The key lime (Citrus aurantifolia), also called the mexican or west indian lime, is a small, yellow, aromatic citrus fruit grown year-round in warm climates all over the world. Indigenous to Malaysia, it was brought to North Africa by Arab traders, to Europe by the Crusaders, and to the Caribbean by the Spanish. Around 1835, botanist Henry Perrine planted the first groves in the Florida Keys, and Americans came to know the fruit as the key lime.
In 1853, a struggling inventor named Gail Borden created condensed milk, which became popular in remote places where fresh milk was unavailable—like Key West, for instance. Key lime pie evolved after somebody in the area got the idea of making a "custard" with Borden's invention by combining it with the Keys' lip-puckering limes and putting it all into a pastry crust.
After severe hurricanes wiped out commercial key lime production in southern Florida in 1926, groves were replanted with the persian lime (Citrus latifolia), which is the common supermarket lime of today (and may be a key lime hybrid). Except for a few locals who still had key lime trees in their backyards, American pie makers started using those limes instead. Most of the world still grows what we call key limes, though—and, responding to new demand, markets have recently begun selling the real thing again, imported from Mexico and Central America.
Today, variations on key lime pie are found across America. Some dress up the custard with meringue or whipped cream, and most now use a graham cracker—because this is usually a refrigerator pie, and pastry crust gets soggy in the fridge. Many bakers cook their pies, but in Key West locals tend to insist that the pie doesn't need baking, because "the lime does the cookin'," and all agree: no green food coloring, ever. True key lime pie is mellow yellow.