A plausible end-of-the-world scenario you've probably never thought of

A plausible end-of-the-world scenario you've probably never thought of: "You've seen the world end with fire, earthquakes, and WTF in movies like 2012 and The Core. But to find out about truly horrific, plausible scenarios you need to ask a scientist. We talked to Cal Tech's Joe Kirschvink, a geobiologist who studies the ancient history of life — and mass death. He gave us a nightmare scenario where one evolutionary shift in ocean microbes leaves humans unable to breathe."