Killing each other has an evolutionary origin

Humans likely inherited lethal interpersonal violence from primates, new study finds.  

Today’s Nature study finds that killing each other has an evolutionary origin. The closer two species are on the evolutionary tree, the closer the levels of violence. For humans, José María Gómez, Estación Experimental de Zonas Áridas and University of Granada, Spain and his team reconstructed probable rates of lethal violence at the time of our origin. The team compiled information on humans from 600 studies spanning Paleolithic samples (50,000 to 12,000 years ago) and Iron Age samples (3,200 to 1,300 years ago) to sources from the past few centuries. Humans come from an evolutionary lineage that has a long history of higher-than-average levels of lethal violence towards each other, they find. They estimate that at the time of our origin, humans were killing each other six times more often than the average mammal.

For other mammals, Gomez and his team compiled information on more than four million deaths from 1,024 mammal species drawn from 137 mammalian families. They used this data to calculate the proportion of deaths as a result of violence from a member of the same species.

We spoke to José María Gómez about the study.

ResearchGate: How did this study come about?

José María Gómez: We were originally interested in finding an objective method to explore the evolution of lethal violence in humans. We started naively thinking that if man is a wolf for other men, maybe we have to think first to what extent wolfs are, in fact, wolfs to other wolfs. This eventually led to a comparative approach to this longstanding question... can we find out how violent humans are by looking at how violent other species are? Evolutionary biology offered a very useful conceptual framework here because it has developed many tools to compare traits among related species

RG: What were your results?

Gómez: We found that human lethal violence has an evolutionary origin but can be modulated by some ecological and cultural factors, like the type of sociopolitical organization.

RG: How did you get these results?

Gómez: We got the results after compiling an exhaustive database containing the levels of lethal violence in more than 1000 mammal species. After tracking the evolution of this trait in the phylogenetic tree of mammals, we realized that closely related species tend to have similar violence levels. This allowed us to reconstruct the evolutionary origin of lethal violence in humans. Later, we compared this evolutionarily inherited level of violence with those observed for human societies with different sociopolitical organization.

RG: Why do you think we inherited this? How does it benefit us from an evolutionary perspective?

Gómez: “Evolutionary inheritance” is a widespread characteristic of many traits across all the organisms on the Earth. The explanation is very simple; closely related species have evolved from a common ancestor from which they have acquired the traits. And consequently, closely related species tend to have similar traits. The greater the phylogenetic relatedness between two species, the more similar they are.

This happens very frequently to many traits from many different organisms: related plants tend to have similar flowers, related mammals tend to have similar trophic habits, related birds tend to have similar courtship behavior. We use the same theoretical framework to study the lethal violence in humans, assuming that related mammals tend to have similar levels of violence.

It is hard, if not impossible, to understand the benefit of having a given level of violence. It is true that many authors, mostly behavioral ecologists, have discussed the adaptive value of lethal violence in humans and other mammals. However, as with other biological traits, adaptation is context-dependent. This means that there are costs and benefits of some traits, or behaviors, and the net balance can be different in different contexts. So, there is not a single ‘best’ strategy, a single level of violence that is the best. According to this, the levels of human violence have changed along with history, depending on the sociopolitical context. In summary, there is no overall benefit in humans having a particular level of lethal violence.

RG: How does culture and socio-political change affect this evolutionarily trait?

Gómez: We found low levels of lethal violence in prehistoric bands and tribes and high levels in present-day bands and tribes. Chiefdoms also showed high violence levels while state societies were the less violent sociopolitical organizations.

RG: Is lethal interpersonal violence just something we need to accept as human? Is there anything we can do to influence this?

Gómez: Understanding human nature can help us take advantage of our evolutionary history. At the same time, that evolutionary history is not a total straitjacket on the human condition; humans have changed and will continue to change in surprising ways. In a sense, our study has philosophical implications for how we picture human nature. The main message of the study, from our point of view, is that no matter how violent or pacific we are in the origin, we can modulate the level of interpersonal violence by changing our social environment. We can build a more pacific society if we wish.

RG: Which other types of animals kill each other?

Gómez: There are many mammals than kill each other. In fact, the biggest surprise of our study was the realization of just how widespread lethal violence is among mammals, about 40 percent of the 1024 studied species of mammals committed intraspecific killings. Of course, some mammals show little or no lethal violence, but its presence in the mammals was more common than we were expected at the beginning. It was also striking that lethal violence was not concentrated in those groups that we would tend to consider as "violent" a priori, such as carnivores. We found high levels of lethal violence in rhinos, ground squirrels, marmots, or horses, just to mention some examples.

RG: What do you want people to take from your study?

Gómez: The main message of this study is not whether lethal violence has changed along our history, something that other scholars working in social sciences can explore more accurately than us. The main message is that lethal violence has an important phylogenetic component. This is at the core of our rationale: If many traits have been proven to have significant phylogenetic signal, why not the lethal violence? And if this occurs, then we have the tools to infer the expected value of lethal violence in any species, including humans, just knowing how lethal violence is distributed across all other related species. Ours is just an attempt to use modern tools to shed light on an old problem.

The phylogenetic component in lethal violence does not imply that lethal violence is only due to a genetic component. Evolutionary legacy is not only a genetic legacy. This phylogenetic effect is more than a mere genetic inclination to violence. Other traits affecting lethal violence are also similar between related species. For example, social behaviour and territoriality, two behavioural traits shared with our relatives, seem to have also contributed to the level of lethal violence phylogenetically inherited in humans.

In the course of our study, we have compiled a vast volume of information that is available in the Supplementary Information file of our article. Our hope is that other scholars could use our data, and may even augment it, in order to see if our conclusions hold or change when expanding our knowledge about lethal violence both in humans and other mammals.

Featured image courtesy of stuckincustoms.