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