Like the community, the family is a social institution. Long ago, human beings lived in loosely-related groups. Each group had a common ancestor (a family member from the distant past). But for over a millennium (a thousand years), there have been two main types of families in the world: the extended form and the nuclear form. The extended family may include grandparents, parents, and children (and sometimes aunts, uncles, and cousins)--in other words, relatives living in the same house or close together on the same street or in the same area. In contrast, the nuclear family consists of only parents and their biological or adopted children. Because of the, industrialization in the nineteenth century, the nuclear family became the most common family structure.