LIfe`s Beginnings on Earth

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Transcript LIfe`s Beginnings on Earth

CHAPTER 25 CAMPBELL and REECE

Conditions on early Earth made the Origin of Life possible

Macroevolution : evolutionary change above the species level

examples: 1.

2.

3.

emergence of terrestrial vertebrates mass extinctions impact on diversity of life origin of key adaptations like flight in birds

Where did 1st cell come from?

4 main stages could have produced very simple cells: 1.

The abiotic synthesis of small organic molecules 2.

3.

4.

Joining of these small molecules into macromolecules (proteins, nucleic acids) Packaging of these macromolecules into protocells, droplets with membranes that maintained internal chemistry different from their surroundings Origin of self-replicating molecules that eventually made inheritance possible

Synthesis of Organic Cpds on early Earth

Planets of our solar system formed ~ 4.6 billion yrs ago

1 st few hundred million yrs conditions would not have allowed life on Earth

1

st

Atmosphere

Collisions would have vaporized any water preventing seas from forming

Atmosphere thick with gases released from volcanic activity

1st Atmosphere

1920’s: Oparin (Russian chemist) and Haldane (British scientist) each came to conclusion early atmosphere was reducing environment (gain e-) in which organic compounds could have formed from simpler molecules

1

st

Organic Compounds

Energy sources:

Lightning

Thermal energy

Intense UV radiation

Primordial Soup

Haldane had hypothesized the early seas site of 1 st organic compounds

1 st cells

Miller & Urey (Univ. of Chicago) in 1950’s

Tested Oparin & Haldane ‘s premise

Created a reducing atmosphere

Added compounds considered to have been found existing on early earth (hydrogen gas – H2, ammonia – NH3, methane – CH4, and H2O vapor)

  

Miller & Urey

Experiment: In 1953, Stanley Miller set up a closed system to simulate conditions thought to have existed on early Earth 1.

2.

Water mixture in “sea” flask was heated; vapor entered atmosphere flask “Atmosphere” flask contained mix of hydrogen gas, methane, ammonia, and water vapor (believed to mimic early Earth’s atmosphere) 3.

4.

5.

Sparks were discharged to mimic lightning Condenser cooled the “atmosphere”, “raining” water and any dissolved molecules down into sea flask As material cycled through apparatus, Miller periodically collected samples for analysis Results: Miller identified variety of organic molecules common in organisms (including simple molecules like formaldehyde (CH2O) and hydrogen cyanide (HCN) and more complex molecules (like amino acids and hydrocarbons) Conclusion: Organic molecules (a 1st step in the origin of life) may have been synthesized abiotically on early Earth

Miller & Urey’s Experiment

Miller & Urey’s Results

Miller-Urey Experiment Clip

Miller & Urey’s Results

Have been repeated using same or similar ingredients, different recipes for the atmosphere and they also produced organic compounds

Still ?s about amounts of methane, ammonia (was there really enough to make it a reducing environment?)

Some repeated experiment in non-reducing, non oxidizing conditions & still produce organic compounds

Miller-Urey Experiment demonstrates:

1.

Abiotic synthesis of organic molecules is possible under various assumptions about the composition of Earth’s early atmosphere 2.

Meterorites may also have been source of minerals and organic molecules Contain amino acids, lipids, simple sugars, uracil

Murchison Meteorite

Murchison Meteorite

Fell to Earth in so named town in Australia in 1969

large (100 kg) and was quickly retrieved

2010 article published in Scientific American: results of mass spectrometry (separating compounds based on charge & size) have revealed at least 14,000 unique molecules

Abiotic Synthesis of Macromolecules

2009 study showed the abiotic synthesis of RNA monomers can occur spontaneously from simpler precursor molecules

Drip solutions with amino acids (aa) or RNA nucleotides onto hot sand, rock, or clay

polymers of aa & RNA (w/out using enzymes or ribosomes)

Protocells (Protobionts)

Basic characteristics of life : reproduction & metabolism:

So 1 st cells would have had to be able to reproduce which would have required them to have a source of nitrogenous bases, sugars, phosphate groups

Now complex enzymes make this all happen

Vesicles as 1

st

step?

When lipids & other organic molecules added to water

vesicles spontaneously form

lipid bilayer (separation of hydrophilic & hydrophobic molecules)

These abiotically produced vesicles “reproduce” and grow on their own.

Clay, like from early Earth will be absorbed into the vesicles

some vesicles demonstrate semi permeability

Self-Replicating RNA

RNA (when folded)can act as enzyme

RNA catalysts called: ribozymes

Some can make complimentary strands of short pieces of RNA

mutations

more stable &/or successful

Ribozyme

Once self-replicating RNA possible much easier for further changes to happen.

Once double-stranded DNA appeared it would have been more stable so RNA left with role we see today