Other forms of early life had been producing waste products such as iron, which built up in the early ocean. As oxygen began to be produced, a peculiar thing happened. Large amounts of iron which had accumulated in the early ocean were attacked by the accumulating oxygen. When oxygen reacts with iron, iron ores are produced. Today, iron ores are taken out of the ground by miners, and the iron they contain is used by human beings to make lots of things.
For a billion years, the oxygen produced by early plant life attacked leftover iron in the ocean, and huge deposits of iron ores were laid down at the bottom of the sea. This activity took place between 3.5 and 2.5 billion years ago. Iron ores mined today in the United States, Australia, and South Africa, are part of the huge deposits laid down at that time. Once the oceans were swept clean of iron, then the oxygen could begin to build up in the atmosphere, and more sophisticated life forms could develop. But the advent of more sophisticated life had to wait for another era, the Proterozoic. It took a billion years for the iron ore process to complete. When it was finished, it closed the period in the history of the Earth which we call the Archean.
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