"What evidence exists for the assumption made by Stanley Miller that the atmosphere of early Earth was reducing and not oxidizing?"
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What were the earliest forms of life on earth?
We don't know, because we have almost no surviving rock from earlier than 4.0 billion years ago, at the time life was forming on Earth. Most astrobiologists think that the earliest life forms have not survived to today. They may have been similar to viruses, and there is also evidence suggesting the existence of a "RNA World" before there were life forms that used DNA to reproduce and regulate their chemical processes. By about 3.9 billion years ago there is an indication from the chemistry of the rocks formed then that life might have been extensive in those ancient oceans. The oldest reliably identified fossils date from about 3.4 billion years, and many astrobiologists think these were probably similar to the cyanobacteria that we have today. For an entertaining account of the search for evidence of this ancient life, I recommend the book by J. William Schopf called "Cradle of Life: The Discovery of Earth's Oldest Fossils." David Morrison
NAI Senior Scientist
March 28, 2007
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