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Formation of Earth
What substances made up Earth's early atmosphere?
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Formation of Earth
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 four billion years ago, Earth cooled and solid rocks formed on its surface.
Millions of years later, volcanic activity shook Earth’s crust.
About 3.8 billion years ago, Earth’s surface cooled enough for water to remain a liquid, and oceans covered much of the surface.
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4. 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.
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5. The First Organic Molecules
What did Miller and Urey's experiments show?
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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
Miller and 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.
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8. The Puzzle of Life's Origins
Evidence suggests that 200–300 million years after Earth had liquid water, cells similar to modern bacteria were common.
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Free Oxygen
Free Oxygen
Microscopic fossils, or microfossils, of unicellular prokaryotic organisms resembling modern bacteria have been found in rocks over 3.5 billion years old.
These first life-forms evolved without oxygen.
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Free Oxygen
About 2.2 billion years ago, photosynthetic bacteria began to pump oxygen into the oceans.
Next, oxygen gas accumulated in the atmosphere.
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Free Oxygen
What occurred when oxygen was added to Earth's atmosphere?
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Free Oxygen
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
What hypothesis explains the origin of eukaryotic cells?
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14. Origin of Eukaryotic Cells
The Endosymbiotic Theory
The endosymbiotic theory proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms.
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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
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 symbiosis among several different prokaryotes.
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17. Origin of Eukaryotic Cells
Aerobic
bacteria
Ancient Prokaryotes
Nuclear envelope
evolving
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.
Ancient Anaerobic Prokaryote
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18. Origin of Eukaryotic Cells
Prokaryotes that use oxygen to generate energy-rich molecules of ATP evolved into mitochondria.
Mitochondrion
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.
Primitive Aerobic Eukaryote
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19. Origin of Eukaryotic Cells
Prokaryotes that carried out photosynthesis evolved into chloroplasts.
Chloroplast
Photosynthetic bacteria
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.
Primitive Photosynthetic Eukaryote
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