A Deep Evolutionary History of Family Relationships

Knowing where we came from helps us to better understand where we are today. Our experiences within our family of origin can have an important influence on various aspects of our adult life, such as how much school we complete or our expectations about adult relationships. Stepping back even farther and putting the question into a longer-term evolutionary context also helps us understand where we are today, and why certain patterns emerge across human cultures.

The most common assumption these days is that our deep ancestors, going back four million or so years, were similar to modern-day chimpanzees. This is a reasonable assumption in many ways, because chimpanzees are our closest living relatives and because there are certain key behavioral similarities between humans and chimpanzees. The most common focus of the human-chimpanzee comparison is coalitional male-male competition, where groups of males cooperate with one another to improve their status within the local community and to go on patrols to try and locate and attack males from neighboring communities. When successful, these male coalitions can eliminate the males of neighboring communities and expand their territory size, to their reproductive benefit and that of the females within their community. Although this potential is kept in check these days, a brief tour of human history confirms the pattern in our more recent ancestors.

At the same time, there are other aspects of chimpanzee behavior that call this assumption into question. The most critical of these differences are the lack of male involvement in parenting, the lack of long-term male-female relationships, and the sexual behavior of females. When they are ready to conceive, female chimpanzees will have sex with multiple males, meaning that none of them knows who the father is and thus none of them invest much in parenting. The absence of male parenting or long-term male-female relationships is not a surprise, because these don’t typically occur in mammals. The surprise is that they are common in humans, and found in one form or another across human societies. This is not to say that all men invest in their children or that all relationships that produce children are long-term, but these are more common than not. So, where did this come from, evolutionarily speaking? 

Another issue with the chimpanzee assumption is the degree of physical sex differences in our deep ancestors, the australopithecines (again, going back four million or so years). Unlike chimpanzees or humans, male australopithecines were much bigger than females (they weighed between 50% and 100% more than females). The magnitude of this sex difference suggests intense one-on-one male-male competition, which in turn is most consistent with either single-male harems or solitary males that controlled territories that encompassed the territories of several females.

On the basis of these and other factors, Mark Flinn and I proposed that our ancestors might have been more similar to our distant cousin, the gorilla. Among the key similarities are male involvement with and protection of offspring, long-term male-female relationships, and more discerning female sexuality. With these as an anchor, moving from a gorilla-like pattern to the current human pattern would require fewer evolutionary changes than those needed to move from a chimpanzee-like pattern to the human pattern. This is not to say that our ancestors were exactly like modern-day gorillas, but rather than may have shared certain social features with gorillas that shed light on some aspects of human behavior that are absent in chimpanzees.

First consider that males are twice the size of females, similar to the sex difference in australopithecines. For similarities to modern humans, let’s take a brief look at some key features of gorilla relationships. Their modal social organization is often described as isolated single-male harems which typically include one reproductive male, two to four females, and their offspring. These can be considered families, albeit polygynous ones. At least in mountain gorillas, these groups sometimes expand to include two or even three often-related males. The primary benefit that males provide to females and their offspring is protection from infanticide, which is a common and important form of paternal protection (when male investment is found) in primates more generally. Encounters between groups of mountain gorillas occur about once every five weeks and provide females their only opportunity to transfer from one group to another. During these encounters, physical male-male competition over females and male mate guarding are common, as is occasional infanticide by extra-group males; thankfully, male-perpetrated infanticide was lost at some point during our evolutionary history.

Groups of Western lowland gorillas more consistently maintain single-male harems, but the groups are less isolated. Several families will live nearby and encounters between groups are often friendly, especially among the males. DNA fingerprinting indicates that males in neighboring groups are typically related, and females are often related within groups. The kinship organization of male lowland gorillas provides a ready explanation for the friendlier behavior between males.

There are potentially important similarities between families of lowland gorillas and human families. Unlike the unrestricted mating of female chimpanzees (during estrous), and a corresponding low level of paternity certainty, adult male and female gorillas often form long-term social relationships and females often behaviorally elicit copulations with their mate.  DNA fingerprinting indicates that male lowland gorillas show high levels of paternity certainty (> 95%), levels that are consistent with those found in people; in other words, female gorillas, like most women, are pretty loyal to their mate. These types of relationships allow male investment in parenting to evolve, because nearly all parental behavior is directed toward their own offspring. The most important contribution is protection from unrelated males, but as Whitten described there is more to it; “Associated males hold, cuddle, nuzzle, examine, and groom infants, and infants turn to these males in times of distress” (Whitten, 1987, p. 346).

The genetic findings indicate that the male-kinship structure for lowland gorillas is close to that currently found with humans. The primary difference is the degree of cooperation among adult males as related to coalitional competition. Such coalitions could easily evolve from the gorilla social structure. The formation of more closely knit male kinships would result in greater proximity of males and through this the creation of the types of multimale, multifemale communities found in all human societies. Indeed, if gorilla families were placed in closer proximity and if male-kinship bonds were strengthened, the common structure of human families, including polygynous families, in traditional societies would be formed. The formation of male coalitions would lessen the importance of physical size and strength during male-male competition and place a premium on the brain and cognitive systems that support the formation and functioning of long-term coalitions. The predicted result is the observed pattern of an evolutionary reduction in the difference in physical size between males and females and an increase in brain size.

In any case, if our ancestors shared these features with gorillas, then we have a long evolutionary history of family formation, relatively stable male-female relationships, and male investment in offspring.  Now, humans clearly have a wide range of family structures, but it is nevertheless striking that the basic structure found in gorillas is found in all human societies.  

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Bradley, B. J., Doran-Sheehy, D. M., Lukas, D., Boesch, C., & Vigilant, L. (2004). Dispersed male networks in Western gorillas. Current Biology, 14, 510-513.

Duda, P., & Zrzavý, J. (2013). Evolution of life history and behavior in Hominidae: Towards

phylogenetic reconstruction of the chimpanzee–human last common ancestor. Journal of Human Evolution, 65, 424-446.

Geary, D. C.  Bailey, D. H., & Oxford, J. (2011). Reflections on the human family. In C. Salmon & T. Shackelford (Eds.), The Oxford Handbook of Evolutionary Family Psychology (pp. 365-385). New York: Oxford University Press.

Geary, D. C., & Flinn, M. V. (2001). Evolution of human parental behavior and the human family. Parenting: Science and Practice, 1, 5-61.

Harcourt, A. H., & Stewart, K. J. (2007). Gorilla society: Conflict, compromise, and cooperation between the sexes. Chicago, IL: University of Chicago Press.

Opie, C., Atkinson, Q. D., Dunbar, R. I., & Shultz, S. (2013). Male infanticide leads to social monogamy in primates. Proceedings of the National Academy of Sciences of the United States of America, 110, 13328-13332.

Plavcan, J. M., & van Schaik, C. P. (1997). Intrasexual competition and body weight dimorphism in anthropoid primates. American Journal of Physical Anthropology, 103, 37-68.

Robbins, M. M., & Robbins, A. M. (in press). Variation in the social organization of gorillas: Life history and socioecological perspectives. Evolutionary Anthropology.

Whitten, P. L. (1987). Infants and adult males. In B. B. Smuts, D. L. Cheney, R. M. Seyfarth, R. W. Wrangham, & T. T. Struhsaker (Eds.), Primate societies (pp. 343-357). Chicago, IL: The University of Chicago Press.

Wrangham, R. W. (1999). Evolution of coalitionary killing. Yearbook of Physical Anthropology, 42, 1-30.



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