What is the theme in A Short History of Nearly Everything by Bill Bryson?

Bryson points out that at this stage scientists know far more about the topography of Mars than about the center of the Earth or the bottom of the Earth's seas. What humans do know about the Earth is often frightening. Throughout much of Earth's history, it has been too hot for humans to exist. Then, about 150 billion years ago, for an unknown reason, the Earth's climate cooled. It entered a series of ice ages, interspersed with warmer eras. Ice caps on the Polar Regions are a recent development in the planet's history, geologically speaking.