Even after the Neolithic Revolution, many people continued to lead a nomadic or seminomadic existence, hunting and gathering or herding domesticated animals. People produced or gathered only enough materials to meet the immediate food, shelter, and clothing needs of their family unit. Even in societies that adopted farming as a way of life, people grew only enough for their own survival. Moreover, the family unit was self-sufficient and relied on its own resources and abilities to meet its needs. No great differences in wealth existed between families, and each person provided necessary support for the group. Group leaders relied primarily on consensus for decision-making. Order and peace were maintained by negotiations between community elders such as warriors and religious leaders. Stability also became dependent on peaceful relationships with neighboring societies, often built on trade. Early civilizations, by contrast, arose where large numbers of people lived in a relatively small, concentrated area and worked to produce a surplus of food and other materials, which they distributed through a system of exchange. For farming communities, this food surplus meant family size grew to six or seven children and caused the global human population to skyrocket. Population growth rooted in agricultural production led to larger cities, in which the food produced by farmers in outlying rural areas was distributed among the population of the urban center, where food was not produced. This system of specialization was a key feature of early civilizations and what distinguished them from previous societies. Individuals performed specific tasks such as farming, writing, or performing religious rituals. People came to rely on the exchange of goods and services to obtain necessary supplies. For example, artisans specializing in craft production relied on farmers to cultivate the food they needed to thrive. In turn, farmers depended upon artisans to produce tools and clothing for them. A weaver acquired wool from a shepherd and produced cloth that might then be given to a physician in exchange for medicine or a priest as payment for conducting a religious ritual. The system of exchange, however, created hierarchies within society. Those who could accumulate more goods became wealthy, and they passed that wealth from one generation to the next. This wealth led, in turn, to the accumulation of political and religious power, while those who continued to labor in production remained lower on the social scale. This social stratification, another characteristic of early civilizations, means that families and individuals could vary greatly in their wealth and status. Those who share the same level of wealth and status make up a distinct class or strata, and these strata or classes are ordered from highest to lowest based on their social standing. The nature of government also changed as populations grew. In smaller groups, decisions about war and migration were made in concert because no individual or family was likely to survive without the others. Also, in small communities, order and peace were often enforced at the family level. If someone acted badly, the customs of the society were brought to bear on them to correct the offending behavior. For example, the San of South Africa held a ritual dance to contact their elders for advice on how to correct a difficult situation. The act of coming together was often enough for the community to heal. In larger civilizations, officials such as priests and kings possessed the authority to command the obedience of subjects, who relied on the powerful to protect them. In return for physical protection and the promise of prosperity, farmers and artisans provided food and goods and, eventually, paid taxes. This exchange served to reinforce both the developing social hierarchy and the specialization of labor. As civilizations developed around the world in this way, they shared the features noted. Their existence did not mean the end of older ways of living, however. Nomadic and seminomadic peoples not only remained an integral part of the ancient world, they also provided crucial resources and a vehicle for the exchange of knowledge and culture. They were particularly important as a means of connecting one large city to another.