Supernaturalists frequently insist that social cooperation, like morality, cannot possibly have any natural cause and thus couldn't have evolved naturally. Of course, social cooperation exists in many other animals aside from humans and no one argues for divine intervention to explain their behavior.
What's more, recent research has hit upon an interesting natural explanation for high levels of social cooperation: monogamy
Recent work has suggested a possible resolution to this debate. arguing that strict lifetime monogamy in which females only mate with one male in their entire life is crucial for the evolution of eusociality. Monogamy leads to a potential worker being equally related (r = 0.5) to her own offspring and to the offspring of her mother (siblings). In this case, any small efficiency benefit for rearing siblings over their own offspring (b/c > 1) will favor eusociality.
In contrast, even a low probability of multiple mating means that potential workers would be more related to their own offspring. In this case, costly helping would require a significant efficiency advantage to rearing siblings over own offspring. Until group living is established, allowing the evolution of specialized cooperative behavior and division of labor, the ratio b/c cannot be expected to greatly exceed 1. For example, feeding a sibling is unlikely to be hugely more beneficial than feeding an offspring by the same amount. Consequently, in the absence of strict monogamy the population cannot even get started on the road to eusociality.
In support of this hypothesis. comparative studies have found monogamy to be the ancestral state in all the independent origins of eusociality studied. Monogamy originated first, giving a high relatedness, and then when ecological conditions led to a high enough b/c, eusociality evolved. Important ecological conditions include "life insurance" benefits of allowing helpers to complete parental care after the death of the mother (for example, ants, bees, and wasps) and "fortress defense" benefits of remaining to help use or defend a food source, when opportunities for successful migration are low (for example. aphids, beetles, termites, thrips, and shrimps).
Source: Science, March 19, 2010
It is interesting that monogamous relationships would create such strong evolutionary benefits, even if indirectly through the development of eusociality and extensive social cooperation. It's true of course that polygamous systems have existed in many human societies, but it's also true that those systems and societies haven't survived over the long term.
How much of that might be because those systems can't compete very well against (mostly) monogamous systems when it comes to social cooperation and internal efficiency?
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