There is some disagreement as to the date which should be assigned to the beginning of Great-Pueblo times, for cultural development was not equal in all sections of the Plateau. In some areas, people were still living as they had in Developmental-Pueblo times, while, in others, Great-Pueblo traits were well established. Since specialization became so marked that various cultural centers must be considered separately, it is best, in most cases, to give dates for this period in terms of specific areas. There is, however, some agreement as to the ending date. In general it may be said that Great-Pueblo times began, in most places, about 1050 A.D. and lasted until the end of the thirteenth century, when the whole northern portion of the Plateau was abandoned. The greatest change from the preceding period was in the realm of architecture. There were a great many unit houses, in which a fairly large percentage of the population lived, but big “apartment houses,” up to five stories in height and containing hundreds of rooms, were also built. This change naturally affected not only the living conditions of the people, but influenced their whole life, for people living together in a closely-knit community will develop differently from the way they would in widely scattered settlements. [Illustration: Fig. 27—Great-Pueblo diorama in the Museum at Mesa Verde National Park showing Spruce Tree House. (Courtesy Mesa Verde National Park.)] When a population is broken up into small independent units and scattered over a wide area, there is not likely to be any need or desire for overall government, and authority is usually vested in the person of the head of the family or clan. As the size of the group increases and life becomes increasingly complex, some centralization of power is inevitable. Cooperation between individuals and groups of individuals becomes not only desirable but essential. In such an undertaking as the building of a huge structure, containing hundreds of rooms, there must be cooperation. With the occupation of such a building, when as many as a thousand people may be living under one roof, the need for working together continued. With greater cooperation, leisure is likely to increase, although sometimes this greater freedom is limited to a ruling caste which makes great demands on the time of other individuals. This does not appear to have been the case among the ancient Pueblo people as they seem to have had an essentially democratic form of government.