The answer to this question lies in a scientific accident.
The origin of DNA, which has been scientifically tested, proven, and repeated, is found in the environment of the Archean Eon, 4000 mya-2500 mya. The atmosphere at this time was mainly due to volcanic outgassing, composed mostly of water vapor, carbon dioxide, methane, ammonia, and a variety of other gases. Also, there is evidence of surface water, a higher surface temperature than is found today, and a higher level of ultra-violet radiation. These factors, when combined, result in the spontaneous creation of simple monomers, including amino acids. When dehydrated, such as when the surface water is evaporated from the higher level of heat, polymers can spontaneously form, inluding proteins and DNA. This theory was tested in what is now referred to as the Miller Experiment; gases were pumped into a sealed glass container, along with water. The glass container was then heated and bombarded with UV radiation. Later on, amino acids were found to have developed on the sides of the glass. Then the accident occurred; someone left the heat on the glass too long, and the water evaporated. When examined, however, the amino acids that had collected on the inside of the glass had spontaneously polymerized into primitive DNA.