Short answers, pulled from the story.
Jazz pianist Jelly Roll Morton claimed that comedian Joe Sims from Vicksburg, Mississippi was the first man to ever do a scat number in the history of the United States. Morton recalled that he and Tony Jackson were using scat for novelty back in 1906 and 1907. This predates Louis Armstrong's famous 1926 recording of Heebie Jeebies.
Louis Armstrong recorded Heebie Jeebies in 1926 during a session where his sheet music fell to the floor. He filled the silence with nonsense syllables, creating an accidental invention that became a national bestseller. This event popularized the form of scat singing despite earlier examples existing.
Ella Fitzgerald's improvisation mimics the sounds of swing-era big bands with which she performed. Sarah Vaughan preferred fricatives and plosives such as shoo-doo-shoo-bee-ooo-bee and mimics that of her accompanying bop-era small combos. These choices allowed singers to mimic the sounds of different instruments to differentiate personal styles.
Scat singing evolved independently in the United States rather than originating from African musical traditions. The technique arose from jazz musicians' practice of formulating riffs vocally before performing them instrumentally. This practice was common in the early New Orleans jazz scene where soloists like Louis Armstrong switched between instrumental solos and scatting.
Jazz artist John Paul Larkin, better known as Scatman John, renewed interest in the genre in the mid-1990s. He scored a world-wide hit with the song Scatman Ski Ba Bop Ba Dop Bop in 1994. He began fusing jazz singing with pop music and eurodance to create this popular track.
Hip hop artists use scat singing to generate rhythmic skeletons that serve as a foundation for their lyrics. Rapper Tajai of the group Souls of Mischief states that he makes a scat kind of skeleton and then fills in the words. This method allows for a fluid transition between singing and rapping while creating a seamless blend of vocal improvisation and spoken word poetry.