Short answers, pulled from the story.
The Screen Actors Guild was founded on the 17th of March 1933 by six actors including Berton Churchill, Charles Miller, Grant Mitchell, Ralph Morgan, Alden Gay, and Kenneth Thomson. These individuals gathered to address multi-year contracts that allowed producers to dictate every aspect of an actor's public and private life without limits.
In October 1947, ten individuals known as the Hollywood Ten appeared before the House Committee on Un-American Activities and refused to cooperate with investigations into Communist influence within Hollywood labor unions. The Screen Actors Guild voted on the 17th of November 1947 to force its officers to take a non-communist pledge following this event.
The Screen Actors Guild Women's Committee formed in 1972 after Brigham Young University conducted a study revealing that 81.7% of television roles went to men while only 18.3% went to women. A 1989 report showed the combined income of male actors reached $644 million compared to just $296 million for female performers according to a 1989 report.
On the 30th of March 2012, the Screen Actors Guild announced that membership had voted to merge with the American Federation of Television and Radio Artists. This created SAG-AFTRA combining over 100,000 film and television principal and background performers worldwide.
Financial core status allowed individuals to work within union environments without full membership but they could not represent themselves as Screen Actors Guild members. The initiation fee for Los Angeles, New York, or Miami locals reached $3,000 plus first semi-annual dues of $58 with maximum annual dues never exceeding $6,566 regardless of income level.