It is believed that the first organisms to appear on Earth resembled singled-celled life forms known as Archaea, which are similar to bacteria but lack nuclei. Modern-day Archaea can live in extreme environments such as hot springs and deep-sea vents. These conditions replicate some of those on early Earth.
Earth in its infancy was a molten, hostile world that was incapable of supporting life. Only when the planet's surface cooled and water vapor from volcanoes condensed as rain in the atmosphere could life begin to develop. Volcanoes are thought to have provided the most basic components of life, including hydrogen, oxygen, carbon and nitrogen.