AP BIOLOGYTHE HISTORY of EARTH

CHAPTER 25CAMPBELL and REECE

Conditions on early Earth made the Origin of Life possibleMacroevolution : evolutionary change

above the species levelexamples:1. emergence of terrestrial vertebrates2. mass extinctions impact on diversity of

life3. 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

molecules2. Joining of these small molecules into

macromolecules (proteins, nucleic acids)3. Packaging of these macromolecules into

protocells, droplets with membranes that maintained internal chemistry different from their surroundings

4. Origin of self-replicating molecules that eventually made inheritance possible

Synthesis of Organic Cpds on early EarthPlanets of our

solar system formed ~ 4.6 billion yrs ago

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

1st AtmosphereCollisions would

have vaporized any water preventing seas from forming

Atmosphere thick with gases released from volcanic activity

1st Atmosphere1920’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

1st Organic CompoundsEnergy sources:

LightningThermal energyIntense UV

radiation

Primordial SoupHaldane had hypothesized the early seas

site of 1st organic compounds 1st cellsMiller & Urey (Univ. of Chicago) in

1950’s Tested Oparin & Haldane ‘s premiseCreated a reducing atmosphereAdded compounds considered to have been

found existing on early earth (hydrogen gas – H2, ammonia – NH3, methane – CH4, and H2O vapor)

Miller & UreyExperiment: In 1953, Stanley Miller set up a closed system to

simulate conditions thought to have existed on early Earth1. Water mixture in “sea” flask was heated; vapor entered atmosphere

flask2. “Atmosphere” flask contained mix of hydrogen gas, methane, ammonia,

and water vapor (believed to mimic early Earth’s atmosphere)3. Sparks were discharged to mimic lightning 4. Condenser cooled the “atmosphere”, “raining” water and any

dissolved molecules down into sea flask5. As material cycled through apparatus, Miller periodically collected

samples for analysisResults: 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 ResultsHave 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 MeteoriteFell to Earth in so named town in

Australia in 1969large (100 kg) and was quickly retrieved2010 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 1st 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 1st step?When lipids & other organic molecules added

to water vesicles spontaneously formlipid 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 RNARNA (when folded)can act

as enzymeRNA catalysts called: ribozymesSome can make complimentary strands of short pieces of RNA mutations more stable &/or successful

RibozymeOnce 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