AP BIOLOGY

Detailed explanation-1: -The rise of oxygen occurred slowly, over hundreds of millions of years, and not without hiccups. Jay Kaufman, a geoscientist at the University of Maryland, points to a series of ice ages’-at least three of them’-that occurred between 2.4 billion and 2.2 billion years ago, when the era of oxygen began.

Detailed explanation-2: -Much of the CO2 dissolved into the oceans. Eventually, a simple form of bacteria developed that could live on energy from the Sun and carbon dioxide in the water, producing oxygen as a waste product. Thus, oxygen began to build up in the atmosphere, while the carbon dioxide levels continued to drop.

Detailed explanation-3: -The early Earth’s atmosphere had a very low concentration of oxygen compared to today. 2.4 billion years ago, the oxygen concentration in the atmosphere was less than one part per billion and the iron concentration in the ocean was much higher than today.

Detailed explanation-4: -Earth’s oxygen supply originated with cyanobacteria, tiny water-dwelling organisms that survive by photosynthesis. In that process, the bacteria convert carbon dioxide and water into organic carbon and free oxygen.