I don't buy into the Viking theory as I have this also and it comes from my Great Grandmother who was supposedly full blooded Cherokee. She passed it along to my dad. I have it, so does my nephew and his son.
In us the first sign is a crooked pinky finger. My dad died in his mid-sixties, never having developed the bumps in the hand or bending of the fingers. I (at age not spoken nor writen aloud) have had bumps for two years.