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The Precambrian era envelopes the major bulk of the history of the Earth, beginning from the creation of the planet approx 4.5 billion years ago and terminating with the origination of composite, multicelled forms of life approximately 4 billion years after.
The Precambrian refers to the earliest of the geologic ages that are signified by the distinct layers of sedimentary rock. The Earth was about more than six hundred million years old when life started. The planet had cooled down from its native molten state, creating a solid crust and oceans formed by water vapor in the atmosphere.
At about three billion years ago, the atmosphere of the Earth was virtually devoid of oxygen. At approximately 2.4 billion years ago, oxygen was discharged from the seas as a waste product of photosynthesis by cyanobacteria. The levels of the gas slowly raised, attaining about 1 percent around two billion years ago.
Approximately 800 million years ago, the levels of oxygen attained about 21 percent and started to breathe life into more composite species. The oxygen-rich ozone layer was also created, protecting the surface of the Earth from the harmful solar radiation.
Answer:
1. Earth's ozone layer is responsible for absorbing infrared and ultraviolet rays from the sun.
2. Changes in the composition of the atmosphere have caused gradual changes in earth's climate, causing changes in plant and animal life that contributed to mass extinctions.
3. Chemical chlorofluorocarbons (CFCs) have been banned and replaced with hydrochlorofluorocarbons (HCFCs).
4. Naturally occurring volcanic eruptions can produce large amounts of gases which contain sulfur. These gases accumulate in the atmosphere in the same way as man-made sulphuroxides.
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Explanation:
