Consanguinity is the condition in which two or more people share a bloodline. Engels describes the earliest form of the family as based on strict notions of consanguinity. In a time when human beings were divided among lines of tribe or kin, reproduction focused on the perpetuation of a bloodline (often determined by a shared ancestor). This required what is now called incest, as only family members shared the desirable qualities for offspring. Once it became apparent that connections with other gene pools facilitated the development of the human brain, and with it the advancement toward civilization, definitions of consanguinity gradually loosened and ultimately prevailed only among a handful of cultures that avoided the transition into the modern age.

The origin of the terms “genetics,” “generation,” and many other familial terms, Engels uses this word to describe a family unit linked by a common ancestor, traditionally but not always female. In his telling, a family based on the principle of gens had as its goal the perpetuation of the bloodline, eventually with the exception of incest since excessive inbreeding weakened their genetic stock. This communal focus allowed the gens to hold property in common and conduct their affairs with a kind of radical democracy.