https://www.wired.com/story/how-a-6000-year-old-dog-cancer-spread-around-the-world/

High in the Himalayas, a heavy-coated dog trots behind the hem of a Buddhist monk’s robes. On the streets of Panama City, another dog collapses into a sliver of shade, escaping the heat of the midday sun. On their bodies a cancer grows. Their tumors each appear unique—their swollen, crumbling contours flush with fresh blood vessels emerging from beneath a tail here or between the legs there. But the cells dividing inside each one, continents apart, are actually the same organism. If you can call a clump of 6,000-year-old cancer cells an organism.

These ancient cells were once part of a dog that roamed the frozen Siberian steppe, a husky-like creature that lived in the time before humans invented the wheel or the plow. Then they mutated, finding a way to evade the canine immune system, a way to outlive their body by finding another. This cancer-cum-sexually transmitted dog parasite still thrives today, the only remnant of that now-extinct Siberian dog race. For millennia, it has been jumping between bodies, spreading like a virus around the world. Canine transmissible venereal tumor, or CTVT, is now found in modern dogs from Malawi to Melbourne to Minneapolis. It’s the longest-lived cancer known to man. But until now, no one had looked deeply into its DNA to trace its evolutionary origins and discover the secrets of its viral success.