Yet traditionally, before the federal government shackled resources to race, most tribes regarded ethnic boundaries as fluid. Social kinship, not biology, was the tie that bound. Marriages between neighboring tribes were often arranged to strengthen political alliances, and a child born of such a couple would become a full member of either the father's or the mother's tribe. Some tribes, like the Tlingit of the Northwest, married outside their clan as a general rule.

By the 18th century, tribes were absorbing non-Natives as well. The Comanches, who were famous raiders, captured and adopted Mexicans and whites, as well as members of other tribes. Quanah Parker, the tribe's last great chief, was the son of a white woman and a Comanche warrior.

"Indian people have always been looking for outside genetic material, way before Europeans showed up," says Wayne Stein, a Native American studies professor at Montana State University. "Indian people were probably the best farmers in the world in the 1400s -- and farmers understand genetics."

I'm left to defend
one lonely drop of blood.
I might terminate
if I get nosebleed.

Excerpt from "Cheeky Moon," a poem by
Ojibwe Indian Marie Annharte Baker. ("Termination" was a policy of the 1950s in which the federal
government sought to dissolve the Indian tribes.)

Roberta Garfield's modest tribal housing unit on the east edge of Poplar is a cheerful bedlam. Neighbors, relatives and pets walk in and out like extras in a musical. A teenager in basketball shorts rummages through the fridge. In the living room, a couple of kids sprawl on the couch watching cartoons over the crackle of a police scanner. A girl pours herself a glass of milk and departs, cradling a tower of Oreos. A tangle of small dogs bursts in and has to be chased out again. In the middle of it all, Roberta Garfield, a comfortably plump 73-year-old with a proud, direct manner, sips a cup of tea. 

"People say, ‘Why do you give so much away?' " she says, gesturing at her raucous household. "And I say, ‘So I can have enough.' My grandmother always said if you stop sharing, you'll never have nothing."

Garfield -- who is half Sioux and a quarter Assiniboine -- is equally generous about tribal membership. She has seven children, 24 grandchildren and 15 great-grandchildren. Of these, at least a dozen can't be enrolled. Garfield has become one of the vocal few who advocate using lineal descent as the enrollment standard at Fort Peck.

Robert McAnally is another; his support is fueled by the fact that his two sons are only associate members.

"You know, being an Indian is not in your quantity of blood," he says. "DNA is not transferred through your blood. Personality is not transferred through your blood. What makes a person an Indian comes from your heart, your mind, your soul, your practices."