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The Environment of the Earth in the Past
Calculations show that the Earth had large oceans very early in its history. During this time the Earth should have been frozen because of the weak luminosity of the sun at that time, nevertheless the fact that there was a large and vigorous ocean suggests that the Earth must have had a large and vigorous atmosphere in place to keep the surface warm. The warm early ocean of Earth was ideal for the development of life. The earliest fossils show that there was life on Earth at least 3.8 billion years ago (see the geologic record for the corresponding epochs of the Earth's history).
The atmosphere of the Earth came from, and continues to come from volcanoes, which produce a great deal of water vapor, carbon dioxide and other gases. Over the course of time the composition of the atmosphere has changed significantly. In particular, the earliest atmosphere was very rich in carbon dioxide (like present Mars and Venus). The present atmosphere is 80% Nitrogen and 20% Oxygen. It was life on Earth which was largely responsible for transforming the content of the Earth's atmosphere to its present composition.
The changes in the atmosphere, as well as the changes to the locations of the continents, have contributed to very significant changes in the climate of the Earth. The surface of the Earth has seen extremely high temperatures, as well as extremely low temperatures. Today's concern about global warming is part of a long history of Earth's variability with regard to climate.
