The Evolution of Life
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Planet Earth is 4.5 billion years old. Of that time, life has existed on it for 3.86 billion years, according to the latest geological evidence. The main theory of how life evolved on this planet is that minerals in rocks on the ocean floor near oceanic vents served as the first cells and that the hot chemicals coming out of the vents provided the necessary energy to combine existing molecules and therefore make them bigger. The origin of the genetic code coincides with the origin of life. Ribonucleic acids (RNA) and cells can’t have formed at different times, but most scientists think that proteins and self-replicating molecules evolved first. Natural selection started when small changes in molecules made better copiers (nucleic acids) and then dominated over all of the other molecules. The ability to join and copy molecules, for example, was a key step in the origin of life. This ability eventually developed into complex chemical DNA. After the DNA formed, membranes developed to cover it, making these prototypes of the first bacteria. 3.8 billion years ago, the first simple-celled bacteria had evolved and were protected from harmful rays from space by the ozone layer.