I know this bridges into anthropology and science a bit, but the title basically says it all: in prehistoric times was marriage (meaning here a lifelong or substantial commitment to a sexual partner) practiced? Did homo sapiens participate in these relationships from their beginnings, or did the practice "start" at some point? Going back further, did other species in genus homo participate in marriage?
The Code of Ur-Nammu clearly references some specific kinds of marriage ~2100 BCE, meaning the practice was common in that time and place. What else do we know?
You are correct to assume that this might be a question more fitting for an anthropologist, archeologist or even a sociologist. Within these fields, the evidence suggest that marriage predates even agriculture. It's obviously not easy to interpret this data, but there is evidence which suggests that hunter-gatherers inter-married with those from budding agricultural settlements.
Many sociologists and anthropologists claim that both the prohibition of incest with relatives of the first degree and marriage are both present in nearly all societies known to us. This prohibition of incest combined with the institution of marriage, cements the formation of alliances and connections to those who aren't next of kin. This practice has proven to be an essential practice for human societies throughout history and prehistory.
I'm not clear on what the earliest reference to marriage is, that depends on your definition of what a clear reference is. The Code of Ur-Nammu is one of the earliest examples of written law, but it is possible that there is some reference to marriage in religious texts predating this. That's something that should be answered by someone who knows a great deal more about early writings than I do.
sources :