[dragonwriter]

> Marriage was adopted by the Church and redefined along the lines you describe - it existed long before that as a legal process to control inheritance.

For property purpose more generally: controlling inheritance was part of that, but a related part (and perhaps a more significant part in early societies where inheritances were small) was economic support the other direction, up the family tree rather than down. Where children (and grandchildren, and lateral relatives) care for their parents (and grandparents, etc.) in their old age instead of their existing nonfamily social support networks, establishing broader family bonds through formal unions is important to that.