Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS

snowball earth graphic
  Some scientists believe our planet was once completely encased in ice
Life appeared on this planet some 3.8 billion years ago, but consisted entirely of microbes until the Cambrian explosion just 600 million years ago- practically yesterday on the geologic time scale. What triggered this huge and rapid leap from single- to multicellular life?

In “Noah’s Snowball,” Drs. Paul Hoffman and Daniel Schrag of Harvard present what they call the “Snowball Earth” hypothesis. Based on geologic evidence from Namibia, China and New England, Hoffman and Schrag propose that 600 million years ago, the Earth cooled a few degrees, setting off a feedback loop that left the entire planet entombed in ice miles thick. The Earth's volcanoes, however, reversed the trend by belching carbon dioxide, the now notorious greenhouse gas, into the atmosphere. Within a hundred years, Hoffman and Schrag hypothesize, the planet's climate flip-flopped from the coldest it's ever been to the warmest.

photo of alan and fossil
Alan points to ancient evidence of Earth's biggest climate change  

Could this rapid reversal have spurred on animal evolution, resulting in the parade of life that ranges from plankton to people? Scientific American Frontiers travels from Boston to Yellowstone National Park to China to pinpoint the origin of animals.


For more on this topic, see the web feature:
Deep Freeze

return to show page




Noah's Snowball Handmade Humans Robot Independence Alien Worlds I, Robot Resources Teaching guide Science hotline video trailer The Sight of Touch Grow your own brain True or False What's in a dream Monastery of the Mind The Power of Half Contact Search Homepage video trailer Science hotline Teaching guide Resources Profile: Robert Edelman The Knowledge Michelle Geller The brain game