Short answers, pulled from the story.
A cappella means singing without instrumental accompaniment. The term was originally used to distinguish between Renaissance polyphonic and Baroque concertato styles. In the 19th century, a renewed interest in Renaissance polyphony led the term to come to mean unaccompanied vocal music specifically.
The oldest piece of music surviving in its entirety is the Seikilos epitaph, a piece from Greece dating to the first century AD. Instructions for performing music appear on a cuneiform tablet from around 2000 BC, though that earlier example is not a complete surviving work.
Peter Christian Lutkin, dean of the Northwestern University School of Music, founded the Northwestern A Cappella Choir in 1906, described as the first permanent organization of its kind in America. F. Melius Christiansen launched a separate tradition in 1911 at St. Olaf College in Northfield, Minnesota, which spread to colleges across the region.
In Transit became the first a cappella musical on Broadway in December 2016. It had originally premiered Off-Broadway on the 5th of October 2010 in a production by Primary Stages, winning the Drama Desk award for Outstanding Ensemble Performance in 2011.
John Legend sang with the Counterparts at the University of Pennsylvania. Sara Bareilles was a member of Awaken A Cappella at UCLA. Mindy Kaling sang with the Rockapellas at Dartmouth College. Mira Sorvino was a member of the Harvard-Radcliffe Veritones at Harvard University.
The Barbershop Harmony Society has approximately 22,000 members across roughly 800 chapters in the United States and Canada. The organization was founded in 1938 under a longer name and rebranded to Barbershop Harmony Society in 2004.