A typical hunter-gatherer band in the rainforest in the Philippines in 1981
We make many jokes about living with mother-in-laws, but it may be just that sort of living arrangement which allowed us to grow culturally and to evolve as humans. After studying 32 existing hunter-gatherer societies, researchers have found out how our species has lived for most of it history, and discovered that in any given group, the majority are not related to one other.
An Agta hunter-gatherer group in the Philippines in 1981
This goes against many, if not most, long-held assumptions gained by observing our closest ancestors, primate groups. In current human foraging societies, individuals are generally well-mixed, with neither a mother- nor father-dominated lineage, and siblings as well as in-laws living together. Most of the societies studied were monogamous, and both male and females were able to leave to join another group.
Hadza hunter-gatherers in Tanzania
No other vertebrate societies have the same sort of social structure, according to what Kim Hill and his colleagues at Arizona State University's School of Human Evolution and Social Change suggest, as outlined in the March 11, 2011 issue of Science:
"The researchers explain that the large network of people who interact together peaceably have frequent opportunities to observe innovative behavior and imitate traits judged to be successful or common. The development of such social structures among early humans could help to explain why humans – but not other animals – evolved some of their costly learning mechanisms and were able to grow culturally, as a group."
Sources: 1. Science/AAAS Press release, 2