Egg

The oldest portable writing known dates from 60,000 years ago, and is on ostrich eggshell fragments. So I feel it is fairly reasonable to be writing the names of instances of love/sex and death on this kind of ground - quails' eggs in this case, since this trope is fairly old. I could not say what the earliest instance is, but Adam and Eve is a good one to start with in European culture. The skull and heart design is a decal, a memento mori, and a devil of a job to work with.

There's also some similarity between the shape of an egg and that of a human skull seen from above.