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17-2 Earth's Early History
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Earth is the only known planet known to support life!
Conditions Necessary for Life:
1.Liquid water
2.A moderate temperature
3.Free oxygen in atmosphere
4.Sunlight
5.Absence of toxic substances in atmosphere
6.Absence of lethal radiationCopyright Pearson Prentice Hall
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17-2 Earth's Early History
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Copyright Pearson Prentice Hall
Formation of Earth
Formation of Earth
• Earth is made mostly of rock!
• High temps on Earth caused most rock to melt and separate.
• Dense materials (iron and nickel) sank and formed the core.
• Lighter materials settled above the core forming the mantle and crust.
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The active volcanoes on Earth released water vapor which built up in the atmosphere.
The water vapor condensed and fell back to Earth as rain.
Over millions of years, the liquid water collected in depressions on Earth’s rocky surface and formed oceans.
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In addition to water vapor, the erupting volcanoes released other gases forming Earth’s early atmosphere (gases that surround Earth).
Formation of Earth
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Formation of Earth
These gases were methane, hydrogen, nitrogen, ammonia, carbon dioxide, and carbon monoxide.
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Today’s atmosphere contains free oxygen (oxygen gas that is not combined with other elements) and a layer of ozone that protects living things from harmful radiation.
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The First Organic Molecules
The Beginning of Life on Earth
The lack of free oxygen in Earth’s early atmosphere would have prevented the survival of most modern organisms so…..
In the 1950s, Stanley Miller and Harold Urey tried to model early Earth Conditions.
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The First Organic Molecules
Miller and Urey's experiments suggested how mixtures of organic compounds needed for life could have formed from simpler compounds present on an early Earth.
Their experiment showed that amino acids could form from matter present on early Earth.
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The First Organic Molecules
Miller and Urey’s Experiment
Mixture of gases simulatingatmosphere of early Earth
Condensationchamber
Spark simulatinglightning storms
Watervapor
Liquid containing amino acids and other organiccompounds
Cold water cools chamber, causing droplets to form.
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Free Oxygen
Primitive Organisms and the Endosymbiotic Theory
• Fossil evidence indicates that the first organisms appeared in Earth’s oceans about 3.5 billion years ago
• These organisms formed without Oxygen
• They were prokaryotes!
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Fossils showed that cyanobacteria (prokaryotes that make food by photosynthesis) became common in shallow areas of Earth’s oceans about 3 billion years ago.
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Photosynthesis by cyanobacteria gradually increased the amount of oxygen in the atmosphere creating free oxygen.
This free oxygen bred life to more complex, oxygen breathing life forms!
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Origin of Eukaryotic Cells
What hypothesis explains the origin of eukaryotic cells?
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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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Origin of Eukaryotic Cells
Endosymbiotic Theory
Mitochondrion
Aerobicbacteria
Nuclear envelopeevolving
Ancient Prokaryotes
Plants and plantlike protists
Primitive PhotosyntheticEukaryote
Primitive AerobicEukaryote
Ancient AnaerobicProkaryote
Chloroplast
Animals, fungi, and
non-plantlike protists
Photosynthetic bacteria
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Early Eukaryotes
• The first eukaryotes evolved around 2 billion years ago.
• Included algae.
• Photosynthesis by algae added more oxygen to the atmosphere.
• Sexual reproduction evolved around 1.2 billion years ago
• Life became much more varied and complex about 540 million years ago.
• This event is known as the Cambrian Explosion!
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• All the major animal groups first appeared during the Cambrian Period.
• Fossil evidence shows plant life started around 480 million years ago.
• Land plants are the major source of atmospheric oxygen today.
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Order of life on EarthAnaerobic Prokaryotes
Photosynthetic Prokaryotes
Unicellular Eukaryotes
Multicellular Eukaryotes
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