4 ideas
A colonial port abruptly acquired a court, ministries, a royal library, a botanical garden, a press and European manners. The colony became the metropolis, and Rio's sense of itself as a capital dates from this reversal.
Singing was hushed and guitar playing was intimate, which suited small rooms and close neighbours better than the loud, percussion-driven samba of the streets. The dwelling pattern of the new beachfront Rio helped set the music's volume and texture.
The criminals absorbed the militants' methods of cell organisation, discipline and collective solidarity, and this gave rise to the Comando Vermelho. That group later organised drug trafficking in Rio's favelas, so a state policy of incarceration helped produce the armed structure that now fights the state.
Wealthy Rio lives on the flat strip between mountain and sea, while the favelas climb the hillsides directly above it, often with the best views in the city. Rich and poor are separated by altitude rather than by distance, so they share beaches, streets and Carnival while living in parallel worlds. Violence and festivity therefore occur in the same few square kilometres.