Who is the earliest known society that had a formal marriage agreement and/or ceremony?

-Just a guess here, but the cavemen were most likely the earliest society. There were no lawyers, or pre-nups the male would simply club the female gently than drag her off to the cave where they would live untill one or the other, or both were eaten by a dinosaur.It seems to go with Humanity, there is Archaeological evidence of males being specifically buried with a female of a similar age, but at different times, leading to the inference that they were a couple, in other words married, as we have no way of knowing if a marriage ceremony took place, this is pure surmise.



However the marriage ceremonies all pre date the written word, so we can only conclude they certainly go back a long way.
Most ancient societies needed a secure environment for the perpetuation of the species,a system of rules to handle the granting of property rights, and the protection of bloodlines. The institution of marriage handled these needs. There is vague evidence that primitive people from the Stone Age all over the world already practiced a form of partner bonding. By the time of the Classical period however, documentation exist of marriage contracts.



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All ancient societies from which records survive had marriage, so we can guess that it existed as an institution before then. Also, modern hunter/gather societies, our best guide to how we lived before cities, writing, and so on, have marriage (the importance of ceremony may vary among groups).
Marriage has existed as long as humanity has existed, as far as we know. Even the most primitive tribes have marriage ceremonies and rituals. It is likely that marriage as an institution existed in distant prehistory.