The greatest extinction event in 66 million years?

Biological communities are changing rapidly in response to human activities, with the high rate of vertebrate species extinction leading many to propose that we are in the midst of a sixth mass extinction event. Five past mass extinction events have most commonly been emphasised across the Phanerozoic, with the last occurring at the end of the Cretaceous, 66 million years ago. Life on Earth has, however, always changed and evolved, with most of the species known to have existed now extinct. The question is, are human activities increasing the rate and magnitude of extinction into realms rarely seen in life history? Considering the literature on extinctions, predominantly over the last 66 million years, we ask what comparisons can be meaningfully drawn and when the Earth last witnessed an extinction event on this scale. We conclude that the available evidence suggests that the current event has likely surpassed most other events during the Cenozoic with the exception of the Eocene-Oligocene event for which much uncertainty remains. Currently we are still a long way short of the devastation caused by the bolide impact 66 million years ago. Given the number of endangered and at risk species, the current event is still in progress and much will depend on how humanity interacts with the rest of the biosphere over the coming millennia.