Rafael Cosme was at a Rio de Janeiro antique fair six years ago when he found a pile of film negatives on the ground. No one wanted them, the vendor said. They were $2.

“I carried home two bags of negatives thinking: What am I doing with my life?” he recalled.

So began Mr. Cosme’s obsession with the lost and discarded photos of his city’s past. Since that morning in 2018, he has collected more than 150,000 film photos and negatives, nearly all shot by amateurs, that tell the story of Rio de Janeiro from the 1890s to the 1980s, one flash in time at a time.

In his work, he has noticed that one theme keeps popping up more than any other.

Carnival.

It is Rio’s annual collective exhalation — a four-day eruption of art and music, costumes and joy — that began again on Saturday.

The celebration has come to define Rio around the world, while also becoming an influential driver of the city’s culture.

“There is no researching this city without going through Carnival,” Mr. Cosme said.

But through the photos, taken over decades by photographers whose names are lost to history, he could see how Carnival had changed with the city, and vice versa.

From 100-year-old prints with a sepia tint to 60-year-old saturated Kodachrome slides, the images revealed changing trends in society, humor, fashion, drug use and sexual liberalization.

Taken by amateurs with the cameras of their day, the photos often have a ragged beauty to them, compared with today’s digital perfection, and also a special intimacy.

“I realized there are endless stories I could tell about this city,” Mr. Cosme said about his discovery of Rio’s lost photos. “Because inside every house, inside every closet, there is a box with revelations.”

Carnival, a days-long celebration ahead of the Christian observance of Lent, arrived in Brazil with the Portuguese colonizers, and for centuries retained traditions from Europe. It was a costume party of sorts, where revelers would hide their identities to play pranks on neighbors.

By the middle of the 19th century, Brazilians began adding music, dancing and revelry in the street. By the turn of the 20th century, it was a full-fledged party.

Around that time, Rio’s rich elites began parading around the city during Carnival in open cars, according to Maria Clementina Pereira Cunha, a historian who has written books about Rio’s Carnival.

It was partly a way to show off their wealth, she said. But when suburbanites began pooling money to rent cars to parade around, too, the trend fell out of fashion with elites and died in the 1930s.

Even with its constant evolution, Carnival remained a costume party. The photos show that many people, particularly among Brazil’s poor, crafted creative outfits at home using what they could find.

“Mothers sewed and embroidered so their children would look well presented at Carnival,” Ms. Pereira Cunha said. “That’s why they wanted their photograph taken.”

Costumes also were satirical and playful, sometimes referring to pop culture and current events — references that are not always so clear today.

One of the most popular costumes was men dressing as women. They were designed to be a joke, often playing up sexist tropes, and the costumes fell out of favor over time.

Clown costumes were long popular, but over the decades they grew more sinister. People who wore them often tried to scare other revelers.

Eventually, men from Rio’s suburbs created a style called “bate bola,” or roughly “slam ball,” a costume that involved menacing clowns who slammed balls tied to ropes against the street. This type of costume, seen in the fifth image below, became renowned for frightening children and is still common today.

By the 1910s, people began carrying glass bottles of a scented ether-based liquid that provided a brief euphoric high. Later the bottles gave way to pressurized cans. They were called “lança perfume,” or “perfume throwers.”

Revelers would spray the concoction into crowds or at strangers, often to flirt, said Felipe Ferreira, a Carnival historian at the State University of Rio de Janeiro.

The government banned the sprays in 1961, but a stronger version is still used illegally today.

Look closely at these photos to see people carrying the bottles and cans.

The 20th century also brought “blocos,” or street bands, which became an integral part of Brazilian Carnival, and still are today. They are each a social club of sorts that play music on the street, with drums, horns and often matching outfits.

They frequently marched through the city, fueling impromptu parties, with different blocos offering differing styles of music, costumes and themes.

By the late 1920s, the so-called samba schools arrived. These were formal groups of samba musicians and dancers who performed increasingly elaborate shows that told stories through costumes, lyrics and dance.

They were made up of largely Black residents of poorer neighborhoods, and they focused on celebrating their Afro-Brazilian heritage.

As they became Rio’s most popular Carnival attraction, the city shut down a main avenue for the schools’ parades, adding large decorations and bleachers, as seen in the photos below. The schools, meanwhile, adopted even more extravagant costumes and floats.

Today the parade remains the centerpiece of Rio’s Carnival, held in a dedicated stadium built in 1984.