Evaluating Claims: Mass Extinctions

4 Humans have existed for 200,000 years on Earth. Biologists and paleoecologists (who study ancient ecosystems) estimate that humans have driven roughly 1,000 species extinct. Early man helped hunt the biggest animals — woolly mammoths, giant kangaroos and giant sloths — to extinction. Since 1500, we have killed off at least 322 types of animals. The passenger pigeon, the Tasmanian tiger and, most recently, the baiji, a freshwater dolphin in China are all gone forever.

Thousands Of Species Threatened

5 Another 20,000 or more species are now threatened with extinction, according to the International Union for the Conservation of Nature. The group keeps a list of all the known endangered plants and animals on the planet. There are 5 million or so species on the planet. Thanks to humans, they are all 28 percent smaller on average. And as many as one third of all animals are either threatened or endangered, a new study in Science finds.

6 Scientists call this sixth mass extinction the "Anthropocene defaunation." The Anthropocene is a name some geologists call the period of time that humans have ruled the Earth — and changed it.

7 They can't be sure how quickly it's happening. Perhaps that's because much of it is happening to beetles and other insects that go overlooked. But according to the study in Science, their numbers fell by half over the past 35 years. At the same time, the human population doubled. Other recent studies suggest that the current extinction rate is roughly 1,000 times faster than the average pace in Earth's history. That makes this the fastest extinction event on record, even if it is not yet a mass die-off.

8 The biggest animals still left on the planet — elephants, tigers, whales, among others — are most at risk. And we humans have shown no desire to stop the activities that drive extinction.