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 weaker than today. Because there was a large ocean, the Earth must have had a large atmosphere in place to keep the surface warm. The warm early ocean was a friendly place for life to develop.
In the past the atmosphere of the Earth came from volcanoes, and today volcanoes continue to produce water vapor and other gases for the atmosphere.
Over time the atmosphere of Earth has changed. Life on Earth was what transformed the content of the Earth's atmosphere to it's present composition. Over time there has also been big changes in the climate (temperature and weather) of the Earth. We have seen extremely high temperatures, and extremely low temperatures. Scientists are concerned about global warming because these changes have the potential to hurt life on Earth.