Afterthought 2 February 2013 :
Be aware that induction on which our mathematics and science is based is a trap for fools, and I will illustrate it this way :
A martian comes to our planet and find a lot of replicas of mankind. He sees the replication as a law of our planet, where every specimen of mankind is a copy of half of itself, being the left and right of two identical sides. Each person has these two identical sides with identical parts like two identical arms, two legs, two eyes, two ears, two brains, two middle fingers, two big toes ... and so on.
By induction he is allowed to extrapolate that what he cannot see in the inside of us are also identical and he confirms his postulate on a 95% probablity that we also have two hearts.
Just to bad. Science is always right. We should have two hearts.