What Makes Us Different?

While sitting at a stop light I happened to look over and noticed the passenger in the car next to me. We were doing pretty much the same things, being a passenger,drinking from a straw and waiting for the light to change.
As I turned away the strangest thought flitted through my mind. How could two people doing similar things ever be thought of as different or better. The small casual actions of slurping from a straw showed us to be foundationaly the same.
And yet it caused me to question the humanity of man. How could slavery, the horror of concentration camps,the mistreatment of any minority be justified? Were our ancestors any different than we are today? Didn't they want the same for their children and families as we do and have succeeded in acquiring? Aren't we breathing the same air? Bleeding the same blood? Wearing the same skin? What makes us different today than then?