The heart shape is recognized the world over as a symbol of romantic love and affection, but its historical origins are difficult to pin down. Some believe the iconic pictogram is derived from the shape of ivy leaves, which are associated with fidelity, while others contend it was modeled after breasts, buttocks or other parts of the human anatomy.

Perhaps the most unusual theory concerns silphium, a species of giant fennel that once grew on the North African coastline near the Greek colony of Cyrene. The ancient Greeks and Romans used silphium as both a food flavoring and a medicine—it supposedly worked wonders as a cough syrup—but it was most famous as an early form of birth control.

Ancient writers and poets hailed the plant for its contraceptive powers, and it became so popular that it was cultivated into extinction by the first century A.D. (legend has it that the Roman Emperor Nero was presented with the last surviving stalk). Silphium’s seedpod bore a striking resemblance to the modern Valentine’s heart, leading many to speculate that the herb’s associations with love and sex may have been what first helped popularize the symbol. The ancient city of Cyrene, which grew rich from the silphium trade, even put the heart shape on its money.

While the silphium theory is compelling, the true origins of the heart shape may be more straightforward. Scholars such as Pierre Vinken and Martin Kemp have argued that the symbol has its roots in the writings of Galen and the philosopher Aristotle, who described the human heart as having three chambers with a small dent in the middle.

According to this theory, the heart shape may have been born when artists and scientists from the Middle Ages attempted to draw representations of ancient medical texts. In the 14th century, for example, the Italian physicist Guido da Vigevano made a series of anatomical drawings featuring a heart that closely resembles the one described by Aristotle.

Since the human heart has long been associated with emotion and pleasure, the shape was eventually co-opted as a symbol of romance and medieval courtly love. It grew especially popular during the Renaissance, when it was used in religious art depicting the Sacred Heart of Christ and as one of the four suits in playing cards. By the 18th and 19th centuries, meanwhile, it had become a recurring motif in love notes and Valentine’s Day cards.