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 indicates that the first eukaryotes evolved at least
1.5 billion years ago. Eukaryotes are distinguished from prokaryotes by their
larger size, the separation of nucleus from cytoplasm by a nuclear envelope, the
association of DNA with histone proteins and its organization into a number off
distinct chromosomes, and complex organelles, among which are chloroplasts and
mitochondria. Scientists believe that eukaryotic organisms such as the protists
evolved from the prokaryotes. There are two main theories which describe how
this transition may have occurred. The first is the endosymbiotic theory, or
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