In some sense, Parkour has been around as long as man's need to hunt and avoid being hunted. Humans naturally have an astounding range of motion and range of options for how to move through a given set of obstacles. Further, children naturally move about with grace and ease, we only start to lose this later in life as we start to move with conscience. There have been several people throughout history to work on concepts of human movement and development, however, to look specifically at Parkour, we need to look at Raymond Belle, a French soldier in the Vietnam war. He and his companions worked to develop efficient methods "to reach or escape". These were then handed down to David Belle, who has spent a majority of his young (34) life working on these principles specifically as Parkour. There were some others who worked with him in the developing stages of Parkour in Lisses, Sebastien Foucan is probably the most well known.

On the first day, David Belle created parkour.
On the second day, David Belle introduced parkour to Sebastien Foucan.
On the third day, both Belle and Foucan trained together.
On the fourth day, Belle and Foucan realized the extent of their abilities.  As they approached the opposite sex, panties instantly dropped.  Much merriment was achieved.
On the fifth day, Foucan brought parkour to England whereas Belle stayed in France.
On the sixth day, Foucan created free running, a more acrobatic version of parkour.
On the seventh day, both men slept.

This is how parkour was created.  No if, ands, or buts.

 lolz JAYKAY!