Chocolate starts with Theobroma cacao, the cacao tree. Usually about 20 feet tall, it's an evergreen native to the Central and South American tropics now cultivated worldwide within 10 to 20 degrees of the equator. (The Ivory Coast, Ghana and Indonesia produce more cacao by ton than the rest of the world combined.) Most cacao comes from millions of farmers who each have a few trees, often organized into cooperatives.
The cacao tree begins bearing its fruit, known as cacao pods, at around 4 or 5 years old. A typical tree might produce 20 pods each year, each pod containing 20-60 seeds — or what we call cacao beans. In many countries, the pulp surrounding the seeds is made into juice; it's believed that the tasty pulp led humans to first domesticate the cacao tree after the last ice age. The ancient Mayan word kakaw was transliterated by the Spanish into "cacao" in the 1500s.
There are three major bean varietals: Criollo, Forastero, and Trinatario. Criollo was first domesticated by the Maya, in Mexico. The beans are among the highest quality, but the trees grow slowly and yield fewer pods. Most chocolate is made from Forastero, which grows faster and more abundantly. Trinitario is a hybrid of Criollo and Forastero, with the aroma of the former and the hardiness of the latter.
Differences between species, photographed by Christine Delsol for SFGate