The full wersion of the post is here.
It is not clear where the familiar shape originates. There was a plant widely used as a contraceptive in ancient Egypt (Silphium) which seeds were heart-shaped and even decorated coins (see the image).
The Heart shape was widely explored in Folk art, especially Dutch, Mexican, German and Polish, Welsh (lovespoons).
Today this symbol continues to evolve. For example the modern celtic knot style heart tatoos, the Polyamory hearts with either the eternity symbol or the “love outside the box” one, peace and love symbols merging together, etc.
I also did a Pinterest board dedicated to heart symbols: http://www.pinterest.com/artishfolk/know-your-hearts/