Changes in the Atmosphere Causing Multicellularity About 2.5 billion years ago, oxygen began slowly to accumulate in the atmosphere, as a result of the photosynthetic activity of the cyanobacteria. Those prokaryotes that were able to use oxygen in ATP production gained a strong advantage, and so they began to prosper and increase. Some of these cells may have evolved into modern forms of aerobic bacteria. Other cells may have become symbionts with larger cells and evolved into mitochondria. As the amount of oxygen and other atmospheric gasses increased, they started blocking out deadly u.v. rays from the sun. The sun’s rays made life outside of water nearly impossible. These changes made life on land possible and evolution occurred as prokaryotes gave rise to land living eukaryotes. The microfossil record indicat....

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