Notes on Geologic Periods of the Earth

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Precambrian
Geologic
The Precambrian era is about 90% of the earth’s geologic life. It refers to all geologic time before 600 million years ago. During the Precambrian era, the earth formed along with the oceans and atmosphere. Originally, the earth was in a molten state, but as it cooled down, it developed a hard crust and oceans that developed water vapor to form an atmosphere. About five hundred million years after the earth was formed, small continents started to form. The plate tectonics also started to build up. As the earth grew older within this period, the layers of the earth started to form. The continents were thought to be joined together in one super continent. It is thought that the end of the Precambrian era was the start of a global ice age.
Biologic
The earth was more than six hundred million years old when the first signs of life started to show. The planet had cooled down from its molten state enough to support life. Scientists believe that the water was the reason that the earth formed its first life. The earliest living organisms were microscopic bacteria. There is fossil evidence of them from as early as 3.4 billion years ago. The first mulitcelled organisms have fossil evidence that dates back to about six hundred million years ago. The main forms of life consisted of sponges, cnidarians, and annelids.

Paleozoic
Cambrian
Geologic
The Cambrian time period was the first in the Paleozoic era. It lasted about fifty-three million years. As the period started, the continents started to pull apart. Land masses were scattered. During this time period the oceans started to oxygenate. The Cambrian was thought to be in the middle of two ice ages; however, there were no significant ice formations during this tim...

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...lision, sea levels were lower. The poles began to gain ice caps. Due to all the ice, the sea levels dropped even more. With the sea levels decreasing, there was more land between continents for animals to travel across. Due to the continually cooling climates, the earth entered an Ice Age. The lower sea levels and new mountain ranges contributed to it. There were a lot more ice caps than there are today.
Biologic
Due to the cooling climates, the forests began to transform into grasslands. New predators began evolving to be fit to chase animals in the grasslands. Cheetahs and leopards were among those animals. They had to adapt or starve. There was also new plant life in the oceans. Kelp began to grow in cooler waters. Sharks also began to develop new species. One species grew to be about fifty feet long. However, it became extinct about 1.6 million years ago.

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