Short answers, pulled from the story.
The earliest piece of a cappella music that has survived in its entirety is the Seikilos epitaph from Greece, which dates to the first century AD. This ancient artifact demonstrates that humans used their voices to create art long before the invention of complex instruments.
The term a cappella came to mean unaccompanied vocal music in the 19th century due to a renewed interest in Renaissance polyphony and a misunderstanding that vocal parts were often doubled by instrumentalists. This historical misunderstanding transformed a specific stylistic distinction into a broad definition of vocal-only performance.
The Society for the Preservation and Encouragement of Barber Shop Quartet Singing in America was formed in 1938 as the first formal men's barbershop organization. This group officially changed its public name to the Barbershop Harmony Society in 2004 and now has about 22,000 members in approximately 800 chapters across the United States and Canada.
The Rensselyrics of Rensselaer Polytechnic Institute, formerly known as the RPI Glee Club, was established in 1873 and is perhaps the oldest known collegiate a cappella group. The longest continuously singing group is probably The Whiffenpoofs of Yale University, which was formed in 1909 and once included Cole Porter as a member.
The first South Asian a cappella group was Penn Masala, an all-male group founded in 1996 at the University of Pennsylvania. The first co-ed South Asian a cappella group was Anokha, from the University of Maryland, which was formed in 2001.
In Transit became the first a cappella musical on Broadway in December 2016. This production premiered on the 5th of October 2010 and was produced by Primary Stages with book, music, and lyrics by Kristen Anderson-Lopez, James-Allen Ford, Russ Kaplan, and Sara Wordsworth.