Round 34-35000 BC, at the height of Stone Age, much of the water was frozen, the Barents strait 1500 km wide was like a road from Siberia to Alaska, what enabled the first people to come to America. They were hunting for food - they followed the game to Alaska in order to survive, once in Alaska it would take thousands of years to get through the North America
Later on, round 12000 BC - life was probably established on the western hemisphere at the time, the mammoth was dying out, the bison took their place as a principle source of food, overtime they started to eat plants - they enriched their nutrition.
By 3000 BC a primitive corn was being grown in New Mexico and Arizona - a strange culture, they buried their dead people in pyramids of earth.
By that time in Mexico, irrigation and canal system had already existed.