Dogs have been used for sledding in the Arctic as far back as ∼9500 years ago. However, the relationships among the earliest sled dogs, other dog populations, and wolves are unknown. Sinding et al. sequenced an ancient sled dog, 10 modern sled dogs, and an ancient wolf and analyzed their genetic relationships with other modern dogs. This analysis indicates that sled dogs represent an ancient lineage going back at least 9500 years and that wolves bred with the ancestors of sled dogs and precontact American dogs. However, gene flow between sled dogs and wolves likely stopped before ∼9500 years ago.