Ryan Padraza Comes Last is a full-blooded Indian, Sioux and Cheyenne on his father's side and Assiniboine on his mother's. He will soon receive his Lakota name: "A Rope." (Comes Last raises rodeo horses and always has a rope in his right hand. He likes to call Ryan his "right-hand man.") But despite his traditional roots and his Native heritage, Ryan may be one of the last of the Comes Last line allowed to enroll as a member of the Fort Peck Tribe.

According to the tribal Constitution, enrolled members must be at least one-quarter Assiniboine or Sioux, or a combination of the two. (Fort Peck is home to both groups, who share one government.) This method of measuring Native American ethnicity by percentage is known as the "blood quantum," and most Indian tribes use it to determine who can be admitted. A few use a different method, called "lineal descent," under which applicants need only prove they have an ancestor on the early tribal rolls. Before 1960, Fort Peck used lineal descent as well...