Then God said, “Let the waters swarm with fish and other life. Let the skies be filled with birds of every kind.” So God created great sea creatures and every living thing that scurries and swarms in the water, and every sort of bird—each producing offspring of the same kind. And God saw that it was good. Then God blessed them, saying, “Be fruitful and multiply. Let the fish fill the seas, and let the birds multiply on the earth.”(Genesis 1:20-22)
Earth was able to support life only after the planet had cooled enough for a rocky crust to solidify. Once that happened, water vapor from volcanoes condensed in the atmosphere, fell as rain, and collected on the Earth’s surface. Besides water vapor, volcanoes also produced gases rich in the basic ingredients of life: carbon, hydrogen, oxygen, and nitrogen. Toxic gases such as ammonia and methane were common. At this point, Earth's early atmosphere consisted entirely of these volcanic gases, and there was no free oxygen. In the primordial “soup” of the early seas, organic molecules concentrated, formed more complex molecules, and became simple cells.
The transition from complex organic molecules to living cells could have occurred in several environments. 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. These environments contain the chemicals and the source of energy needed to synthesize more complex organic structures. Although scientists have not succeeded in creating life from organic molecules in the laboratory, they have reproduced many of the intermediate steps.
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. Many Archaea now live in hot springs, deep-sea vents, saline water, and other harsh environments. 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. The oldest evidence for life may be 3.5-billion-year-old sedimentary structures from Australia that resemble stromatolites. Stromatolites are created today by living mats of microorganisms (mostly cyanobacteria, or blue-green algae). These primitive organisms trap thin layers of sediment with their sticky filaments and grow upward to get light for photosynthesis. Modern-day examples of stromatolites can be found in waters off Australia, the Bahamas, and Belize.
In the Archean structures, layers similar to those seen in living stromatolites are evident, and secondary structures interpreted as simple filamentous microfossils have been recovered from the layers. The biotic origin of the structures has, however, been questioned. Both the supposed Archean stromatolites and the microfossils may have been produced by inorganic processes. Regardless, uncontested microfossils and chemical traces of life were present at least by 2.7 billion years ago. Stromatolites that were produced by microorganisms are abundant later in the Archean and throughout the Proterozoic. These sedimentary structures, formed by organic processes, provide important evidence of early life. At present, we can say with certainty that life had evolved by 2.7 billion years ago, and possibly as early as 3.5 billion years ago.