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Chemical Evolution
Proposes life began with the formation of a self-replicating molecule
Origin-of-life researchers are testing the steps of the theory by simulating conditions found in atmosphere and ocean of the early Earth
Miller’s experiments
Start with simple molecules like CH4, NH3, CO2 and CO
In the presence of ultraviolet radiation or lightning, these simple molecules can form more complex organic molecules like H2C0 and HCN
The results suggest these nonspontaneous reactions could have occurred on ancient Earth
Stopcockfor takingsamples
Water
Heat
Glasstubing
Electrode
Gases
Trap
Water droplets
Condenser
Sparkdischarge
Large glassflask
Figure 3.1
More complex organic molecules
Chemical evolution continues as HCN and H2CO react to form amino acids, sugars and nucleotides
These molecules are the building blocks or monomers needed to make the complex macromolecules found in living organisms
Experiments have shown that amino acids, sugars and purines are readily produced under early Earth conditions
HO
HO
Condensation reaction: monomer in, water out
HO
H
H OH+ Water
H
HMonomer
Monomer
Figure 3.8a
Polymerization of monomers into macromolecules
These condensation reactions occur readily when growing polymers stick to clay particles
Researchers have found polypeptide formation and RNA formation from the polymerization of amino acids and ribonucleic acids, respectively
Nucleotide
Ribose Deoxyribose
P O
O
O–
–O
Phosphategroup
ON
4
3 2
1
5 Nitrogenousbase
5-carbonsugar
O OHH
HOHHH
OHHO5CH2
OH
2C
HH
HOH OH
H
N
O
H
N
NH2
Pyrimidines
NH
O
H
N
O
NH2
NN
N
H
N
Cytosine(C)
Adenine(A)
Uracil(U)
Purines
Thymine(T)
Guanine(G)
H3CNH
N
H
H
N N NH2
NH
ON
C3
4C 1C2CC3
1C4C
HO5CH2
O
O
Figure 3.6a-c
Final step - a self-replicating molecule
A self-replicating molecule must:
• Be able to catalyze polymerization reactions
• Furnish a mechanism for making a copy
Most origin-of-life researchers propose the first self-replicating molecule was RNA
• RNA can catalyze a variety of chemical reactions
• Complementary base pairing provides a mechanism for making a copy
Other candidates for first self-replicating molecule
Proteins are excellent catalysts but are not capable of self-replication
DNA is an excellent template for its own replication but has no catalytic abilities
1. Complementary bases pair.
5´
3´
3´
3´
5´
5´G
G
GC
C
C
U
U
A
A
RNA FORMS A TEMPLATE FOR ITS SYNYTHESIS
Tem
pla
te s
tra
nd C
A
G
U
G
5´
5´
3´
3´
C
G
U
C
A
2. Copied strand polymerizes.
Tem
pla
te s
tra
nd
Co
pie
d s
tran
d5´3´
5´ 3´
3. Copy and template separate.
G
G
C
A
U
G
U
C
A
C
Tem
pla
te s
tra
nd
Co
pie
d s
tran
d
Figure 3.16, left
4. Copy serves as new template.
G
U
G
C
AG
U
C
A
C5´3´
3´5´
3´
5´
3´
5´
Co
pie
d
str
and
= n
ew t
emp
late
5´ 3´
5´3´
C
A
G
U
G
G
U
C
A
C
5. New copy polymerizes.
New
tem
pla
te s
tran
d
New
co
py
stra
nd
6. New copy is identical to original template.
5´3´
3´5´
C
A
C
U
GC
A
G
U
GNew
co
py
stra
nd
New
te
mp
late
str
and
Figure 3.16, right
What Constitutes Life?
First living organism was a cell in which controlled reactions could occur in an enclosed environment, leading to replication of the cell.
Those simple organisms would have been acted on by natural selection
Biological evolution overtakes chemical evolution
The earth’s environment - 4.6 billions years ago until 2.5 billion years ago
The earth’s interior was much hotter and volcanism more frequent
Materials like iron and other unoxidized minerals were brought to the earth’s surface
• These materials were were quickly oxidized
• Little free oxygen remained in the environment
The oldest fossils
Taken from rocks in Greenland 3.8 billion years old
The Greenland fossils appear to be Archaea
Filamentous cyanobacteria - 3.5 billion years old
Fossil bacteria - 3.5 billion years old
Stromatolites
Stromatolites
Work of communities of photosynthetic bacteria
The bacteria secrete a sticky gel that protects them from uv radiation
Gel also causes sediment to stick
Periodically, the bacteria have to creep outward to be exposed to sufficient sunlight
The result is a cabbage-like formation
Stromatolites
The earth’s environment - beginning 2.5 billion years ago
Heat production tapered off
Crustal movements slowed and larger land masses began to form and persist
Shallow seas spread over these continental expanses, providing habitat for photosynthetic cyanobacteria
These bacteria relentlessly pumped out free oxgyen
The result of oxygen availability was that the earth rusted, visible in banded iron formations
Once the iron was all oxidized, oxygen began to accumulate
• first dissolved in water
• then escaping into the atmosphere
These organisms fundamentally changed the earth:
• The earlier atmosphere of methane and hydrogen sulfide was replaced with an atmosphere of oxygen
The oxygen boom drove organisms without oxygen-handling enzymes into anaerobic habitats (stagnant waters, dead organic material, sediments)
Other bacteria evolved the ability to use oxygen to break down food into carbon dioxide and water