Here are a few nuggets from Marriage, a History [1], the wonderfully written and encyclopedic book by Stephanie Coontz [2].
Among the Mbuti Pygmies of the Congo, a couple is considered married if they have lived together for two seasons.
In small scale societies, if a man and a woman are seen eating together alone, they are considered married.
The idea that in prehistoric times a man would spend his life hunting only for the benefit of his own wife and children, who were dependent solely upon his hunting prowess for survival, is simply a project of 1950s marital norms onto the past. The male/female pair was a good way to organize sexual companionship, share child rearing, and divide daily work. A man who was a skilled hunter might have been an attractive mate, as would have been a woman who was skilled at foraging or making cooking implements, but marrying a good hunter was not the main way that a woman and her child got access to food and protection.
Says, historian John Modell of the 1950s, “the ‘sorting’ of women into the marriageable and the future spinsters occurred early and vigorously.” The small, and suspect, minority of women who did not marry at the same age as their peers had less chance of ever getting married than their counterparts a hundred years earlier. They were what the Japanese called Christmas cake, likely to stay on the shelf after the twenty-fifth.