For decades, the radical Snowball Earth Theory had been dismissed as impossible. In this program, geologist Brian Harland; meteorologist Mikhail Budyko; Joseph Kirschvink, of the California Institute of Technology; Harvard University’s Paul Hoffman and Daniel Schrag; Guy Narbonne, of Queen’s University, Ontario; and Chris McKay, of NASA Ames Research Center, explain the step-by-step scientific detective work that convinced them that 600 million years ago, global glaciation really did occur—and that such a freeze/thaw sequence may be a vital step in the evolution of complex life forms. Original BBCW broadcast title: Snowball Earth. (50 minutes)