By definition a social animal has some organized interaction between individuals. In the human condition there are complex social interactions within and between several organizational categories: individual, in-group, family, polity and ethnic culture. See also Hierarchy.
We prefer to avoid the term “society” as being too vague, but it can be understood as the sum total of social interactions that an individual participates in. In this definition, no two individuals live in the same society. Part of the reason for deprecating the concept of society is that due to its vagueness, society has no real organizational integrity, therefore can't be said to survive or not survive, and therefore only evolves as a consequence of the evolution of its component parts.
The individual is the highest level of biological organization, and the smallest visible unit in social organization.
We'll use the term in-group to refer to all of the many different social groupings that an individual participates in and identifies with. Family, polity and culture can be considered in-groups, but we will use these more specific terms when we can. Out-group is defined by negation to be everything outside the in-group. In-group is a real social organization—in-groups can thrive or fail, and so are subject to evolutionary pressure. The out-group is a construct of the in-group, and doesn't have any actual social reality, however the dynamics of in-group/out-group conflict and social enforcement of boundaries between the in-group and out-group seem to be a basic aspect of human nature present at all levels of social organization.
Family is the smallest unit of social organization. Because of the close biological relationships in the family and the crucial role that the family plays in human reproduction family organization is much more tightly controlled by evolved biological instincts than other social organizations.
Political organization has to do with power, authority, law, territory, and negotiation or conflict with other political organizations. If we use the term “polity” without further qualification, we mean the highest level of effective political organization, which in ancient times would be the village or tribe, and in modern times is the nation-state.
The state is a type of polity that first arose about 6000 years ago, and is characterized by territorial acquisitiveness, militarism, strict chain of command with a supreme leader, rule of law, taxes, money, grain as the dietary staple, written language and bureaucracy. As such, the state is a cultural idea. States that discovered the power of these cultural practices quickly overwhelmed neighboring non-state polities with their military power and economic productivity, so we now all live in states. This is cultural evolution in action. No individual chose to subordinate him or herself to the state out of enlightened self-interest.
See Culture for a discussion of the broad sense of culture as the sea of evolving ideas and behaviors that individuals navigate. We use ethnic culture to mean “a culture” in the anthropological sense of “there's this tribe where…” From the origin of modern man perhaps 100,000 years ago to about 6000 years ago (when the state appeared), culture was a complete package that you inherited from your tribe. It told you everything you needed to know about how to live and explained everything that mattered. It gave you your language, your dress, your music, your spiritual beliefs, your worldview, your tribal and family organization, and your tools and techniques for getting food, clothing and shelter. Also, due to common ancestry and intermarriage within the culture, you were more related to those in your culture, and so physically resembled them.
Because ethnic culture was so comprehensive and so closely aligned with important political and ecological boundaries, ethnic culture had clear social and geographic boundaries. You knew who you were, and you knew which tribes were “like us” and which tribes weren't. Because of this, ethnic culture formed the largest group of social organization.
It is important to distinguish between the contributions of culture and polity to society because the boundaries of culture and polity overlap in complex ways. Culture frequently spans multiple polities. In non-state cultures, adjacent villages may have basically the same language, lifeways, behavioral norms, and ethnic traditions in dress, music, etc., but have no higher authority enforcing cooperative behavior. In these cases we often see that cultures evolve behavioral norms such as stylized warfare that minimize the damaging effects of conflict between polities sharing the same culture. However conflict with polities from another culture quickly becomes total warfare: there are no shared social norms and the other tribe is demonized as the out-group.
Polities can also span multiple ethnic cultures, which is particularly likely in states. It is the nature of states to expand and engulf adjacent polities. The state may subordinate existing political structures or replace them entirely, but this is not possible with culture. Political alliances are intrinsically fluid and opportunistic, but we don't choose our culture. There is no cultural chain-of-command to coopt.
States impose an official culture from the homeland, invariably emphasizing state-centric cultural values of the rightness, honor and glory of military conquest and obedience to the state, but this is slow going. It is characteristic of the evolution from empire to state that people increasingly learn the state language, identify with the state, adopt new lifeways and state-centric values, and downgrade their previously vital culture to a historic ethnic tradition. Ethnicity is what is left when a culture degenerates to an origin myth.
Based on their own experience, Americans, Japanese or Australians may suppose that it is normal for a state to be culturally homogeneous, but it is a peculiar property of states that arose recently from military conquest and the marginalization or genocide of indigenous cultures. The degree of cultural homogeneity seen in Europe is partially a result of the less recent Indo-European cultural invasion (displacing almost all indigenous languages and imposing military and pastoral values), and also remnants of the Roman empire: the Catholic church, etc.