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17-2 Earth's Early History
Photo credit: Jackie Beckett/American Museum of Natural History
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Formation of Earth
Formation of Earth
Hypotheses about Earth’s early history are based on a relatively small amount of evidence.
Gaps and uncertainties make it likely that scientific ideas about the origin of life will change.
Evidence shows Earth was not “born” in one event.
Pieces of cosmic debris were probably attracted to one another over the course of _____________years.
When Earth was young, it was struck by one or more objects, producing enough heat to________the entire globe.
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Formation of Earth
Once Earth melted, its elements rearranged themselves according to____________.
The most dense elements formed the planet’s_____.
Moderately dense elements floated to the surface, cooled, and formed a ___________________.
The least dense elements formed the first __________________.
Earth's early atmosphere probably contained hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water.
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Formation of Earth
Scientists infer that about 4 billion years ago, Earth cooled and solid rocks formed on its surface.
For millions of years later, __________activity shook Earth’s crust.
Comets and asteroids bombarded its surface.
About 3.8 billion years ago, Earth’s surface cooled enough for water to remain a_________, and oceans covered much of the surface.
Those primitive oceans were_________because they contained lots of dissolved iron.
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5. The First Organic Molecules
Could organic molecules have evolved under conditions on early Earth?
In the 1950s, Stanley Miller and Harold Urey tried to answer that question by simulating conditions on the early Earth in a laboratory setting.
They filled a flask with hydrogen, methane, ammonia, and water to represent the__________and they made certain no microorganisms could contaminate results.
Then they passed electric sparks through the mixture to simulate lightning.
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6. The First Organic Molecules
Miller and Urey’s Experiment
Mixture of gases simulating
atmosphere of early Earth
Spark simulating
lightning storms
Condensation
chamber
Water
vapor
Cold water cools chamber, causing droplets to form.
Miller and Urey produced amino acids, which are needed to make proteins, by passing sparks through a mixture of hydrogen, methane, ammonia, and water. This and other experiments suggested how simple compounds found on the early Earth could have combined to form the organic compounds needed for life.
Liquid containing amino acids and other organic
compounds
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7. The First Organic Molecules
The results were spectacular: over a few days, several ____________-the building blocks of proteins-began to accumulate.
Miller & Urey's experiments suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on a primitive Earth.
Although their simulations of early Earth were not accurate, experiments with current knowledge yielded similar results.
One of Miller’s experiments in 1995 produced cytosine and uracil!
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8. The Puzzle of Life's Origin
Evidence suggests that 200–300 million years after Earth had liquid water, cells similar to modern bacteria were common.
Formation of Microspheres 
In certain conditions, large organic molecules form tiny bubbles called________________microspheres.
Microspheres are not cells, but they have selectively permeable membranes and store and release______.
Hypotheses suggest that structures similar to microspheres might have acquired more characteristics of living cells.
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9. The Puzzle of Life's Origin
Evolution of RNA and DNA 
How could DNA and RNA have evolved? Several hypotheses suggest:
Some RNA sequences can help DNA_________ under the right conditions.
Some RNA molecules can even grow and duplicate themselves suggesting RNA might have existed___________DNA.
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10. The Puzzle of Life's Origin
Proteins build cell
structures and catalyze
chemical reactions
RNA and the Origin of Life
RNA nucleotides
Simple organic
molecules
RNA helps in
protein synthesis
Abiotic “stew” of
inorganic matter
One hypothesis about the origin of life, illustrated here, suggests that RNA could have evolved before DNA. Scientists have not yet demonstrated the later stages of this process in a laboratory setting. 
RNA able to replicate itself,
synthesize proteins, and
function in information
storage
DNA functions in
information storage
and retrieval
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Free Oxygen
Free Oxygen
Microscopic fossils, or______________, of unicellular prokaryotic organisms resembling modern bacteria have been found in rocks over 3.5 billion years old.
These first life-forms evolved without_____________.
About 2.2 billion years ago, photosynthetic bacteria began to pump oxygen into the oceans.
Next, oxygen gas accumulated in the atmosphere.
Concentrations of methane and hydrogen sulfide began to decrease, the_____________began to form, and the skies turned from a pink-orange to the present shade of blue.
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Free Oxygen
The increase in oxygen, a highly reactive gas, most likely created the first global “_____________” crisis.
To the first cells, oxygen was a _________________.
The rise of oxygen in the atmosphere drove some life forms to extinction, while other life forms evolved new, more efficient metabolic pathways that used oxygen for respiration.
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13. Origin of Eukaryotic Cells
The Endosymbiotic Theory
About 2 billion years ago, prokaryotic cells began evolving internal cell membranes. The result was the ancestor of all eukaryotic cells.
According to the endosymbiotic theory, eukaryotic cells formed from a_____________among several different _____________________.
The endosymbiotic theory proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms.
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14. Origin of Eukaryotic Cells
One group of prokaryotes had the ability to use oxygen to generate molecules of ________.
These evolved into______________that are now in the cells of all multicellular organisms.
Other prokaryotes that carried out photosynthesis evolved into the_______________of plants and algae.
This hypothesis was proposed more than a century ago, when microscopists saw that the membranes of mitochondria and chloroplasts resembled the plasma membranes of prokaryotes.
Until the_________, the endosymbiotic theory did not receive much support.
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15. Origin of Eukaryotic Cells
Endosymbiotic Theory
Ancient Prokaryotes
Chloroplast
Plants and plantlike protists
Aerobic
bacteria
Photosynthetic bacteria
Nuclear envelope
evolving
Mitochondrion
Primitive Photosynthetic
Eukaryote
The endosymbiotic theory proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms. Ancient prokaryotes may have entered primitive eukaryotic cells and remained there as organelles.
Animals, fungi, and
non-plantlike protists
Ancient Anaerobic
Prokaryote
Primitive Aerobic
Eukaryote
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16. Origin of Eukaryotic Cells
The Evidence
Lynn__________and her supporters built an argument for the endosymbiotic theory on several pieces of evidence including:
Mitochondria and chloroplasts contain DNA similar to bacterial DNA.
Mitochondria and chloroplasts have_____________ whose size and structure closely resemble those of bacteria.
Mitochondria and chloroplasts reproduce by_______ __________, like bacteria.
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17. Sexual Reproduction and Multicellularity
Most prokaryotes reproduce asexually. Asexual reproduction:
yields daughter cells that are_____________of the parent cell.
restricts genetic variation to_________in DNA.
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18. Sexual Reproduction and Multicellularity
Sexual reproduction shuffles genes in each generation.
In sexual reproduction:
offspring never resemble parents exactly
there is an increased probability that favorable combinations will be produced
there is an increased chance of evolutionary change due to _____________________
A few hundred million years later, evolving life forms developed___________________organisms.
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