Bryson continues stressing how much our existence depends upon luck: without a molten core to disrupt the “super ice age,” Earth would have stayed permanently frozen over, and human life would likely never have evolved. As before, however, Bryson shows how this is a double-edged sword, since Earth’s molten core also puts humans in peril on a daily basis. Once again, he wants the reader not to take the relative tranquility of Earth’s recent climate for granted. Bryson also indicates that scientific knowledge about Earth’s historical climate is highly limited.