When people ventured across the land bridge connecting Siberia to North America, dogs trotted by their sides. Canines and their human companions spread throughout the continent for thousands of years, settling from California to Nova Scotia and down to Peru. These dogs ranged from the size of squat bull-terriers to hulking malamutes. By all accounts, these early American hounds were thriving. That was, until the arrival of European colonists, when the continent of dogs was snuffed out, leaving hardly a genetic trace, according to research published last week in Science.