Native Americans, unknown

This is an iconic Edward Curtis photo titled "The Oath." Here is what www.edwardcurtis.com has to say about it:

The Oath, 1908
This is an important Curtis image and was chosen as the frontispiece for Volume IV of The North American Indian. This is also one of Curtis’ most sought after goldtones and the large area of sky helps create a most compelling goldtone. In this photograph the Apsaroke warrior, Picket Pin, takes an oath of truth and honor while two other warriors act as witnesses. The Oath requires the man to thrust an arrow through a piece of meat, place it upon a red-painted buffalo skull, raise it toward the sun and, if his words are true, touch the meat to his mouth. The ceremony was intended for use with difficult or controversial communications to help insure both the veracity and good intentions of the speaker.