Robert Flaherty's 1922 portrait of an Inuk family. The first feature-length documentary; complicated by the staging revealed decades later.
From Wikipedia
Nanook of the North is a 1922 American silent film that combines elements of documentary and docudrama/docufiction, at a time when the concept of separating films into documentary and drama did not yet exist. In the tradition of what would later be called salvage ethnography, the film follows the struggles of the Inuk man named Nanook and his family in the Canadian Arctic. It is written and directed by Robert J. Flaherty, who also served as cinematographer, editor, and producer.