Short answers, pulled from the story.
The earliest well-documented instance of improvisational theatre in Western history dates back to 391 BC with the Atellan Farce. These performances emerged from ancient Roman traditions where actors relied on masks and stock characters to create spontaneous scenes without a fixed script.
Paul Sills co-founded The Compass Players in Chicago alongside David Shepherd before establishing The Second City troupe. The Second City opened its doors on the 16th of December 1959 under Paul Sills direction.
Longform improvisation thrives across major American cities including Chicago, New York City, Los Angeles, Austin, Atlanta, Dallas, Boston, Minneapolis, Phoenix, Philadelphia, San Francisco, Seattle, Detroit, Toronto, Vancouver, and Washington D.C. International presence grows in the United Kingdom particularly within London, Bristol, Glasgow, and at the Edinburgh Festival Fringe.
Beginning in the late 1990s applied improvisation emerged as a method to utilize improv principles outside performance spaces for education and business training. Drama therapy utilizes psychodramatic improvisation techniques extensively alongside other methods developed specifically for therapeutic contexts.
Many silent filmmakers including Charlie Chaplin and Buster Keaton used improvisation during film production developing gags while filming and altering plots to fit emerging needs. The Marx Brothers became notorious for deviating from scripts they received with ad-libs becoming part of standard routines and making their way into final films.