I added one too many zeroes. Sue me.
Marriage, as we, the west, specifically Europe & the Americas, know it, has been practiced, basically as is, since the 13th century. Polygamy is still practiced in many (most?) non-western traditions. Polygamy tends to lend from (primarily) polygyny which is as more property rights than state of wedded bliss. More animal than evolved human social trait.
Marriage is one of the basic foundations of social traits, sending us up the ladder to civilization. It creates a bond & allows for the species to move past sperm spreading &/or safety structure to more advanced pass times, like creating nukes.