Few traces remain of the domesticated dogs that populated the Americas before the arrival of Europeans in the 15th century. On page 81 of this issue, Ní Leathlobhair et al. (1) shed light on the origins of the elusive precontact dog population through genetic analysis of ancient and modern dogs. Building on earlier work, they show that American dogs alive today have almost no ancestry from precontact dogs, a monophyletic lineage descended from Arctic dogs that accompanied human migrations from Asia. Instead, the authors found that their closest remaining relative is a global transmissible cancer carrying the DNA of a long-deceased dog. It remains unclear why precontact dogs survived and thrived for thousands of years in the Americas only to swiftly and almost completely disappear with the arrival of Europeans.