My Four Apocalypses

Paleobotanist Kirk Johnson has studied the effects of the dinosaur-killing asteroid and led expeditions that resulted in the discovery of more than 1,400 fossil sites on all continents. His work has been featured in NOVA documentaries, including Ice Age Death Trap and the upcoming Making of North America. Most recently, Johnson has overseen the acquisition of the Nations T. rex, a 66 million-year-old skeleton that will be the centerpiece of the Smithsonians new dinosaur hall that will open in 2019. (15:54)