Origin of Life
Bacteria appeared on earth between 3.4 and 4 billion years ago. Precambrian evidence of filamentous bacteria-like structures, found making up fossilized sediments called stromalites, has been discovered in Australia and southern Africa. These fossils date back between 3.4 and 3.5 billion years old. Environmental conditions of earth 4 billion years ago were much different than those of today. It has been theorized that the atmosphere was a reducing atmosphere with intense lightning, tremendous volcanic activity, very little oxygen, intense ultraviolet radiation, and meteorite bombardment. These conditions made it impossible for any type of aerobic life to exist. The first cells may have originated by chemical evolution in an anaerobic environment.
Abiotic Synthesis Theory of Life: A.I.Oparin postulated, in the 1920's, that primitive earth with its reducing atmosphere favored chemical reactions that produced simple monomers that eventually formed complex organic compounds necessary for life. Stanley Miller and Harold Urey tested this hypothesis by building an apparatus (pictured below) to simulate the conditions of the earth at that time.
Their experiments produced 20 amino acids and other organic compounds including: ATP, lipids, some sugars, and the bases of RNA and DNA. The main problem of organic synthesis is how did these molecules develop without the aid of enzymes to speed up the reactions? Sidney Fox, of the University of Miami, dripped a dilute solution of organic monomers onto hot sand, clay , and rock. The water vaporized and left behind polypeptides he called proteinoids. Clay was abundant in prebiotic earth. Clay has the ability to act as a substrate for this type of chemical reaction. The charged sites on the clay attracted monomers in such concentrations to bring them into close proximity for chemical binding. Once these organic compounds were produced, aggregates of these proteinoids self-assembled into small spheres called protobionts. These small spheres were capable of osmotic swelling and shrinking and even able to produce a membrane potential.
RNA the first Genetic Material: Evidence to support the idea that RNA was the first genetic material includes: short polymers of self-replicating RNA have been abiotically produced in a test tube without enzymes. RNA has the ability to act as a catalyst to help make mRNA, tRNA or rRNA. The RNA folding is unique depending on its sequence. This along with mutations, creates a variety of closely related molecules. Through chance this self-replicating material was captured by the protobionts and the entire structure developed under the guide of natural selection.
Other Origin Theories:
Panspermia. Extraterrestrial organic compounds located on the surface of meteorites could well have contributed to the pool of material found on prebiotic earth. Modern meteorites have given up specimens of simple organic compounds such as amino acids.
Some researchers believe that life began on the sea floor away from the harsh surface environments. The discovery of deep sea vents showed the scientific community that the materials needed for life to begin were present away from the harsh surface.
Julius Rebek produced a simple organic molecule in 1991 that served as a template for self-replication. This discovery suggests that RNA was too complex to have come first and a much simpler self-replicating molecule preceded it.