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Scientists have formulated many theories about the origin of life and how it evolved into the various forms known today These ideas come from the evidence of the fossil record from laboratory simulations of conditions on the earth and from consideration of the structure and function of cells The earth was created more than 4 billion years ago although more than 2 billion years probably passed before life was developed Scientists believe that the atmosphere of the young earth was mostly water vapor methane and ammonia with very little gaseous oxygen Laboratory simulations have shown that all major classes of organic molecules could have been generated from this atmosphere by the energy of the sun or by lightning and that the lack of oxygen would prevent newly formed organic molecules from being broken down by oxidation Randomly formed aggregations that could harness energy to grow and reproduce themselves would eventually far outnumber other combinations DNA may have been an important component of the self-reproducing aggregates and RNA is the only organic molecule able to duplicate themselves The absence of oxygen from the atmosphere of the young earth meant that no ozone layer existed to screen out ultraviolet radiation and no oxygen was available for aerobic respiration Therefore the first cells were probably photosynthetic and used ultraviolet light Because photosynthesis generates oxygen the oxygen content of the atmosphere gradually increased As a result cells that could use this oxygen to generate energy and photosynthetic cells that could use light other than ultraviolet eventually became predominant Eukaryotes may have evolved from prokaryotes This idea comes from speculation about the origin of mitochondria and chloroplasts These organelles may be the descendants of aerobic and photosynthetic prokaryotes that were engulfed by larger prokaryotes but remained alive within them endosymbiosis Over the years the host cell became dependent on the endosymbionts for energy ATP while they in turn became dependent on the host for most other cell functions The
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