Origin of the heart shape

The traditional "heart shape" - two curved lobes tapering to a point at the bottom - has an unclear origin, possibly coming to us from several differing sources. Here are some popular theories, roughly organized in chronological order of their appearance:

* a representation of female genitalia
* a medieval attempt to represent what the scientifically-unknown heart organ looked like
* a representation of the imprint that a lipstick kiss makes on a piece of paper