River dolphins have no predators, except for humans. In December 2006 the Yangtze river dolphin, called the baiji, succumbed to pollution, propeller blades, dams, and overfishing; it became the first cetacean to be declared "functionally" extinct—meaning the species cannot renew itself, even if a specimen or two still exist. "Losing the baiji is like taking a chain saw to the cetacean tree of life," says Hamilton. "We've lost 20 million years of independent evolution." The Ganges river dolphin is also in grave danger; only a few thousand remain, and they live in some of the most polluted rivers on Earth.