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The First Eukaryotes
- 2.1 billion years ago
- endosymbiont theory- one celll engulfed cyanobcteria and then it evolved as a symbiotic relationship
- Key evidence :
- Inner membranes are similar to plasma membranes of prokaryotes
- Division is similar in these organelles and some prokaryotes
- These organelles transcribe and translate their own DNA
- Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes
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Spontaneous generation
- Spontaneous generation is the proposal that living organisms can arise from nonliving matter
- Medieval beliefs reflected the concept of spontaneous generation
- Maggots were thought to arise from meat
- Microbes were thought to arise from broth
- Mice were thought to arise from mixtures of sweaty shirts and wheat
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Conditions on early Earth made the origin of life possible
- 1. Abiotic synthesis of small organic molecules
- 2. Joining of these small molecules into macromolecules
- 3. Packaging of molecules into protocells
- 4. Origin of self-replicating molecules
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Oparin and Haldane
- 1920s, Oparin and Haldane: early atmosphere was a reducing environment
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Miller and Urey
- 1920s, Oparin and Haldane: early atmosphere was a reducing environment
- 1953, Miller and Urey: abiotic synthesis of organic molecules in a reducing atmosphere is possible
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Protocells
- Replication and metabolism are key properties of life
- Protocells may have been fluid-filled vesicles with a membrane-like structure
- lipids and other organic molecules can spontaneously form vesicles
- Adding clay can increase the rate of vesicle formation
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Self-Replicating RNA and the Dawn of Natural Selection
- The first genetic material was probably RNA, not DNA
- ribozymes catalyze many different reactions
- The early genetic material might have formed an “RNA world”
- RNA are single stranded- can form many different shapes
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Figure 25.7 Clock analogy for some key events in Earth’s history.
- Archeans- oldest. Formed prokaryotes. Then photosynthesizing prokaryotes (no membrane or nucleus) introduced oxygen. Evidence- rusty rocks because rust only forms when oxygen is present.
- Evantually single and multicellular eukaryotes (plants) and then animals (cnidarians or other things in water) and then
- Colonization of Land-
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The First Single-Celled Organisms
- The oldest known fossils are stromatolites
- 3.5 billion years ago
- Prokaryotes were Earth’s sole inhabitants from 3.5 to about 2.1 billion years ago
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stromatolites
- The oldest known fossils are stromatolites
- 3.5 billion years ago
- Prokaryotes were Earth’s sole inhabitants from 3.5 to about 2.1 billion years ago
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cyanobacteria
- The early rise in O2 was likely caused by ancient cyanobacteria
- A later increase in the rise of O2 might have been caused by the evolution of eukaryotic cells containing chloroplasts
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endosymbiont theory- one celll engulfed cyanobcteria and then it evolved as a symbiotic relationship
Key evidence :
- Inner membranes are similar to plasma membranes of prokaryotes
- Division is similar in these organelles and some prokaryotes
- These organelles transcribe and translate their own DNA
- Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes
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The Cambrian Explosion
- sudden appearance of fossils resembling modern animal phyla (animals that look like us)
- provides the first evidence of predator-prey interactions
- Fossils in China provide evidence of modern animal phyla tens of millions of years before the Cambrian explosion
- The Chinese fossils suggest that
- “the Cambrian explosion had a long fuse
- (happened over a long period of time) ”
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The Colonization of Land
- Fungi, plants, and animals began to colonize land about 500 million years ago
- Arthropods (can invade air) and tetrapods are the most widespread and diverse land animals
- Tetrapods evolved from lobe-finned fishes around 365 million years ago- has gills and lungs
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Plate Tectonics
- At three points in time, the land masses of Earth have formed a supercontinent
- Earth’s crust is composed of plates floating on Earth’s mantle
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Continental Drift (Pangea)
- Formation of the supercontinent Pangaea about 250 million years ago had many effects
- A deepening of ocean basins (when Eurasia combined there was less water for organisms to live in- colder and drier)
- A reduction in shallow water habitat
- A colder and drier climate inland
- Continental drift has many effects on living organisms
- A continent’s climate can change as it moves north or south
- Separation of land masses can lead to allopatric speciation
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Formation of the supercontinent Pangaea about 250 million years ago had many effects
- A deepening of ocean basins (when Eurasia combined there was less water for organisms to live in- colder and drier)
- A reduction in shallow water habitat
- A colder and drier climate inland
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Continental drift has many effects on living organisms
- A continent’s climate can change as it moves north or south
- Separation of land masses can lead to allopatric speciation
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Mass Extinctions
- The fossil record shows that most species that have ever lived are now extinct
- can be caused by changes to environment
- result of disruptive global environmental changes
- >50% of species became extinct
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A number of factors might have contributed to these extinctions
- Intense volcanism
- Global warming: CO2 from the volcanoes
- Reduced temperature gradient from equator to poles
- Oceanic anoxia (less 02) from reduced mixing of ocean waters
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How Did Humans Evolve?
- The oldest hominin fossils are from Africa
- Hominins: include humans and extinct humanlike primates
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