Not Nth Man's origin, per se, but the origin of his earlier incarnation, Blacksun.
Raymond Lightner conducted an experiment in harnessing stellar energy. Decades later, his adult son Thomas Lightner recreated his father's work to perform the same experiment. The end result was time and space folding in on itself, causing both father and son to merge into a single super-powered being named Blacksun, with the power of a black hole. Blacksun fought against heroes assembled against him simultaneously in both time frames (Doc Savage in the 30's, Thing and Human Torch in the 70's), until his/their powers overloaded, returning everyone to their proper place in time and leaving the Lightners powerless.