From all indications, we are not about to be dethroned. A new study published in a journal of the National Academy of Sciences says nearly 200 species have vanished in the past century, and 9,000 have seen substantial reductions in their numbers. Only 7,000 cheetahs are left, and the population of West African lions is down to 400. Scientists suggest that Earth is well into the sixth mass extinction of the last half-billion years.