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The Big Bang is how the universe came together, our Earth is of course part of the universe and formed along with the rest of our Solar System about 4.6 bya. This is where our geologic timescale begins.
Early in the Earth's Precambrian Period it was constantly agitated and bombarded by asteroids. It was without atmosphere, water, or much of anything we have on Earth today other than molten rock.
As the Earth slowly cooled, it became cold enough for Earth's crust to finally form. Water vapor brought from asteroid impacts also condensed and it started to rain.
Before this point, Earth's had an atmosphere that was completely devoid of oxygen. Now that the Earth had water though, the first unicellular organisms started to appear and they eventually performed photosynthesis.
This filled the Earth with oxygen, giving it an ozone layer within its atmosphere which protected form the Sun's rays. It was also cool enough for large land masses such as Pangea to form.
Now in the Paleozoic Era, the first fish and complex organisms are starting to appear. Plants life also diversifies very much. In the Mesozoic Era our first dinosaurs started to appear, as well as our first mammals. We know all of this because of the great Fossil record that these animals left behind. We can also date them knowing the Law of Superposition, which states that older rock layers will be located below new ones. The Law of Superposition also let's us know the age of all the rocks located beneath us including sedimentary, igneous and metamorphic rock.
In the Cenozoic Era, the one we are currently in, mammals are thriving and our atmosphere is 21 percent oxygen. Mammals are very diverse on the planet and it is full of life. It is still changing, as plates are slowly drifting away from each other as convergent and divergent plate boundaries push the continents further and further away from each other.
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