These primitive organisms trap thin layers of sediment with their sticky filaments and grow upward to get light for photosynthesis. Stromatolites are created today by living mats of microorganisms (mostly cyanobacteria, or blue-green algae). The oldest evidence for life may be 3.5-billion-year-old sedimentary structures from Australia that resemble stromatolites. If the first organisms resembled modern Archaea, they also may have lived in such places, but direct evidence for early life is controversial because it is difficult to distinguish between complex inorganic structures and simple biological ones in the geologic record. Many Archaea now live in hot springs, deep-sea vents, saline water, and other harsh environments. So what were the first living things and when did they appear? Studies of genetic material indicate that a living group of single-celled organisms called Archaea may share many features with early life on Earth. Although scientists have not succeeded in creating life from organic molecules in the laboratory, they have reproduced many of the intermediate steps. These environments contain the chemicals and the source of energy needed to synthesize more complex organic structures. Small, warm ponds are one possibility, but recent work has suggested that deep-sea hydrothermal vents, such as those found along mid-ocean spreading centers today, may have been the cradle of Earth's life. The transition from complex organic molecules to living cells could have occurred in several environments.

In the primordial “soup” of the early seas, organic molecules concentrated, formed more complex molecules, and became simple cells. At this point, Earth's early atmosphere consisted entirely of these volcanic gases, and there was no free oxygen. Toxic gases such as ammonia and methane were common.

Besides water vapor, volcanoes also produced gases rich in the basic ingredients of life: carbon, hydrogen, oxygen, and nitrogen. Once that happened, water vapor from volcanoes condensed in the atmosphere, fell as rain, and collected on the Earth’s surface. Earth was able to support life only after the planet had cooled enough for a rocky crust to solidify.

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