Charles Darwin's theory of evolution by natural selection is the subject here, and Andrew Marr argues it has done more than reshape biology: it has changed what people think it means to be human. This is the first episode of a three-part series, and Marr traces the 150 years since "On the Origin of Species" was published, showing how Darwin's ideas chipped away at the need for a creator and put religious authority on the defensive. He follows the theory past the science into territory Darwin never intended, tracking its influence on debates over human morality and where our ethical instincts come from. Marr presents this less as settled history than as an idea still working itself out in culture, still generating arguments in classrooms, churches, and courtrooms. The episode stays focused on consequences rather than mechanics: not how natural selection works, but what happens to a society once it accepts that explanation for its own existence.