AP BIOLOGY

Detailed explanation-1: -The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen.

Detailed explanation-2: -For the most part, scientists agree that oxygen, though lacking in the atmosphere, was likely brewing in the oceans as a byproduct of cyanobacterial photosynthesis as early as 3 billion years ago.

Detailed explanation-3: -Oxygen (O2) is evolved during photosynthetic electron transport when water is split by the oxygen-evolving complex to provide protons and electrons to the chloroplastic electron chain, thereby generating ATP and NADPH-the energy source and reducing power for plant metabolism.

Detailed explanation-4: -When cyanobacteria evolved at least 2.4 billion years ago, they set the stage for a remarkable transformation. They became Earth’s first photo-synthesizers, making food using water and the Sun’s energy, and releasing oxygen as a result.

Detailed explanation-5: -Caption: MIT scientists estimate that oxygenic photosynthesis-the ability to turn light and water into energy, releasing oxygen-first evolved on Earth between 3.4 and 2.9 billion years ago.