Many past societies did not have atomic families like we do today.
In fact, even couple-marriage is not _that_ old (several thousands of years)? I remember reading about this stuff in "Origin of the Family, Private Property and the State", which, while somewhat dated (125 years ago or so?) is quite interesting. Matrilineal societies, group marriage (i.e. being 'married' to all people in a certain category so that if you travel to another village you have people in your 'family' or 'marriage group' there even if you've never met them) and other arrangements.