Abiogenesis is the process by which a living organism is formed from non living matter. Although closely related to the theory of evolution, abiogenesis deals exclusively with the origin of life, where evolution examines how existing life forms change. Evolution begins only after abiogenesis - the two cover different areas.
While scientists can evaluate the probably of different theories of abiogenesis, it is unlikely that any will be conclusively proven to be the process by which life first formed on Earth. The first organisms would not leave a fossil record, and chemical reactions cannot be traced millions of years after the fact.
Thermal vent cycle theory
One theory of abiogenesis postulates that the first primitive life formed around thermal vents found in the ocean. These vents are areas with exposed magma on the sea floor which generate hot water. The hot water rises, causing the surrounding cold water to sink, which creates cycles of heating and cooling that the water passes through around the vent. If such a vent had fatty acid vesicles, which form spontaneously in water, and any kind of nucleic acid-like polymer inside, which also can form spontaneously, then it would enter a repeating cycle of heating and cooling, similar to modern PCR, which would drive the replication of the nucleic acids. Over time, these random nucleic acid sequences could begin forming crude nucleic acid enzymes, similar to modern ribozymes, which assist the vesicle in surviving or replicating. At this point life has begun, abiogenesis is at an end and evolution takes over in examining how these extremely primitive life forms become more complex.