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Option D: Evolution
D
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Origin of life on Earth
Problems for starting life on Earth
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•
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Life is based on organic molecules
Organisms are organized
Need for reproduction
Water tends to depolymerize molecules

Miller and Urey
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Ground-breaking experiment in 1953:
◦ Reproduction of the Hadean environment in a glass sphere:
▪ Introduction of gases (e
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methane, hydrogen, ammonia)
▪ Water cycle
▪ Warm temperature
▪ Exposure to UV radiation (since there initially was no ozone layer)
▪ Electric sparks (representing lightning)
→ After a week: organic compounds formed = base for life

Could comets have brought organic compounds to Earth?
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Comets can carry organic compounds
Earth was bombarded by a shower of comets about 4 billion years ago
Impact of crashing on Earth's surface could help polymerize certain amino acids into
polypeptides

In what environments could organic compounds have formed?
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In space
◦ Spectral lines contain glycine (= simplest amino acid)
→ Organic molecules can probably form in space

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In alternating wet/dry conditions
◦ Drying of clay particles could have created catalysing reactions and organic molecules

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Near volcanoes
◦ Volcanic eruption spews out water vapour, other gases and minerals which are needed to
form organic matter
◦ Rich sources of raw materials and warmth → favourable conditions for formation of
amino acids and sugars

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In deep oceans
◦ Hydrothermal vents (= underwater geysers) are home to communities, despite being
deprived of sunlight

1

RNA's Role in early life
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•
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Most organisms today store their genetic code in form of DNA
DNA replication needs enzymes
Enzymes were not present in the prebiotic world → DNA not the means of inheritance

• In some conditions: single-strand RNA can replicate without enzymes
→ probable former means of inheritance
• RNA can act as enzyme = ribozyme

Protobionts
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•
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When clay dries out and is heated → up to 200 amino acids can spontaneously join together
as polypeptide chains
These chains can form proteinoid microspheres
In combination with other forming polymers → chemistry can be maintained which is
different from the surroundings
Coacervate (microscopic sphere) can form from lipids in water (selectively permeable)
→ form spontaneously due to hydrophobic forces between water and lipid molecules
Protobionts = significant step to the formation of cells

Where did all the oxygen come from?
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4 billion years ago there was no oxygen in the atmosphere → early life forms were
anaerobic cells

The endosymbiotic theory
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


First eukaryotic cells were formed when larger prokaryotic cells ingested, but did not digest,
smaller prokaryotic cells
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2
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g
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g
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3
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foramen

Dorsal position

Central position

B
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Canine teeth

Long and pointed

Short and less pointed

D
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Superciliary arches

Protruding

Flat

F
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jaws

Large and wide

Small and thin

Possible coexistence of several hominid species
Time-wise:
 A
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africanus
 H
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neanderthalensis and H
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afarensis and A
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neanderthalensis and H
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A
lot of intermediate species are missing in fossil records
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There are only a few bones for each fossil
because conditions for fossilization are rare
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A characteristic from one fossil (one individual) is not necessarily representative for the
species
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Estimations are
imprecise if we only have one fossil
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Changes in brain size during hominid evolution
Bigger brain  more energy needed
Bigger brain  better techniques for development of technology, hunting, etc
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g

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