The Carnival's roots go back to the ancient Romans and Greeks who celebrated the rites of Spring. In the Middle Ages, when the Catholic Church tried to suppress all pagan ideas, it failed when it came to this celebration.
The Church incorporated the rite into its own calendar as a period of thanksgiving. The nations of Europe, especially France, Spain, and Portugal, gave thanks by throwing parties, wearing masks, and dancing in the streets. All three colonizing powers carried the tradition with them to the New World, but in Brazil it landed with a difference. Not only did the Portuguese have a taste for abandoned merriment, (they brought the "entrudo", a prank where merry-makers throw water, flour, face powder, and many other things at each other's faces), but the Negro slaves also took to the celebration.
They would smear their faces with flour, borrow an old wig or frayed shirt of the master, and give themselves over to mad revelry for the three days. Many masters even let their slaves roam freely during the celebration. Since the slaves were grateful for the chance to enjoy themselves, they rarely used the occasion as a chance to run away.
Prior to 1840, the streets of Brazilian towns ran riot during the three-day period leading up to Ash Wednesday with people in masks hurling stink bombs and squirting each other with flour and strong-smelling liquids; even arson was a form of entertainment. In 1840, the Italian wife of a Rio de Janeiro hotel owner changed the carnival celebration forever by sending out invitations, hiring musicians, importing streamers and confetti, and giving a lavish masked ball. In a few years the masked ball became the fashion and the wild pranks played on the streets disappeared.
Today Rio de Janeiro has the biggest and best known pre-Lenten carnival in the world – its most colourful event is the Samba School Parade. The samba schools taking part in the parade – each roughly having three to five thousand participants – are composed overwhelmingly of poor people from the city's sprawling suburbs. Every carnival Rio's samba schools compete with each other and are judged on every aspect of their presentation by a jury. Each samba school must base its effort around a central theme. Sometimes the theme is an historical event or personality.
Other times, it is a story or legend from Brazilian literature. The costumes must reflect the theme's historical time and place. The samba song must recount or develop it, and the huge floats must detail the theme in depth.