This language was long thought to be extinct. Then one man spoke up.

As a boy, Blas Omar Jaime spent many afternoons learning about his ancestors. In addition to yerba mate and torta fritas, his mother, Ederlinda Miguelina Yelón, passed on the knowledge she had stored in Chaná, a guttural language spoken by barely moving the lips or tongue.

The Chaná are an indigenous people of Argentina and Uruguay whose lives were intertwined with the mighty Paraná River, the second longest in South America. They revered silence, considered birds as their guardians, and sang lullabies to their babies: Utalá tapey-'é, uá utalá dioi – sleep little one, the sun has fallen asleep.

Mrs. Miguelina Yelón urged her son to protect their stories by keeping them secret. So it wasn't until decades later, when he was recently retired and looking for people to talk to, that he made a surprising discovery: No one else seemed to speak Chaná. Scholars had long considered the language extinct.

“I said, 'I exist. I am here,” said Mr. Jaime, now 89, sitting in his sparse kitchen on the outskirts of Paraná, a medium-sized city in Argentina's Entre Ríos province.

These words marked the beginning of a journey for Mr. Jaime, who has spent nearly two decades resurrecting Chaná and, in many ways, putting the indigenous group back on the map. For UNESCO, whose mission includes the preservation of languages, he is a crucial repository of knowledge.

His painstaking work with a linguist has produced a dictionary of approximately 1,000 Chaná words. For people of indigenous descent in Argentina, he is a beacon who has inspired many to connect with their history. And for Argentina, he is part of an important, if still fraught, reckoning over its history of colonization and indigenous extermination.

“Language gives you identity,” Mr. Jaime said. “If someone does not know his language, he or she is not a people.”

Along the way, Mr. Jaime has had a touch of celebrity. He is the subject of several documentaries delivered one TED Talk, lent his face and voice to a coffee brand and has appeared in an educational cartoon about the Chaná. Last year there was a recording of him speaking Chaná echoed over downtown Buenos Aires as part of an artist project that sought to honor Argentina's indigenous history.

Now a transition of the guard is underway to his daughter Evangelina Jaime, who learned Chaná from her father and teaches it to others. (It is unclear how many Chaná remain in Argentina.)

“It's generations and generations of silence,” said Ms. Jaime, 46. “But we won't be silent anymore.”

Archaeologists trace the presence of Chaná people back about 2,000 years in what are now the Argentine provinces of Buenos Aires, Santa Fe and Entre Rios, as well as in parts of present-day Uruguay. The first European plate of the Chaná was made in the 16th century by Spanish explorers.

They fished, lived a nomadic life and were skilled clay artisans. Colonization displaced the Chaná, their territory shrank, and their numbers dwindled as they assimilated into the newly created Argentina, which launched military campaigns to exterminate indigenous communities and clear land for settlement.

Before Mr. Jaime revealed his knowledge of Chaná, the last known mention of the language was in 1815, when Dámaso A. Larrañaga, a priest, met three elderly Chaná men in Uruguay and documented in two notebooks what he learned about the language . Only one of those books has been preserved and contains 70 words.

The wealth of information Mr. Jaime received from his mother was much more extensive. Mrs. Miguelina Yelón was an adá oyendén – a “female memory keeper” – someone who traditionally preserved the knowledge of the community.

According to Mr. Jaime, only women were Chaná memories.

“This was a matriarchy,” Ms. Jaime said. “Women were the ones who led the Chaná people. But something happened – we're not sure what – that caused men to take control again. And women agreed to give up that power in exchange for being the sole guardians of that history.”

Mrs. Miguelina Yelón had no daughters to whom she could pass on her knowledge. (Her three daughters all died as children.) So she turned to Mr. Jaime.

That's how he spent his afternoons soaking up stories about the Chaná, learning words that described their world: “atamá” means “river”; “vanatí beáda” is “tree”; “tijuinem” means “god”; 'yogüin' is 'fire'.

His mother warned him not to share his knowledge with anyone. “From the moment we were born, we hid our culture because at that time you were discriminated against for being aboriginal,” he said.

Decades passed. Mr. Jaime led a varied life, working as a delivery boy, in a publishing house, as a traveling jewelry salesman, in a government transportation department, as a taxi driver, and as a Mormon minister. When he was 71 and retired, he was invited to an Indigenous event and pushed into the crowd to tell his story.

Since then, Mr. Jaime hasn't stopped talking.

One of the first to bring him to attention was Daniel Tirso Fiorotto, a journalist working for La Nación, a national newspaper.

“I knew this was a treasure,” said Mr. Fiorotto, who tracked down Mr. Jaime and published his first story in March 2005. “I left there amazed.”

After reading Mr. Fiorotto's article, Pedro Viegas Barros, a linguist, also met with Mr. Jaime and found a man who clearly had fragments of a language, even if it had been eroded from lack of use.

The meeting marked the beginning of many years of collaboration. Mr. Viegas Barros wrote several articles about the process of efforts to restore the language, and he and Mr. Jaime published a dictionary of legends and Chaná rituals.

According to UNESCOIn 2016, at least 40 percent of the world's languages ​​– or more than 2,600 – were at risk of disappearing because they were spoken by a relatively small number of people, the last year for which reliable data is available.

Referring to Mr. Jaime, Serena Heckler, program specialist at UNESCO's regional office in Montevideo, the capital of Uruguay, said: “We are very aware of the importance of what he does.”

While his work to preserve Chaná is not the only case of a language once considered dead suddenly resurfacing, it is exceptionally rare, Ms. Heckler said.

In Argentina, as in other countries in the Americas, indigenous peoples faced systemic repression that contributed to the erosion or disappearance of their languages. In some cases, children were beaten at school for speaking a language other than Spanish, Ms. Heckler said.

Saving a language as rare as Chaná is difficult, she added.

“People have to commit to making it part of their identity,” Ms. Heckler said. “These are completely different grammatical structures and new ways of thinking.”

That challenge resonates with Ms. Jaime, who has had to overcome deeply held beliefs among the Chaná.

“It was passed down from generation to generation: don't cry. Don't show yourself. Don't laugh too hard. Speak calmly. Don't say anything to anyone,” she said.

That's how Mrs. Jaime lived for a while.

As a teenager, she shunned her heritage because she was bullied and verbally abused at school by teachers who doubted her when she said she was Chaná.

After her father started speaking publicly, she helped him organize language classes that he offered at a local museum.

During this process, she started learning the language. Now she teaches Chaná online to students all over the world. Many are academics, although some say they have traces of indigenous ancestry, while a small number believe they are descendants of Chaná.

She plans to teach her adult son the language so he can continue their family's work.

Back at Mr. Jaime's kitchen table, the older man wrote down his name in the language he is trying to keep alive. It was a name that he believes reflects the way he has lived. 'Agó Acoé Inó', which means 'dog without an owner'. His daughter leaned over to make sure he spelled it correctly.

“She knows more than me now,” he said, laughing. “We will not lose Chaná.”