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There was almost no atmosphere. The surface was molten and, as Earth cooled, an atmosphere formed mainly from gases that spewed from volcanoes. It included hydrogen sulfide, methane, and 10 to 200 times as much of carbon dioxide as today’s atmosphere.

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There were methane droplets in the air that covered Earth in a global haze. There was no oxygen gas on Earth as oxygen was only in compounds such as water. There were greenhouse gases in the atmosphere, such as methane and carbon dioxide that were able to trap enough of the sun’s heat to keep temperatures above freezing.

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There were tiny photosynthetic organisms that produced enough oxygen to react with the methane in the atmosphere,  causing it to transform permanently. About two billion years ago, the methane haze cleared and the sky turned blue. After photosynthetic bacteria had been making oxygen for about 300 million years, there was enough oxygen built up in the atmosphere to oxidize the iron in rocks and soil to make reddish iron oxide and create vast expanses of red beds.

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Plants had become firmly established on land, and life once again had a major impact on the Earth’s atmosphere. Oxygen made up 20 percent of the atmosphere—about today’s level—around 350 million years ago, and it rose to as much as 35 percent over the next 50 million years. As great masses of dead plants became buried under swamps and out of contact with oxygen, the level of carbon dioxide in the atmosphere actually ended up dropping.

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Earth was much warmer than it is in the present day. Scientists additionally understand that changes in the atmosphere affect Earth’s climate. Between 200 and 45 million years ago polar ice caps were small or absent, and winters were warmer around the globe. In these warm climates, many types of plants and animals lived in the polar regions.