earth's history: geological timeline
Earth was formed about 4.5 billion years ago, or bya. Back then, there was no atmosphere, no life, no water. It is believed to have been similar to our Earth's moon, starting out with no colors either. Water came into the picture about 4 bya. It erupted from volcanoes in puffs with oxygen, nitrogen, etc; and eventually formed the atmosphere. Some water also came onto Earth by meteors crashing onto the surface and leaving behind water, since they have ice in them. Life "appeared" on Earth 3.5 bya, back then, they were simple cells, almost like bubbles. Complex cells started to turn up on Earth about 2 billion years ago. A lot of cell life lived in the water. Earth's history is broken up into four eras: Cenozoic is the period we're in now, then comes Mesozoic, Paleozoic, and everything before was the Precambrian Era. In those eras, there are periods. there is a graph below of all the periods and eras.