Here something that might help
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Most scientists believe that for half of </span>Earth's<span> 4.6-billion-year history, the</span>atmosphere<span> contained almost no </span>oxygen<span>. Cyanobacteria or blue-green algae became the first microbes to produce </span>oxygen<span> by photosynthesis, perhaps as long ago as 3.5 billion years ago and certainly by 2.7 billion years ago.</span>
Enzymes lower the activation energy of a chemical reaction.
Mouth, throat, small and large intestine
The elements are pure forms of matter that cannot be broken down into simpler substances.
