Once tens of millions of bison thundered across the American plains, only to be slaughtered nearly to extinction in the nineteenth century. In all that carnage, one wild herd survived in the remote heart of Yellowstone, and its descendants roam there still.

That makes Yellowstone's bison uniquely precious. They are the only population in the country to have lived wild and free continuously since prehistoric times, never fully domesticated or fenced. To watch them graze the Lamar Valley or trudge single file through winter snow is to glimpse the West as it once was.

The animals are deceptively dangerous. Despite their bulk, bison can run three times faster than a person and turn on a dime, and every year visitors who approach too closely are gored for their carelessness. Rangers plead with tourists to keep their distance from creatures that are wild, powerful, and utterly indifferent to a good photograph.

The herd's survival is not without controversy. When bison wander beyond the park in search of winter forage, conflicts arise with ranchers worried about disease and grazing, leading to contentious management. Yet the animals endure, a conservation triumph and a national symbol, grazing the same valleys their ancestors did across an unbroken line stretching back beyond human memory.