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