The Indian's economics differed as widely as their languages and colors. They ranged from nomadic tribesmen to highly developed village Indians. But on the whole their social system was that of tribal communalism. Its basis was common ownership of the land, whether the tribe made its living by hunting, fishing, farming, or by a combination of all these occupations. The Indians did not grow wheat or rice; they grew Indian corn. There is much argument as to where the Indians originally came from. Some scientists hold that mankind itself originated in the Americas. This is an unlikely supposition, as remains of pre-human "man" have never been found in this hemisphere as in Asia, Africa and Europe. The popular view has it that the Indians came from Asia about 25,000 years ago. It has been estimated that the sea level was then about 225 feet lower than it is today. So the Indians could have crossed on solid land from Siberia to Alaska. From the latter area the newcomers are supposed to have gradually fanned out all over the western hemisphere. Separated by two great oceans, the Indians and the people of the other hemisphere knew nothing about each other and had no contact whatever with each other until the day Christopher Columbus sighted land after a long, dangerous voyage across the Atlantic.