What region and time period are you thinking of?
Depending on how far into prehistory your question is intended, this may be a more appropriate question for /r/askanthropology.
As other people have suggested, you may get more answers in /r/askanthropology.
We have no written records of the prehistoric period, so this question is difficult to answer. There are lots of speculations based on different things:
from hunter/gatherer societies alive today or in historic times - but there is no consensus there since not all hunter/gatherer societies are alike, and in any case, they are a poor proxy for 200,000-odd years of our species' existence.
from comparison to other primates - again, there is no consensus - some people claim we're promiscuous like bonobos, others claim we're not, we're more like chimpanzees. There are no convincing arguments from either side; people choose the primate that favors their own bias. And it's hard to see how good such "evidence" could be, given a cumulative 14 million years of divergence between us and chimps/bonobos.
from various biological parameters that have to do with body size or sexual dimorphism. Some say we ought to be more like chimps because sexual dimorphism is low among us. Others say we ought to be more like gorillas. Some cite the average weight difference between men and women to show our low sexual dimorphism. Others say weight is irrelevant, the chimp/gorilla divide is about strength not weight, and human males have 40% more muscle mass than females, which puts them in gorilla territory.
So there really isn't any consensus opinion from these particular lines of thought, with people having very divergent opinions backed by somewhat plausible stories. To what extent such stories are cherry picked is a tough call to make.
However, there is one line of evidence that I think is more objective, which comes from genetics. Several studies show that if we look at all the ancestors of the people alive today, then we find that we have on average twice as many female ancestors as male ancestors - that is, our ancestors had a higher effective female population size.
This means that while most women reproduced, only about half as many men did. Since each offspring requires both male and female parents, the conclusion is that some men monopolized a lot of women, while other men got none. So at least among lineages surviving among humans today, our prehistory shows strong evidence of polygyny.
Here are a couple references if you want to read further:
Wilder, J. A., Mobasher, Z., & Hammer, M. F. (2004). Genetic evidence for unequal effective population sizes of human females and males. Molecular Biology and Evolution, 21, 2047-2057.
Lippold, S., Xu, H., Ko, A., Li, M., Renaud, G., Butthof, A., … Stoneking, M. (2014). Human paternal and maternal demographic histories: insights from high-resolution Y chromosome and mtDNA sequences. Investigative Genetics, 5, 13.