It's actually quite rare in mammals (5%), so why did it evolve? Was it to be better parents together, to protect offspring from being killed by other males or instead to protect women from competitors? Well, it depends on who you ask.
Being a solitary individual who can't run all over the place protecting females from other males wins the prize, according to Cambridge zoologists Lukas and Clutton-Brock, who looked at 2,000 species of mammals and determined that when breeding females didn't like each other and female density was low, a monogamous mating strategy developed because males were unable to defend multiple females from other males.
Christopher Opie, anthropologist at University College London, and colleagues say instead that the prospect of male infanticide led to social monogamy in primates rather than due to a mate-guarding strategy or because of benefits to bi-parental care. They used a Bayesian analysis of 230 primate species and male infanticide came out on top.
Which is right? No way to know. The Cambridge zoologists say infanticide is not and the anthropologists and psychologists say it is. The only thing they both agree on is that paternal care is a consequence rather than a cause of social monogamy, so bi-parental care is not it. That has to warm the hearts of human fathers everywhere.
Science magazine cleverly yet nonsensically inserted a picture of Prince William, Princess Katherine (or whatever we are supposed to call her) and Baby Boy Prince George in their coverage of the PNAS paper, which can someday tell anthropologists about the evolution of link bait. I will do the same here because they know what they are doing; just the annual subscription fees of Science magazine alone could run Science 2.0 for the next 18,000 years, and I have no reason to claim anything at all about unicorn meat:
Prince William will stick with her because he does not want rival princes whacking his kid, according to a study. Not sure that is really true in 2013, but maybe... Credit: Andrew Winning/REUTERS. Link: Science
So why would monogamy persist after kids are raised? It does, notes the Science article. We are more like serial monogamists than actual monogamists, which explains why wealthier males who can afford to lose 50% of their assets get their kids a younger, prettier mommy in their 40s and 50s. Okay, whatever. We're still better than monkeys, right? Right??
D. Lukas, T. H. Clutton-Brock, 'The Evolution of Social Monogamy in Mammals', Science July 29th, 2013 DOI: 10.1126/science.1238677
Christopher Opie, Quentin D. Atkinson, Robin I. M. Dunbar, and Susanne Shultz, 'Male infanticide leads to social monogamy in primates', PNAS, July 29th, 2013, doi:10.1073/pnas.1307903110