... not a significant amount of free oxygen in the atmosphere. According ("Evolution," Microsoft ® Encarta ® Encyclopedia 99. © 1993-1998 Microsoft Corporation) experiments have shown that complicated organic molecules, including amino acids, can occur spontaneously under certain conditions believed to simulate the earth's primitive environment. Other experiments indicate that concentration of organic molecules may have led to the synthesis of complex ... -celled organisms were formed. At that time, free atmospheric oxygen and food and plant supplies became more stable. About 700 and 570 ...

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