How changes in the atmosphere eukarotes and multicellularit

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 enosymbiosis, and the other is the autogenous theory, or autogenisis. These two theories are not mutually exclusive, meaning one or the other could account for different parts of eukaryotic cells. The endosymbiotic theory states that the formation of eukaryotic cells were symbiotic associations of prokaryotic cells living inside larger prokaryotes. The endosymbiotic hypothesis accou nts for the presence in eukaryotic cells of complex organelles not found in the far simpler prokaryotes. Many modern organisms contain intracellular symbiotic bacteria, cyanobacteria, or photosynthetic protists, indicating that such associations are not difficult to establish and maintain. Endosymbiosis is said to be responsible for the presence of chloroplasts and mitochondria in eukaryotes. Autogenisis, the alternative to the endosymbiotic theory is specialization of internal membranes derived originally from the plasma membrane of a prokaryote. Autogenisis could be responsible for structures like the nuclear membrane and endoplasmic reticulum in eukaryotes. There are two scenarios for which multicellularity may have occurred. The first is unicellular organisms came together to form a colonial organism, then some tissue developed specialized functions and the cells became differentiated, forming a multicellular organism. The other scenario starts with a coencytic organism forming cellulorization with individual cells developing membranes, then tissues became more specialized forming a multicellular organism. There are some advantages of multicellularity such as having specialization of cells which creates a division of labor, leading to greater efficiency. Another advantage is multicellular organisms have a larger size which provides protection from predators. Fungi are large and have a large surface area to volume ratio, allowing them to absorb nutrients more efficiently.