In the final episode, Cox explains what makes Earth a home for life and asks what ingredients were required for complex life to begin.
The fourth episode sees Brian in Australia looking at how the size of life affects each plant and animal, and how long it can survive.
Brian travels to Africa and Madagascar to analyse why Earth is a fertile place and how it allows complex life to exist.
In the second episode, Brian travels to the U.S. to showcase how the laws of science allowed senses to arise.