Answer:
2.7 billion
Explanation:
This period is referred to as the Great Oxygenation Event. This period saw the evolution of photosynthetic organisms (cyanobacteria) that utilized carbon dioxide (that dominated the atmosphere then) and sunlight to manufacture their energy molecules-glucose. Photosynthesis produces oxygen as a byproduct that is evolved into the atmosphere. As these photosynthetic organisms proliferated on earth, the atmosphere composition changed to become partly dominated by oxygen (which is an oxidaton agent) with less and less carbon dioxide.
Around 4.1 billion years ago the first single celled organism appeared
<span>All cells contain the same key elements, such as carbon, nitrogen, and oxygen. </span>
Answer:
where ??? i only see isn letter A-D
