The Evolution of the Earth and Its Atmosphere

Abstract

Earth is believed to have formed about 4.5 billion years ago. In the first 500 million years ago (Ma), a dense atmosphere emerged from vapor and gases that were expelled during degassing of the interior of the planet. These gases may have consisted of hydrogen, water vapor, methane, and carbon oxides. Prior to 3.5 billion years ago the atmosphere probably consisted of carbon dioxide, carbon monoxide, water, nitrogen, and hydrogen. The hydrosphere was formed 4 billion years ago from the condensation of water vapor, resulting in oceans of water in which sedimentation occurred. The most important feature of the ancient environment was the absence of free oxygen, which only began to persist in the atmosphere in small quantities about 50 Ma before the start of the great oxygenation event. This mass oxygenation of the atmosphere resulted in a rapid buildup of free oxygen. The rate of oxygen production by photosynthesis was slower in the Precambrian, and the concentrations were less than 10 % of the oxygen of today and probably fluctuated greatly. These fluctuations in oxygen concentration had little effect on life. The presence of oxygen allowed life with new opportunities. The origin of the mitochondria is related to the evolution of the earth. Mitochondria contain their own DNA, which is circular as in the bacteria. Mitochondrial ribosomes and transfer RNA molecules are also similar to those of bacteria, as are the components of their membrane. These observations led different researchers to propose an extracellular origin for mitochondria. The endosymbiotic hypothesis for the origin of mitochondria suggests that mitochondria are descended from specialized bacteria that somehow survived the endocytosis in another species of prokaryote or some other cell type, and became incorporated into the cytoplasm, providing a considerable evolutionary advantage.