There's been research that indicates that a soup of various relatively simple hydrocarbons and nitrates (such as what might have been found in the ooze or atmosphere of the earth several billion years ago) can, when exposed to electric discharges, spontaneously generate significant quantities of amino and nucleic acids.
It's conceivable that with sufficient time, lightning storms and 'soup' to work with that molecules eventually strung together to the point where proto-life was brought into existance. Millions upon millions of years work of sparks and sludge could conceivably bring that about. This concept is the 'heterotrophic hypothesis'. Google for the Urey-Miller experiment for more insight into this possibility.
Other theories suggest that ice comets or other interstellar projectiles may have 'seeded' our planet with the basic building blocks of life, and some meteors recovered after impact also sometimes contain higher-order molecules, or that these compounds will form spontaneously miles underground from geothermal heat sources that catalyse simpler compounds in clay.
Unless we get sherman and a wayback machine, we may never know.
