Walnuts are the oldest tree fruit that humans have enjoyed, dating from around 7000 BC. They were introduced to the United States around 1770 by Spanish missionaries. Their main area of cultivation moved from Southern to Northern California around 1852, and they were improved as a lighter variety with less bitterness. Thanks to the temperate climate, fertile land and little pest damage, California caters for as much as two-thirds of the entire world's walnut production, making walnuts one of the most important agricultural crops.