these civilizations were seen as highly impressive in many regards and in recent years historians have done well to accentuate those aspects of Central and South American societies (centralized states, impressive infrastructure, monumental architecture, organized trade, technological advances, etc.).

Meanwhile, relatively little attention has been paid to North America and the assumption of backwardness persists. The indigenous peoples of North America are often assumed to have lacked empires and to have been tribal and semi-nomadic or nomadic.

Why is that the case?

First, by the time of European contact, the North American societies appear to have already been in decline. For example, the Mississippian culture centered on Cahokia appears to have peaked sometime between 1200 and 1400, decades before Christopher Columbus' arrival in Hispaniola and more than two hundred years before Jamestown, which was the first permanent English settlement in North America. By contrast, the Spaniards encountered the Aztec and Inca empires near their peak.

Second, the North American natives built structures using earth and wood rather than stone. So, for example, Poverty Point in Louisiana, dating to about three thousand years ago, or Monks Mound in Illinois, dating to about one thousand years ago, must have been very impressive structures, but they have suffered from erosion. These earthworks are more difficult to identify and detect and many could have gradually blended in with the landscape and been lost to history. The process of detection and study is further complicated by the problem of access to private property, as was the case for some time with the Newark Earthworks in Ohio.

Third, for those who inhabit North America, it may be difficult to palate that impressive societies were destroyed through the actions of their forebears. It might easier to look at how the Spaniards ravaged the native cultures of Central and South America, for example, than to dwell particularly on something analogous in our own backyard and consider that something impressive was lost to history.

In short, some of the assumed differenced need to be reexamined, particularly in light of recent findings, but the contexts of the North American natives, both of yesterday and today, present some challenges for the historian.

The Native American societies in South America before European conquest were more advanced and unified than those in North America. Civilizations such as the Aztecs, Incas, and Mayas developed sophisticated societies with  knowledge of fields such as mathematics, medicine, astronomy, and technology. Their societies' advancements rivaled the achievements of progressive civilizations in other parts of the world. These societies were ruled by complex governments that were able to unify large numbers of people.

The societies in North America, on the other hand, were relatively undeveloped with regard to technology, and they existed as loose bands without complex governments. Archeologists have posited that South American societies were more able to become complex because people had access to nutrient-rich plant life, such as maize and beans, while Native Americans in North America had less nutrient-rich forms of agriculture. 

This is a difficult question to answer, not least because there was so much diversity among Native peoples within North and South America. Algonquian peoples who lived in what is today the Northeastern United States had very little in common with Pueblo peoples in the Southwest. It is true that many societies in South and Central America—the Inca, Maya, and Aztec in particular—developed highly complex societies and powerful empires centered in massive urban areas. Yet many Mississippian peoples in North America lived in large cities. Cahokia in what is today St. Louis was a massive city complete with large temple structures and ceremonial mounds. But if one were to generalize in the extreme, South and Central America saw the development of large, expansive, centralized societies to a far greater extent than did North America. Archaeologists and historians have long since dispensed with the notion that all Native people in North America were primitive nomads who had no important political structures. Also, they were connected by far-flung networks of trade that facilitated cultural change. But it is true that few societies in North America matched the power and the scale of the Aztec and Inca peoples.

This is somewhat difficult to answer because we do not know as much as we would like to know about Native American societies, particularly in North America.  However, it is generally correct to say that North American societies were not as urbanized and centralized as those in Central and South America.  

In Central and South America, there were many elaborate societies that left behind many traces of their existence or that were still in existence when the Europeans came.  For example, the Aztecs' capital city of Tenochtitlan was as big as any city in Europe at the time.  There is simply nothing like that in North America.  It is also true that North American societies (so far as we know) did not develop things like hieroglyphics and complicated astronomical calendars such as those of the Maya.

So far as we know, then, North American societies were less elaborate and developed in terms of their material society.  This is not to make value judgements about the societies, simply to say that they did not leave behind the ruins of large cities or writings that gave evidence of long-standing dynasties.