Acrobats were one of the sketches between acts at Astley's circus, although their original skills were a blend of a couple of other acts we see today. While acrobatic performance has evolved into several different fields, it's worth noting that gymnastic or tumbling skills were a circus mainstay from the beginning.
Because British circuses were equestrian-focused, the first acrobats in the ring used their horses as props. In fact, in 1846 an acrobat named John H. Glenroy made circus history by performing the first somersault on horseback [source: Jando]. (If you want an entertaining account of a 7-year-old joining the circus, do look up Glenroy's autobiography.)
The other acrobatic arm of early circus acts was the floor acrobat who did tumbling or balancing acts. These early floor acrobats began incorporating humor into their performances by creating silly and funny characters. Comedy became a pretty lucrative draw once the circus introduced — you guessed it — clowns.