Chapter 22
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Transcript of Chapter 22
Chapter 22
The Origin and History of LifeEarly Evolution
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• 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
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Proposed Stages of Life’s Origins4. Polymers enclosed in
membranes acquired cellular properties
1. Nucleotides and amino acids were produced prior to the existence of cells
2. Nucleotides polymerized to form DNA and RNA, amino acids polymerized to form proteins
3. Polymers became enclosed in membranes protobionts
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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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ElectrodesElectrical discharge
GasesH2OH2CH4NH3
To vacuum Cold water
Condenser
TrapBoiling water
Sample containingorganic moleculessuch as amino acids
Precipitatingdroplets
Miller-Urey experiment
Products included amino acids, purines, pyrimidines
Reducing atmosphere thought to be like early earth
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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
Reaction: Condensation
or Dehydration Synthesis
Monomers are joined to form long chains. Sugars form carbohydrates, amino acids form proteins,
and nucleotides form nucleic acids.
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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– 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 RNA11
57 µm
200 nm
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Stage 4: RNA world
• Most scientists favor RNA as the first macromolecule of protobionts
• Important RNA functions1. Ability to store information2. Capacity for self-replication3. Enzymatic function – ribozymes
• DNA and proteins do not have all 3 functions
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• 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
Advantages of DNA/RNA/protein world
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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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1.8144
354 Devonian
Silurian
Ordovician
Cambrian
4,550
3,800
3,400
3,000
2,500
1,600
543
900
248
443
0
Permian
Triassic
Jurassic
Cretaceous
Quaternary
Tertiary
CarboniferousH
ad
ean
Arc
ha
ean
Pro
tero
zoic
PR
EC
AM
BR
IAN
Ph
ane
rozo
ic
MYA
Late
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
History of life on Earth
Eons
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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 yearsOne 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• 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
March 18th in the evolutionary year
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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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• 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
The First Eukaryotes July 10th in the evolutionary year
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1.5 BYA 1.0 BYA2 BYA2.5 BYA3 BYA3.5 BYA
Aerobicprokaryote
Photosyntheticprokaryote
Infolding ofplasmamembrane
Endosymbioticorigin of
mitochondria Endosymbioticorigin of
chloroplastsOrigin ofProkaryotes
Origin ofEukaryotes
Endosymbiosis Hypothesis
"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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• Also during the Proterzoic Eon a wave of diversification occurred when multicellularity evolved at ~1.5 bya
• Comparisons of DNA sequences give evidence that multicellular ancestors gave rise to algae, plants, fungi, and animals
The Origin of Multicellularity
unicellular alga 8 identical cells 64+ cells, two types 1,000+ cells, 2 types
August 30th in the evolutionary year
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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)
– The Cambrian explosion provides the first evidence of predator-prey interactions
November 17th in the evolutionary year
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• Arthropods are the most abundant land animals• Tetrapods arrived ~365 mya
– Our species arrived ~170,000 years ago
The Colonization of Land ~500 mya
• Adaptations developed for organisms to live on land
–Plants produced waterproof coating and a vascular system for internal transport –Fungi followed plants
November 20th in the evolutionary year
December 1st in the evolutionary year
Last 23 minutes of December 31st!
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Relative Dating of rock layers and fossils/environments: Which rocks and fossils
are the oldest? Why?
A
B
C
DEF
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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 half-life, 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• 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
extinctions
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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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KAUAI5.1
millionyears OAHU
3.7millionyears
HAWAII0.4
millionyears
1.3millionyears
MAUIMOLOKAI
LANAI Argyroxiphium sandwicense
Dubautia linearisDubautia scabra
Dubautia waialealae
Dubautia laxa
N
Descendants of ancestral tarweed that arrived 5 mya from
North America
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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