The Precambrian Eon began 4,600,000,000 years ago and ended 543,000,000 years ago and is known as Evolution's Dark Ages. The climate, which started 2,400,000,000 years ago when oxygen formed out of the sea, varied greatly during the eon. When the climate was warm, there were no glaciers at the poles. When the climate was cold, there were glaciers everywhere including the equator. The climate shifted from being stable to unstable throughout the entire eon. The changes in climate caused the shape of the continents to change during the eon. When they first formed, they were a twenty-sided Supercontinent centered between today’s equator and prime meridian. The continents then broke apart, shifted and reformed to become the Rodinia Supercontinent 1,100,000,000 years ago. The Rodinia Supercontinent stayed the same throughout the remainder of the Precambrian Eon.
During the Precambrian Eon, many types of modern life that we know started to form. First, Earth started out with living cells. The living cells formed 3,800,000,000 years ago out of the oceans. They then started to form marine invertebrates that later evolved into today’s fish and jellyfish. When they were complex enough to actually swim, they inhabited many rocks and became hard shelled invertebrates that may have evolved into today’s snails and sea snails. The hard shelled marine invertebrates ruled the oceans for the rest of the Precambrian Eon. Plant life boomed also during the Precambrian Eon. In the warm periods of time, algae formed at the bottom of the ocean. The algae soon became dominant and after thousands of years, reached land and started the process that we know as photosynthesis. The oxygen produced by photosynthesis helped animals transition to land at the beginning of the Paleozoic Era. After all of the evolution, the unfortunate mass extinction hit. The sea levels rose in some areas, and decreased in some areas. This caused oxygen depletion, and the extinction of most living species. An estimated 250,000 species became extinct and 95% of them were hard shelled marine invertebrates. The Precambrian Extinction marked the end of the Precambrian Eon.