Short answers, pulled from the story.
A cuneiform tablet from around 2000 BC contains the earliest known instructions for performing music. This artifact predates the Seikilos epitaph, which is a Greek piece from the first century AD that survives in its entirety as the oldest complete musical composition.
The phrase originally differentiated Renaissance polyphonic styles from Baroque concertato forms. In the 19th century, renewed interest in Renaissance polyphony led people to believe vocal parts were performed without instruments, causing the misunderstanding to make the phrase evolve into meaning unaccompanied vocal music exclusively.
The Free Methodist Church lifted its ban on organs entirely in 1955 after allowing local decisions since 1943. Written opposition to instruments in Christian worship began near the turn of the 5th century according to church history records.
In 1938, the Society for the Preservation and Encouragement of Barber Shop Quartet Singing in America formed as the first formal men's organization. This group rebranded itself in 2004 to become the Barbershop Harmony Society with about 22,000 members across 800 chapters today.
The Rensselyrics of Rensselaer Polytechnic Institute established in 1873 is perhaps the oldest known collegiate a cappella group. The Whiffenpoofs of Yale University formed in 1909 remains the longest continuously singing group ever created.
Television show The Sing-Off aired on NBC beginning December 2009 featuring eight groups competing for $100,000 and recording contracts. Pitch Perfect released in late 2012 caused coed a cappella groups to rise far more sharply than previous years.