Did Christian fundamentalist always hold to young earth creationism interpretation of Genesis?

In short, no. Christian fundamentalist did not always embrace young earth creationism.

George Frederick Wright wrote a commentary on evolution in his work “The Fundamentals.” Many Christians accepted evolution holding to either a day-age interpretation of Genesis or to a “Gap theory.” Only in the latter half of the Twentieth Century did creationism gain traction in the United States.

The origin of creationism can by tied to Seventh Day Adventist founder Ellen G White.

She had a vision that revealed to her that natural geological phenomenon were created during the global flood of Noah. This idea was tied closely to the Seventh Day Adventist movement until 1959, the one-hundredth anniversary of the publication of Darwin’s Origins of Species. Many Christians did not feel comfortable holding to a Seventh Day Adventist teaching.

In 1959, however there was a push to make the public more aware of Evolutionary Theory, as many public schools did not teach much on the subject. As a result of this push new textbooks were written that unapologetically taught evolution a central organizing principle in Biology. This curriculum change leads many conservative Christians of that time to see this move as an attempt to shove evolution down the throats American of children.

Prior to this a self-taught amateur geologist, George McCready Price wrote a series of convincing books that repackaged White’s vision as science. By the early 1960s Price’s work was updated by Whitcomb and Morris. So when conservative Christians felt they were having evolution shoved down their children’s throats Whitcomb and Morris were ready to offer then an alternative explanation for origins that was both scientific and agreed with a literal reading of Genesis.

Thus the Creation Science movement was born and began gaining traction though the 1970s. Along the way they dropped references to the Bible to make the argument more palatable for teaching in an American public school classroom. Creationist claims about the nature of fossilization have not been proven.  Many of the arguments that they make in support of their ideas do not hold up to scientific scrutiny.