First thing you need to know: Before doing anything else, you should simply click "play" and start watching the video above. It doesn't take long for Robert Sapolsky, one of Stanford's finest teachers, to pull you right into his course. Better to watch him than listen to me.
Second thing to know: Sapolsky is a MacArthur Fellow, a world renowned neurobiologist, and an adept science writer best known for his book, Why Zebras Don’t Get Ulcers. Much of his research focuses on the interplay between the mind and body (how biology affects the mind, and the mind, the body), and that relationship lies at the heart of this course called "Human Behavioral Biology."
Now the third: Human Behavioral Biology is available on YouTube and iTunes for free. The course, consisting of 25 videos spanning 36 hours, is otherwise listed in the Biology section of our big list of Free Online courses (now 1,300 courses in total).