The avocado is an 'everyday' fruit that was once nearly extinct. It evolved to be eaten by giant prehistoric ground sloths, which would eat the fruit whole and disperse the large seed in their droppings. When these giant mammals died out 10,000 years ago, avocados might have vanished if not for human cultivation. The Aztecs called the fruit 'ahuacatl.' Avocados are unique among fruits because they are high in healthy monounsaturated fats rather than sugar. Because of their high fat content, they are often used as a substitute for butter or cream in vegan cooking. They also do not ripen on the tree; they only begin to soften once they are harvested.