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The Environment of the Earth in the Past
The Earth had large oceans very early in its history. During this time the Earth should have been frozen because the sun was weak at that time. Because there was a large and vigorous ocean, the Earth must have had a large and vigorous atmosphere in place to keep the surface warm. This environment was well suited 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).
In the past the atmosphere of the Earth came from volcanoes, and today volcanoes continue to produce water vapor, carbon dioxide and other gases for the atmosphere.
Over the course of time the composition of the atmosphere has changed significantly. The earliest atmosphere was very rich in carbon dioxide (like the present Mars and Venus). The present atmosphere is 80% Nitrogen and 20% Oxygen. Life on Earth was what transformed the content of the Earth's atmosphere to its present composition.
Over the course of time there have also been significant changes in the Earth's climate. The surface of the Earth has seen extremely high temperatures, as well as extremely low temperatures. Large changes in climate can have serious consequences for life on Earth, which is the main reason we are concerned about global warming.
