"[W]hen Marshawn was first born, Delisa Lynch, his mother, was told that her son had absorbed the placenta of a twin brother and had been nourished by both in utero. The midwife told her at the time to not be surprised if Marshawn turned out to be an 'amazingly strong child.'"

http://www.sbnation.com/nfl/2015/1/27/7879507/super-bowl-49-media-day-marshawn-lynch-nfl-seahawks-patriots

I'm gonna guess that the skittles he absorbed through the umbilical cord also played a role.