EARTH SCIENCE

Viewpoint: Yes, human impact is currently causing the greatest mass extinction in Earth's history.

Viewpoint: No, several measures indicate that the current mass extinction, while severe and alarming, is not the largest in Earth's history.

Of all the species that have lived on Earth over the last 3 billion years, only about one in 1,000 is alive today. The rest became extinct, typically within 10 million years of their first planetary appearance, an extinction rate that has contributed to the planet's current biodiversity level. Presumably, all the species alive today will experience the same fate within the next 10 million years or so, making way for our own successors.

Mass extinctions—catastrophic widespread perturbations in which 50% or more of species become extinct in a relatively short...