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

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

Ms. Miguelina Yelón urged her son to protect their stories by keeping them secret. So it was not until decades later, recently retired and seeking out people with whom he could chat, that he made a startling discovery: No one else seemed to speak Chaná. Scholars had long considered the language extinct.