Actually we do have an excellent idea of how our early beginings were where as marriage is conserned.

Odd factoid for you: Marriage customs involving monogamy developed in groups of humans that had little to no contact with each other for thousands of years independently and "predeominated", sugesting that as with many other human behaviors a certian level of hard wireing is involved into our sexual and yes our marriage prefrences.

And perhaps you need to be more specific in your query where as the institution of marriage is conserned if your trying to equate the modern urban westernized version of marrige as stereotypical for marriage in general as nessesarally as a bad thing with all those pendulating children... you may need to re-think your approach.

Divorce rates have always went up in societies that afford personal freedom a higher status over the good of the group as they become more decadent and succesfully urbanized, Rome being one of many excellent examples.