A new article in Nature describes the genetic impact between primates and humans during the course of evolution. Researchers discovered humans lost DNA for "penal spines" allowing us to have larger brains and to have sex longer. The article suggests the significance of having sex longer leads to pair-bonding and eventually monogamy. Without which, human relationships would be very different than they today.
"Sex would be a very different proposition for humans if — like some animals including chimpanzees, macaques and mice — men had penises studded with small, hard spines. Now researchers at Stanford University in California have found a molecular mechanism for how the human penis could have evolved to be so distinctly spine-free. They have pinpointed it as the loss of a particular chunk of non-coding DNA that influences the expression of the androgen receptor gene involved in hormone signalling.
...Other molecular biologists praised the work for its clever approach and said it would open up new avenues of inquiry, particularly for those working on the evolution of the human brain."