Life is valuable to all of us, but how it began is not something many people are interested in discussing. This is a brief summary of one version the history of life on Earth, agreed on by most biologists.
The Earth's crust formed about 4.5 billion years ago. There are various theories as to what it was that caused the first prokaryotic cells to form 700 million years later. Unfortunately, Scientists have not agreed on what it was. Roughly two billion years later, these prokaryotic cells combined to form the first eukaryotic cells.
The Cambrian period started about 543 million years ago, algae diversified and there was a huge evolutionary radiation among our first ancestors. During the Ordovician period (510-440 mya) marine organisms radiated all over the globe, mosses and horsetails colonized wet terrestrial environments. As the Ordovician period came to a close, major environmental changes caused widespread glacial formation, which, in turn, caused 75% of all marine species to become extinct.
During the Silurian period (440-409 mya), marine life bounced back after the major extinction event at the end of the Ordovician. The first terrestrial arthropods appeared during this period. During the Devonian period (409-354 mya), rates of evolutionary change accelerated, the first forests appeared and the Northern and Southern continents slowly moved northward. At the end of the Devonian there was another 75% mass extinction of all marine life due to climatic circumstances. The Carboniferous (354-290 mya) brought large amounts of glacial formation on Gondwana. Terrestrial animals diversified and some of the first reptiles appeared during Carboniferous period. During the Permian period (290-245 mya), all of the continents came together to form the super-continent Pangaea. Volcanic activity blocked out the sun's rays, causing a massive ice age. At the end of the Permian period, massive lava flows...