesis
–Biologically important molecules
may have been formed in the
temperature gradient between
extremely hot vent water and cold
ocean water
–Supported by experiments
–Complex biological communities
found here that derive energy from
chemicals in the vent (not the sun)
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Polymerization
Monomers are joined to form long chains.
Sugars form carbohydrates, amino acids form proteins,
and nucleotides form nucleic acids.
Reaction: Condensation
or Dehydration Synthesis
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Stage 2: Organic polymers
• Experimentally, prebiotic synthesis of
polymers is usually not possible in
aqueous solutions
– Hydrolysis competes with polymerization
• Experiments have shown formation of
nucleic acid polymers and polypeptides
(proteins) on clay surfaces
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Stage 3: Formation of boundaries
• Protobionts are cell-like collections of
polymers
– 4 characteristics
1. Boundary separated external
environment from internal contents
2. Polymers inside the protobiont contained
information
3. Polymers inside the protobiont had
enzymatic function
4. Protobionts capable of self-replication
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Types of Protobionts
• Coacervates
57 µm
– Droplets that form
spontaneously from the
association of charged
polymers
– Enzymes trapped inside
can perform primitive
metabolic functions
• Liposomes
– Vesicles surrounded by a
lipid layer
– Clay can catalyze
formation of liposomes that
grow and divide
– Can enclose RNA
200 nm
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Stage 4: RNA world
•
•
Most scientists favor RNA as the first
macromolecule of protobionts
Important RNA functions
1. Ability to store information
2. Capacity for self-replication
3. Enzymatic function – ribozymes
•
DNA and proteins do not have all 3
functions
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Advantages of DNA/RNA/protein world
• Information storage
– DNA would have relieved RNA of
informational role and allowed RNA to do
other functions
– DNA is less likely to suffer mutations
• Metabolism and other cellular functions
– Proteins have a greater catalytic potential and
efficiency
– Proteins can fulfill other functions such as
transport and stabilizing cell structure
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How did metabolism develop?
• Reactions occurred by chance in
protobionts
• Useful reactions could be retained by
selection for successful protobionts
• Metabolic pathways evolved backward
• Use of ATP and breakdown of glucose by
glycolysis represent early pathways now
shared by all living organisms
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From Protobionts to Living Cells
• In contrast to protobionts, cells contain
– specific and reproducible reaction sequences
to maintain metabolism
– specific macromolecules to maintain cell
structure
– ability to control internal processes
– ability to reproduce
• The transition from protobionts to living cells has
not been demonstrated in the laboratory
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Phanerozoic
MYA Eons
0
1.8
144
248
354
443
543
Quaternary
Tertiary
Cretaceous
Jurassic
Triassic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
3,000
3,400
3,800
4,550
Hadean Archaean
2,500
PRECAMBRIAN
1,600
Proterozoic
Late
900
History of life
on Earth
Middle
Early
Late
Middle
Early
• Geological time scale
– From 4.55 bya to
present
• 4 eons
– Hadean, Archaean,
Proterozoic,
Phanerozoic
– 1st three are called
Precambrian
• Each eon is further
divided into eras
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Earth’s History Condensed
Into a One Year Timeline
Earth’s scientists estimate the age of the earth to
be ~4.6 billion years old. To make this time span
a bit more tangible, I'll be marking events in
earth’s history on a one year timeline using these
scales:
One month represents ~375 million years
One day represents ~12.3 million years
Assuming that the earth formed on January 1st….
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Prokaryotic cells arose during Archaeon Eon
• Archaeon Eon- when diverse microbial
life flourished in primordial oceans
• First cells were prokaryotic
– Includes Bacteria and
Archaea
– Organisms were anaerobic
due to scarcity of free oxygen
– First cells were heterotrophs
• Prokaryotic autotrophs evolved as supply
of organic molecules dwindled
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Stromatolites
March 18th in the
evolutionary year
• First known fossils from 3.5 bya
• Autotrophic cyanobacteria were
preserved while heterotrophic
ancestors were not
– Stromatolite formation involves
layers of calcium carbonate
• Cyanobacteria produce O2 as a
by-product of photosynthesis
• Release of O2 spelled doom for
many prokaryotic groups that
were anaerobic
• Allowed the evolution of aerobic
species
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Photosynthesis and the Oxygen
Revolution
• Oxygen began accumulating in the atmosphere
about 2.7 billion years ago.
• Banded iron formations are evidence of the age of
oxygenic photosynthesis – approximately 2 bya in
photo
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The First Eukaryotes
July 10th in the
evolutionary year
• During the Proterozoic Eon evidence of fossils
of eukaryotic cells appears ~2.1 bya
– Note the presence of
a nucleus
– Both bacteria and archaea
contributed substantially
to nuclear genome
• Endosymbiosis hypothesis
– mitochondria and plastids (chloroplast precursors)
were formerly small prokaryotes living within larger
host cells
• An endosymbiont is a cell that lives within a host cell
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Endosymbiosis Hypothesis
3.5 BYA
Infolding of
plasma
membrane
3 BYA
2.5 BYA
1.5 BYA
1.0 BYA
Origin of
Eukaryotes
Aerobic
prokaryote
Origin of
Prokaryotes
2 BYA
Photosynthetic
prokaryote
Endosymbiotic
origin of
mitochondria
Endosymbiotic
origin of
chloroplasts
"Serial endosymbiosis"
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• Key evidence supporting an
endosymbiotic origin of mitochondria and
plastids:
– Similarities in inner membrane structures and
functions
– Division is similar in these organelles and some
prokaryotes
– These organelles transcribe their own DNA into
RNA and produce proteins from this RNA
– Their ribosomes are more similar to prokaryotic
ribosomes than to eukaryotic ribosomes
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The Origin of Multicellularity
• Also during the Proterzoic Eon a wave of
diversification occurred when multicellularity
August 30th in the
evolved at ~1.5 bya
evolutionary year
• Comparisons of DNA sequences give evidence
that multicellular ancestors gave rise to algae,
plants, fungi, and animals
unicellular alga
8 identical cells
64+ cells, two types 1,000+ cells, 2 types
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Phanerzoic Eon
• Proliferation of multicellular eukaryotic life
was extensive (from 543 mya to today)
• Includes the Cambrian explosion
– The Cambrian explosion refers to the
sudden appearance of fossils resembling
modern phyla in the Cambrian period
(533 to 525 mya)
November 17th in the
evolutionary year
– The Cambrian explosion provides the first
evidence of predator-prey interactions
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The Colonization of Land ~500 mya
November 20th in the
evolutionary year
• Adaptations developed
for organisms to live on
land
–Plants produced
waterproof coating and
a vascular system for
internal transport
–Fungi followed plants
• Arthropods are the most abundant land animals
December 1st in the
• Tetrapods arrived ~365 mya
evolutionary year
– Our species arrived ~170,000 years ago
Last 23 minutes26of
December 31st!
Relative Dating of rock layers and
fossils/environments: Which rocks and fossils
are the oldest? Why?
A
B
C
D
E
F
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Distribution of Fossils
• The lowest stratum, or layer, in a cross section
of Earth is oldest, while the top stratum is the
most recent.
• Fossils found within a single stratum are of the
same approximate age.
• Relative age of a fossil says that a given fossil
is younger or older than another based on what
stratum it is found
• Absolute age could be estimated from
radioisotope dating
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Radioisotope dating
• Fossils can be dated using elemental
isotopes in accompanying rock
• Half-life – length of time required for
exactly one-half of original isotope to
decay
• Measure amount of a given isotope as well
as the amount of the decay product
• As paleontologists are unlikely to find the
first member of a species, expect fossil
record to underestimate actual date
species came into existence
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How Rocks and Fossils Are Dated
• After every halflife, the amount of
parent material
decreases by
one-half.
• C-14 has a ½ life
of ~5,730 years
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Major environmental changes
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Climate/temperature
Atmosphere
Land masses
Continental drift
Flood
Glaciation
Volcanic eruptions
Meteoric impacts
These environmental changes
• Can allow for new types of
organisms
• Responsible for many
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extinctions
Five Mass Extinctions so far
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Is a Sixth Mass Extinction Under
Way?
• Scientists estimate that the current rate of
extinction is 100 to 1,000 times the typical
background rate
• Data suggest that a sixth human-caused
mass extinction is likely to occur unless
dramatic action is taken
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Consequences of Mass Extinctions
• Mass extinction can alter ecological
communities and the niches available to
organisms
• It can take from 5 to 100 million years for
diversity to recover following a mass
extinction
• Mass extinction can pave the way for
adaptive radiations
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Adaptive radiation
• An organism’s movement into a variety of
different environments or exploitation of a variety
of different food sources leads to adaptive
radiation.
• Adaptive radiation produces a wide array of
descendant species from one type of ancestor.
• The mass extinction of dinosaurs gave way to
adaptive radiation of mammals 65 million years
ago.
• Hawaii is an excellent laboratory to study
adaptive radiation.
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Descendants of ancestral tarweed that arrived
5 mya from
North America
Dubautia laxa
KAUAI
5.1
million
years
1.3
million
years
MOLOKAI
MAUI
OAHU
3.7 LANAI
million
years
Argyroxiphium sandwicense
HAWAII
0.4
million
years
Dubautia waialealae
Dubautia scabra
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Dubautia linearis
Mammalian adaptive radiation
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Evolution is not goal oriented
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Evolution is not goal oriented
• Evolution is like tinkering—it is a process in
which new forms arise by the slight
modification of existing forms
• Leads to species that are adapted to a
specific environment
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