Some forms of bacteria are known as Archaea
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To date, the oldest known life forms are certain types of bacteria that have been found in
Australian rocks that date back 3.5 billion years ago. At this time, oxygen was not present
in the Earth's atmosphere, thus these bacteria must have been able to live without breathing oxygen.
These bacteria have been found within rock itself, miles underneath the surface.
Bacteria such as this give off ammonia as a waste product. Thus one way to detect increased activity by such creatures is to measure levels of nitrogen in the atmosphere.
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