Early Ideas
Spontaneous generation is the supposed production of living organisms from nonliving matter, as inferred from the apparent appearance of life in some supposedly sterile environments.
Francesco Redi's experiment was based of the belief of spontaneous generation. He wanted to see if flies and maggots really did generate from rotting food. In his experiment he took 3 samples of raw meat and put them each in their own individual jar. He put a cork in one, covered another with a gauze, and left one open. When he waited a little while flies/ maggots were only in the open jar so he concluded that animals cannot randomly generate from food and only appear on food that is left out.
Louis Pasteur's experiment was to confirm Francesco's conclusion that animals indeed cannot be spontaneously generated. In his experiment Louis boiled some broth in a flask to sterilize it. Next he broke the neck of one flask and left the other intact then set the flasks in an open area. After a while mold began to form in the open flask while the closed had nothing. Louis then concluded the living organisms have to come from other living organisms.
Bio-genesis is the synthesis of substances by living organisms
Modern Ideas
Scientists hypothesize that two things happened before life could appear on Earth. The first is that there had to be organic molecules-molecules containing carbon. The second is that the organic molecules must have formed into more complex molecules such as proteins, carbohydrates, and nucleic acids. These are the materials that are absolutely necessary for life.
Miller and Urey conducted an experiment to see if they could spontaneously generate organic molecules. In their experiment they used several connected flasks with different elements and compounds to simulate the Earth atmosphere. In the end of their experiement organic material actually formed. They then concluded that, given the right conditions, you can spontaneously generate some organic molecules.
The Evolution Of Cells
The earliest organism on earth happened to be single celled Archaea.
Archaea has changed our atmosphere by removing carbon dioxide from the air and then returning it to the atmosphere and circiling carbon and nitrogen around.
The Emdosymbiont Theory
The emdosymbiont theory is the theory that eukaryotes evolved from prokaryotes. What is believed is that a host cell engulfed a prokaryotic cell that can photosynthesize which would give the new cell chloroplasts and other plastids.