AP BIOLOGY

Detailed explanation-1: -Early Earth atmospheres (> 3 billion years ago) did not contain the 21% oxygen content that we have today. When life on Earth originated (  3.5 billion years ago) the first prokaryotic cells were anaerobic chemoautotrophs, most likely occupying deep ocean and subterranean habitats.

Detailed explanation-2: -The first fossils of prokaryotic (bacterial) cells are known from 3.5 and 3.4 billion years ago. These bacteria were photosynthetic (although non-oxygen producing) so it is likely that simpler non-photosynthetic bacteria evolved prior to this (Schopf, 1987; Beukes, 2004).

Detailed explanation-3: -The first life form, bacteria like prokaryotic cell, originated from protocell in the Archaeozoic era (3800 million yrs ago, the era of the dawn of life). The first life forms were anaerobic as the primitive atmosphere was reducing and oxygen was absent.

Detailed explanation-4: -When life first arose (likely more than four billion years ago), there was no free oxygen in the atmosphere at all. Life was anaerobic, meaning that it did not need oxygen to live and grow.

Detailed explanation-5: -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.