First, let's not forget about the waves of immigrants that stayed in the eastern seaboard cities after coming out of rural conditions. Some remained parochial, others were influenced by the cosmopolitan environs into which they came.

The constitution was drafted to bring together a group of very disparate states that were failing miserably under the articles of confederation. It was drafted in the summer of 1788 in Philadelphia as a reaction to the previous 13 years of paralysis under the articles.

In the 18th century, ideas like "liberty" meant different things than we think of today(See Bailyn, Idoelogical Origins of the American Revolution). Liberty to the founders was the liberty to own property, including slaves. "Life liberty and the pursuit of happiness" was really a lot more like "life liberty and the pursuit of profit" (see Jack Greene, Pursuits of Happiness). So, there's really not a question of knowing better--they knew, but it was really outside the realm of 18th century thinking on the subject. This is not an excuse, merely an explanation.

Even so, people like Jefferson accepted slavery as a necessary evil. However, as the South became increasingly alienated from the rest of the country, many enthusiastic but misguided defenses were made as to the "positive good" of slavery--e.g. had a civilizing effect, etc. These "positive good" defenses were only undertaken by Southern politicians at the very end, right before the beginning of the Civil War. The cost paid to end slavery in the US was indeed high, in much more than lives and dollars.

The origins of slavery in the US are complex. One of the best books on the subject is Edmund Morgan's American Slavery American Freedom oer 30 years old and still a seminal text. One of the key points that Morgan makes is that until indentured servants revolted in 1676 in Virginia (Bacon's Rebellion) race was a much more fluid concept in the colonies. For example, Anthony Johnson, a black man, owned a plantation complete with slaves on the eastern shore of MAryland in the 17th century (Breen and Innis, Myne Owne Ground)until "black" became associated with "slave" in the late 17th/early 18th century.

Finally, the indians of Virginia didn't make very effective slaves--they had the home field advantage and could easily remove themselves from the tobacco fields of early Virginia.

Keep readings, boys, keep reading!