Archeologists theorize that the social structure of the extended family was imported to hunting and gathering societies because

When you think about it, there is a very good reason why this was so. During hunter-gatherer societies the mortality rate was very high, children could die very fast and not surviving the age of five was common.  The extended family played a very important role here as they enabled a structure of support, help and compassion that each could be useful in different, critical times. 

Attack on the titian and seven deadly sins