Has the Earth’s sixth mass extinction already arrived?

Palaeontologists recognize five major extinction events from the fossil record, with the most recent, the Cretaceous mass extinction, ending some 65 million years ago. Given the many species known to have disappeared in the past few thousand years, some biologists suggest that a sixth such event is now under way. Barnosky et al. set out to review the evidence for that claim, and conclude that the recent loss of species is dramatic and serious, but not yet in the mass extinction category — usually defined as a loss of at least 75% of Earth's species in a geologically short time frame. But that said, there are clear indications that the loss of species now classed as 'critically endangered' would soon propel the world into its sixth mass extinction.