The prologue describes the changes taking place in the once all-black neigborhood (the Bottom) above the once all-white town of Medallion, Ohio.
I - The Bottom of Heaven - a nigger joke
Once a slave owner promised his slave that he would freee him if he completed some dificult tasks. In addition he promised him a plot of good "bottom land" int he valley. When the time came, the slave owner didn't want to part with any of his good land. He gave the slave a plot in the hills, stating that it was the "bottom of heaven" because it was closer to God. The slave was delighted to accept the "gift". But in fact it was a nigger joke : Only later did he realize taht hilly land was extremely difficult to farm. He became a slave again, the land's slave.
II - The inverted order - how the bottom became a desirable place for white people.
Morrison begins the novel with a short prologue that focuses on change: the leveling of a black neighborhood in order to create a golf course for white people. In the 1920's, only white people lived in Medallion and only black people lived in the Bottom. The once-worthless land is now being metamorphosed into a socially desirable locale for white people. This inverted order is not merely an ironic setting but it's an essential theme of the novel : What may seem good initially may prove to be not so good after all, and what may seem evel on the surfacemay later prove to be value. the demolition of Bottom's old shacks seems like an improvement. However, Morrison states the Bottom was once a vibrant community filled with laughing voices and a paradde of unique, interesting people.
III - Humor and narration
Humor was especially abundant then, and it even influenced—ironically—the names of businesses—for example, the Time and a Half Pool Hall, which is intentionally humorous and part of Morrison’s interweaving of the change/reversal motif. Although the phrase "time and a half" means that someone working at that scale gets more money for working overtime, the men in the pool hall aren’t working overtime; actually, they aren’t working at all. Another business that is humorously named is Irene’s Palace of Cosmetology, a take-off on the names of pretentious beauty parlors—or salons—for rich white women. This particular beauty parlor is hardly a "palace." Palaces are generally associated with rich people, but the only rich people in the valley are the whites who live down in Medallion, not up in the steep-hilled Bottom.
Morrison ends the prologue with a couple of items of suspense. Back in 1920, she says, the black population of the Bottom was wondering what a man named Shadrack "was all about," and what a little girl named Sula "was all about." In fact, the community wondered what it itself was "all about." The community is itself a character of the novel. By beginning at the end, Morrison shows us what will eventually happen to this black community, which gains and then loses its identity. The circularity of Morrison's narrative, beginning as it does with the Bottom's end, takes us all the way to the novel's closing sentence : it had no top, just circles and circles of sorrow.
The Bottom is a metaphore of life at that time. It refers to the segreagation after the emancipation. Even if lynching was officially stopped, ther were still a lot af troubles. Sula deals with all the things that were done to black people. The prologue introduces an other important theme of the novel : For as Morrison has said, "Evil is as useful as good. Sometimes good looks like evil and evil looks like good."