Sunflowers grew wild all over North America. From the Paleo-Indian time on, many different groups of people picked sunflowers and ate the little seeds. Sunflower seeds are a good source of fat. People who live by hunting and gathering always need more fat to eat, because wild animals and fish have very little fat. Some people just ate the seeds. Other people ground the seeds up into flour and mixed the flour with water to bake a flat bread, like pita bread.
By about 3000-2000 BC, in the Late Archaic period, people in south-western North America (modern Mexico) began to farm sunflowers (grew them on purpose) and were encouraging the flowers to evolve bigger and bigger seeds, and more of them. About the same time, people like the Cherokee along the East Coast of North America also independently began to farm sunflowers.
This picture shows a domesticated (farmed) sunflower. It has a bigger middle than the wild ones, and bigger seeds. Sunflowers, like olives in Ancient Greece and West Asia and North Africa, became an important food for people in North America, because of the fat they provided. Further south, in southern Mexico, the Aztecs also ate a lot of sunflower seeds. At Aztec temples to the Sun the priestesses wore crowns made of sunflowers.
When the European invaders came to North America and South America in the 1500s AD, people showed them how to grow sunflowers too. Spanish explorers brought sunflower seeds back to Europe with them. At first, people in Europe grew sunflowers mainly for decoration. But by the 1830s people were farming sunflowers in Russia for their oil.
Then Russian people moving to the United States and Canada in the 1900s brought sunflower seeds back with them. In the 1930s, the Canadian government encouraged farmers to grow more sunflowers for food in Canada, and by the 1970s this idea spread south into the states of North Dakota, South Dakota, and Kansas. Today people in Europe also grow a lot of sunflowers for their oil.
Wild sunflowers still grow all over North America today. You may know them as Black-eyed Susans.