Recognized/formalized man+woman monogamy: "oldest" cultural contract?

See (telegraphese) subject.

Excluding intra nuclear family, such as motherhood, siblings;
Excluding, maybe, kinship rituals
Definitely excluding priesthood, military-politico stuff.

I’m getting anthropological here, and I’m sure I’m WAG-ing with my inclusions/exclusions, and would love to know more.

You know, guys and dolls.

Geneticists use statistical analysis tools to map the history of genetic changes. Some linguists have used these tools to track changes in language and culture to determine when they split and changed (based on cognates, words that are retained as language changes). Based on this sort of analysis, some researchers claim that monogamy was the earliest marital relationship, and polygamy followed later with religious or social change. There was a recent article in New Scientist that highlighted this, but it is currently only available to subscribers.