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>
Answer:
When Glucose is in short supply.
Explanation:
Ketone bodies are produced in mitochondria of liver cells to augment the shortage of glucose in the body. Shortage of blood sugar glucose can occur due to fasting, rigorous exercises or medical conditions.
Evidence would the fossils they left in the grounds they found.
