The jam-band tradition begins with the Grateful Dead’s legendary live shows. From their roots in San Francisco’s psychedelic scene of the late ’60s, the band used the stage to thread rock, country, jazz, blues, and folk into tapestries of sound both cosmic and earthy. As for their longevity, it can be chalked up to their improvisational chops. No two versions of “Dark Star” sound the same—and that’s the ever-evolving beauty of the Dead experience.