Is klunking the father of mountain biking? Is attacking downhills with coaster brakes where the sport of downhill mountain biking originated? Or did mountain biking and klunking grow up side by side?
Klunker as a term is synonymous with early mountain bike (as it is currently known). Klunkers were developed by customising early beach cruisers on 26" wheels (Schwinn predominantly) by spreading the rear dropouts to make them multi-geared and adding moto handle bars and brakes due to their durability and strength (brands such as Magura were initially moto brands). 

The term klunker was used to describe these built up bikes on cruiser frames.

This style of bike and mountain riding first appeared in Marin County, California in the mid 70's prior to the first purpose designed mountain bike appearing in 78. If you look at the proponents of the early mountain biking movement and design; Gary Fisher, Joe Breeze, Charlie Kelly, Tom Ritchey and Keith Bontrager (there are going to be many more....) all were active in Marin and it's surrounding counties at the time.