A 33,000-year-old tooth found in Siberia offers new evidence about the prehistoric ancestry of man's best friend, according to a new study out Wednesday in a US journal. Dogs were already domesticated by the advent of agriculture, 10,000 years ago, but scientists still don't know how far back the history goes or just when and where the modern species we know as dogs became distinct from wild wolves. Some studies suggest that split happened more than 100,000 years ago, but the earliest known fossils from modern canines are just 36,000 years old.