Evolutionary thought usually focuses on individuals being selected for, and thus passing on individually valuable traits. The idea that groups can be selected for, thus passing on a trait of "groupishness," is usually dismissed. Jonathan Haidt, in The Righteous Mind, makes a case for group selection. It is precisely groupishness that gives humans an advantage as a species, and more social groups of humans their advantage, in the long run, over anti-social aggregations of people.
We see the same group advantage at the molecular level of society: families. Couples who successfully marry and raise children together have well-documented advantages in their own lives. Their children, likewise, benefit in many ways. The gap between the social class that marries and the class that does not is growing.
What this says to me is that successful marriage is an instance of group selection in an evolutionary sense. Lineages that produce more successful marriages get selected for, generation after generation. These benefits accrue quickly, too. It does not take the tens of thousands of years that random physical mutations do to become part of the dominant fraction of the species. People choose who they marry, and who they have children with, much more than other species do. Thus, those selecting for long marriages can do so deliberately and with pretty good information. And pass their advantage on in human-scaled time to their descendents.