Humans are like many bird species, serially monogomous with ocassional cheating. Our australopithicine ancestors were definitely polygamous, they had a huge amount of sexual dimorphism. The transition from polygamy to monomgamy was a result of our brain expansion. Humans are unusual in that babies continue the rapid fetal rate of brain development for a year after birth, this is why human infants are much more helpless that those of other apes. Under such a situation, it becomes evolutionary advantageous for the father to stay with the the mother and offspring because the female would be burdunded with a helpless infant. This is easily seen in the age of weening. In hunter-gatherer societies a child is usually weened at the age of 4, other apes ween at age 5 or 6. This situation is also the start of the sexual double standard and patriarchy, since the male would want to be sure the offspring were his, and also why sexuality is much more private in humans than in other apes and is why male and female brains work differently.