History of the Big 5, no Big 7, no Big 6, no Big 7, no Big 10, no Big 6, no Big 8. OK, we'll just call it the Big 12 and leave it at that...what? no? it's going to keep changing?...whatever.
The Early Years
In 1890, University of Kansas, Kansas State Agriculture College (now Kansas State University), and Washburn University helped form the Kansas Intercollegiate Athletic Association. It was the first successful attempt to organize Kansas colleges for the purposes of promoting and regulating amateur intercollegiate athletics.
Two years later, in 1892, Universities of Iowa, Kansas, Missouri, and Nebraska formed the Western Interstate University Football Association. Early WIUFA play led to the famous rivalry between Kansas and Missouri.
Small Blip On The Radar
From 1898 through 1906, Kansas was an independent football school.
Now We're Getting Somewhere
Missouri Valley Intercollegiate Athletic Association (MVIAA) was founded in 1907 by its charter member schools: University of Kansas, University of Missouri, University of Nebraska, and Washington University; University of Iowa was a joint member of both the MVIAA and the Western Conference (now the Big Ten).
In 1908, Drake University and Iowa State College (now Iowa State University) joined (up to 7); in 1911, Iowa left (down to 6); in 1913, Kansas State University joined (back to 7). Nebraska left in 1919 to play two seasons as an independent. That year, the conference added Grinnell College, and University of Oklahoma joined a year later; its intrastate rival, Oklahoma A&M, joined in 1925 (up to 10).
In 1928, the larger state schools--Iowa State, Kansas, Kansas State, Missouri, Nebraska and Oklahoma--remained together as the MVIAA, which became known informally as the "Big Six Conference", while the smaller schools formed a new conference, the Missouri Valley Conference (MVC). (There was some bad blood in the breakup, and the similarity of "Missouri Valley Intercollegiate Athletic Association" and "Missouri Valley Conference" is no coincidence; each new conference maintained that it was the original -- for instance, according to them, did you know that Kansas won the MVC football league title in 1907?)
Two decades later, University of Colorado joined in 1948 from the Mountain States Conference (also known as Skyline Conference, a forerunner of the Western Athletic Conference), creating what was known for about ten years as the "Big Seven". Oklahoma A&M, now called Oklahoma State, rejoined in 1958, and the MVIAA would officially rename itself the "Big Eight" in 1964.
Opening Pandora's Box
The alluring box remained closed for nearly 40 years, until 1996, when four former members of the now-defunct Southwest Conference (Baylor University, University of Texas, Texas A&M University, and Texas Tech University) were added to form the Big 12 Conference.
The Circle Of Life
So there you have it: KU, KSU, CU, and ISU reestablish the MVIAA, aka the "Big Four".
(btw, half-adapted, half-plagiarized from internet sources.)