Something I've always wondered about. It's neat to see a quantitative answer, finally. "To reach the new figure, Dr Lunter and his colleagues took advantage of the ability of evolution to discern which activities matter and which do not. They identified how much of our genome has avoided accumulating changes over 100 million years of mammalian evolution – a clear indication that this DNA matters, it has some important function that needs to be retained." http://www.sci-news.com/genetics/science-only-8-2-human-dna-functional-02083.html Not sure how robust their condition for 'functional' is though.