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Comment: 10:50 - 12:11 (01:21)
Source: Annenberg/CPB Resources - Earth Revealed - 11. Evolution Through Time
Keywords: "Dee Trent", life, atmosphere, oxygen, photosynthesis, stromatolite, algae, ozone, "carbon dioxide", "solar radiation", Precambrian, Paleozoic, limestone, ocean, horse, camel, humans, sycamore, "carbon cycle"

Our transcription: The first appearance of life out of the seas was possible in large part because the planet's atmosphere had become oxygen rich due to photosynthesis of stromatolitic algae.
As oxygen built up, an ozone layer formed high in the atmosphere shielding the Earth's surface from deadly solar radiation.
Although this process was actually well underway in the Precambrian, it was not until the mid Paleozoic that life was sufficiently developed to take advantage of the dry land habitats the ozone layer protected.
At the same time oxygen increased in the atmosphere, another atmospheric gas, carbon dioxide, decreased because carbon dioxide had become an important building block of life.
And a lot of the carbon dioxide is trapped in organisms in seawater and actually eventually becomes limes oozes on the sea floor, which may eventually become limestones.
Organisms are very much a part of the control of our environment.
The plants take in the carbon dioxide and give off oxygen.
The plants, the limestones, the ocean, the atmosphere, human beings, horses and camels, the sycamore tree outside are all interrelated in a big cycle called the "carbon cycle."

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