Cosmologist and astrophysicist Martin Rees discusses the future of humanity on Earth and beyond, foreseeing a time when space pioneers may use gene splicing and AI to adapt to living on Mars, thus becoming "post-human." What concerns him most? "My worry, particularly about bio and cyber, is that whatever can be done will be done somewhere by someone. That is a scary prospect and is going to be a big challenge to governance, in my opinion."