hoffman: there were stresses associated with the glaciation. there were obviously stresses associated with the greenhouse aftermath. so you could imagine that that would be an environment in which there would be strong selective pressure and, therefore that might be an incentive for evolutionary change. but the arrival of multicellular animals is not just a change. this is a biological innovation. this is a change to a world in which you had organisms which achieved a level of complexity and behavior that had never been seen previously. we don't have any idea at this point why a snowball glaciation and its greenhouse aftermath might have created a circumstance that would have been selectively favorable for this great change in the course of biological evolution but the coincidence in timing is tantalizing. narrator: while the question of how complex life emerged on our planet persists the work of scientists like paul hoffman and andy knoll brings us closer to the answer shedding light not just on how earth became habitable, but on how it remain

hoffman: there were stresses associated with the glaciation. there were obviously stresses associated with the greenhouse aftermath. so you could imagine that that would be an environment in which there would be strong, selective pressure, and, therefore, that might be an incentive for evolutionary change. but the arrival of multicellular animals is not just a change. this is a biological innovation. this is a change to a world in which you had organisms which achieved a level of complexity and behavior that had never been seen previously. we don't have any idea at this point why a snowball glaciation and its greenhouse aftermath might have created a circumstance that would have been selectively favorable for this great change in the course of biological evolution, but the coincidence in timing is tantalizing. narrator: while the question of how complex life emerged on our planet persists, the work of scientists like paul hoffman and andy knoll brings us closer to the answer, shedding light not just on how earth became habitable, but on how it