Eric Michael Johnson has a fascinating piece in Psychology Today, Sex, Evolution, and the Case of the Missing Polygamists. I want to spotlight a few paragraphs:
Keep in mind that in terms of interpreting such genetic evidence we are of necessity confined to a fairly recent time depth (and remember, by "recent" someone like me means the last 10,000 years or so). For this time period multiple lines of evidence do indeed suggest that humans were moderately to extremely polygynous and that women were moving between groups more than men were. However, humans have been around for far longer than 10,000 years, with conservative estimates placing the emergence of modern Homo sapiens at about 200,000 years ago. A genetic record extending back 10,000 years is remarkable, but it's essentially adding only three more novels to our existing timeline. There is also something very important to consider that dramatically influenced human behavior within the last 10,000 years: the invention of agriculture. Prior to about 12,000 years ago all humans were hunter-gatherers and lived a migratory existence. With the advent of farming some human societies began to remain sedentary for the first time in our history. This change had serious impacts on human life and behavior. Just as Alzheimer's dramatically altered the content of Agatha Christie's work, so agriculture radically transformed human society and, by consequence, sexual behavior.
Cultural norms can be protean, but we humans have short time horizons. One model of human history which I am convinced of is that many "traditional" social arrangements which we view as old-fashioned and timeless are actually innovations which arose during the shift to agriculture, which allowed for the birth of complex societies in many regions of the world. Not only did customary norms shift, but the nature of customary norms varied across the society as class stratification emerged. Solemnized marriage between elite lineages may seem normative through much of the history of civilization, but this was obviously going to be much less of a factor for most of the peasantry. Both hunger-gatherers and farmers lived on the Malthusian margin. Excess population swallowed any gains in economic productivity. But obviously the change in population density and mode of production resulted in qualitative differences between the two classes of societies, especially for agriculture elites who could much more efficiently extract rents to sustain a more affluent lifestyle of leisure. Now much of the world is moving to the next stage: agricultural values combined with the reality of post-Malthusian consumer societies. Institutionalized marriage between a man and a woman in classic a bourgeois sense in the modern West is a hybrid of of values. No longer so much a bond between family lineages, as it was for pre-modern elites, nor is it an ephemeral and common law arrangement as it may have been for the masses (in part because of shorter life expectancies). Finally we should look past the West, and see that changes are occurring all the across the world as cultures are generating values "mash-ups." Over the 20th century in South Asia "Sanskritization" has taken hold and practices such as dowry which were normative among upper class groups have now spread throughout the culture. The decline of matrilineal social structures in parts of Southern India in favor of more conventional national implicit or explicit patrlineage is part of the horizontal homogenization through space which is concomitant with the vertical integration of values across class lines. Similarly, the rise of "Islamic Orthodoxy" and adherence to sharia can only exist in a regime of economic surplus, as punctilious attention to religious law is often not possible for families on the boundary between subsistence and starvation. Our "software" is in many ways a compound of deep evolutionarily encoded instincts, along with more recently crystallized norms and values which "have always been." But we've gotten a massive upgrade in the "hardware," and the software is adjusting and expanding.