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Origin of Life Biogenesis Earth History Earliest Life Forms The Theory of Biogenesis Before the seventeenth century, most people believed that living things could arise from nonliving matter. Flies could come from rotting meat, mice could come from old clothes and dirty straw, etc. This belief was called the Theory of Spontaneous Generation. The combined work of several important scientists disproved this theory. Redi’s Experiment In 1668, Redi designed a controlled experiment that showed that flies did not arise from rotting meat. Control Group Experimental Group Maggots and flies appeared on the uncovered meat, but the net-covered meat produced no flies. The flies came from eggs laid by other flies, not the meat. Spallanzani’s Experiment Spallanzani tested the hypothesis that bacteria arose by spontaneous generation. He boiled broth and allowed it to sit in containers for several days. One group of containers was open to the air, and one group was sealed shut. Only the open containers grew bacteria. Control Group Experimental Group Pasteur’s Experiment Pasteur disproved spontaneous generation conclusively in the mid 1800’s. He used a curved-neck flask containing nutrient broth and boiled it. It was left open to the air for an entire year. Nothing grew. After a year, the curved neck was broken off and one day later, bacteria appeared in the broth. Pasteur deduced that bacteria from the air had fallen into the flask, and that they had NOT arisen spontaneously, but from pre-existing life, in other words, by BIOGENESIS. Broth in open flask is boiled No growth after one year Neck of flask is broken and growth occurs after one day! Earth History About 4.6 b.y.a, the solar system was formed. Out of swirling clouds of gases, the sun and the planets and their moons condensed. Eventually they cooled and attained their present forms. Sediments from the earth’s crust have been dated using radioactive clocks. This technique has been used to accurately estimate the age of the earth. The First Organic Molecules How did the 92 naturally-occurring elements found on earth assemble into organic compounds? An early explanation was proposed by Alexander Oparin in 1923. Oparin said that the atmosphere of the early earth contained no free oxygen. It was a reducing atmosphere made primarily of NH3, H2, H2O, and hydrocarbons such as methane (CH4). The surface of the early earth was above 100oC. At those temperatures, the atoms found in the atmosphere would have had enough energy to form simple organic compounds such as amino acids spontaneously. As the earth cooled, these compounds would have fallen with the rain and collected in the early oceans. Miller-Urey Experiment In 1953, Stanley Miller and Harold Urey tested Oparin’s theory. They attempted to duplicate conditions on the early earth and see if organic compounds would form. They produced many organic compounds, including amino acids. electrode Water vapor forms Water vapor, CH4, H2, NH3 condenser H2O Organic compounds After Miller-Urey More refined experiments since the 1950’s have continued to produce a variety of organic compounds, including amino acids, ATP, and the nucleotides that make up DNA. Other planetary scientists have suggested that the early atmosphere contained large amounts of CO2, which interferes with the formation of organic compounds. Current theories now include the possibility that these organic compounds arose in sheltered areas such as parts of the ocean near undersea volcanic vents or the entrances to hot springs. From Molecules to Cells Under certain conditions, cell-like structures can be formed spontaneously in the lab from solutions of organic molecules. These include microspheres, which are small spheres surrounded by protein molecules organized like a membrane. Coacervates are another type of structure, which are a group of droplets containing such compounds as amino acids, proteins, and sugars. Coacervates and microspheres exhibit some of the properties of living things, such as growing, or budding, or concentrating “nutrients” within their membranes. Thus, some properties of life can occur without the presence of genes. They cannot respond to the pressures of natural selection, however, because they cannot pass on their characteristics. Earliest Life Forms In Australia, fossils of early life forms have been found that are 3.5 billion years ago. Studies comparing DNA and RNA have shown RNA to be more versatile. It can even act as an enzyme in certain cases, catalyzing its own reactions. RNA molecules have “heredity” and can respond to changing environmental pressures (natural selection). RNA plays a critical role in the replication of DNA, protein synthesis, and other basic chemical processes in the cell. Therefore, scientists now believe that life probably started with self-replicating forms of RNA, not DNA. The First Prokaryotes Because the atmosphere of the early earth contained no free oxygen, the earliest organisms must have been anaerobic. Because the early oceans were rich with organic molecules like amino acids and carbohydrates, the early living things were probably heterotrophs, taking those molecules into their cells as nutrients. As the populations of heterotrophs grew, they depleted their environment of nutrients, thus causing an environmental selection for the survival of autotrophs, when they arose. The first autotrophs were chemosynthetic, not photosynthetic. Photosynthesis and Aerobic Respiration When photosynthetic autotrophs appeared about 3.8 b.y.a. and reproduced in large numbers, they produced oxygen in large amounts, which accumulated in the atmosphere. This was toxic to many early organisms. Some organisms, however, used oxygen to bond to other substances in their cells, thus preventing damage. This was the first step in the development of aerobic respiration. It took about a billion years for oxygen to accumulate in the atmosphere to its present concentration of about 21%. The oxygen eventually rose to the upper layers of the atmosphere where it formed ozone (O3). Ozone acts as a shield from UV radiation that allowed life to migrate onto the land. The First Eukaryotes The first eukaryotes probably arose by the process of endosymbiosis, in which a larger cell engulfs but does not digest a smaller cell. The two cells then exist in a relationship that is beneficial to both. It is thought that between 1.5 and 2.0 b.y.a. a small aerobic prokaryote began to live and reproduce inside of a larger anaerobic prokaryote. These small aerobic prokaryotes evolved into mitochondria. Small aerobic prokaryote Large prokaryote with engulfed smaller prokaryotes Eukaryotes Later there was another successful influx of photosynthetic prokaryotes similar to cyanobacteria into larger cells. These small photosynthetic prokaryotes evolved into chloroplasts. Evidence supporting the theory of endosymbiosis includes the following: 1) Mitochondria and chloroplasts each have their own genes on a circular piece of DNA like that of prokaryotes. 2) They also make their own enzymes, and they are also surrounded by two membranes instead of one. 3) They reproduce by binary fission, like prokaryotes do. mitochondria Small photosynthetic prokaryote Photosynthetic Pre-eukaryote