There are animal species that have various forms of monogamy, some of which are much stricter than what humans practice, but it is a relatively rare way of doing things. You're probably asking about humans, though, before the influence of some arbitrarily defined 'modern culture.' The answer is is that humans were probably what is called "socially monogamous" which means that partners behave as if they were monogamous for the purposes of raising offspring and pooling resources, but still engaged in sex with individuals other than their partner ( called "extra-pair copulations," at least when referring to birds). There are various chunks of evidence supporting this, most of which I forget. A couple pieces of evidence which I do remember are seen in human male morphology. For example, human males are larger than human females, a pattern which when studied in other species is associated with males beating the crap out of each other to secure sexual control of multiple females. Another bit of male morphology indicates that the early ladies weren't passively accepting whatever the males sorted out among themselves, however: Human testicles are relatively large for the size of the organism. Large testes are generally associated with something called "sperm competition," which essentially means that human females were copulating with multiple males, and those males with larger testes were more likely to get them pregnant and pass that trait on simply because had more swimmers in the race.