When the dinosaurs go extinct and 75 percent of life goes extinct after a meteor hits the planet, that's an era boundary. That's when we change from the Mesozoic to the Cenozoic.

The Anthropocene essentially would be the time of human influence on the planet. That's controversial though, because geology is a retrospective discipline. The rocks of the Anthropocene haven't been deposited yet, really.

The technical definition of the Holocene has to do with the extinction of a snail species in Sicily.

If you were back in the Cretaceous Period - the last of the time of the dinosaurs - and you were driving from New York to Philadelphia on the New Jersey Turnpike, you would be driving across water.

If we want to know how the Earth's biosphere is going to respond to the things that humans are doing to the planet right now, the only evidence that we have is how biotic systems have responded in the past.

We're changing things, in many cases in irreparable ways, and that will certainly be recorded in the geological record.

When the dinosaurs go extinct and 75 percent of life goes extinct after a meteor hits the planet, that's an era boundary. That's when we change from the Mesozoic to the Cenozoic.