Graph #2 and Background Information
Roughly four billion years ago, Earth’s atmosphere had little to no free oxygen available to support livingorganisms. As ancient bacteria evolved, developing the ability to perform photosynthesis, oxygen levels in Earth’satmosphere gradually increased. The graph below shows a period of time during which two sharp increases inoxygen levels in the atmosphere occurred.
Scientists think that these relatively drastic increases in oxygen levels were the result of rapid increases inphotosynthetic populations of bacteria in oceans. As the graph shows, following both sharp increases in oxygenlevels, there were increases in the body volume (size) of organisms. Organisms use oxygen to perform cellularrespiration to produce energy needed for growth and reproduction. Scientists have concluded that the increase inoxygen available allowed for body sizes to increase, since oxygen was no longer limiting growth.
