A History of Violence

In preparation for my novel Parabellum, I spent a lot of time researching why some humans act violently against other humans. But violence between humans is nothing new. In a study published last year in Nature, José María Gómez, Miguel Verdú, Adela González-Megías and Marcos Méndez discovered that intraspecies lethal violence has existed in humans and many other mammals from the Paleolithic era to the Iron Age and beyond. In this interview, Marcos Méndez expounds on some of the group’s major findings. Their original paper “The phylogenetic roots of human lethal violence” is available at www.nature.com/nature/journal/v538/n7624/full/nature19758.html

Greg: In your study, you found that, at the time of the origin of humanity, humans killed each other six times more frequently than the average mammal. Were humans the most common intraspecies killer? What other mammals ranked high on the list?

Marcos: No, ancestral humans did not have the highest level of intraspecific lethal violence. Mammals such as meerkats and several species of monkeys or carnivores (sea lions, lions, pumas) or even chinchillas had a higher frequency of intraspecific violent deaths.

An ancient cave painting of humans and animal violence

Greg: You also compiled data about how lethal human violence has changed over time and in different socio-political systems. What were some of these notable changes?

Marcos: Regarding changes with time, we found an increase in lethal violence during the Iron Age and the Middle Ages (in the Old World—similar trends were present in the New World), and a strong decrease (even below the levels of ancestral humans) for modern and contemporaneous ages (from 500 years BP [before present] to current times). Regarding changes with socio-political organization, we found that very simple societies such as ancient hunter-gatherers had a relatively low level of intraspecific lethal violence and chiefdoms had a much larger level of lethal violence. This lethal violence was lowest in modern and historical states, where socio-political organization is more complex.

Greg: Just because there has always been some level of violence between humans, that doesn’t mean humans can’t change their violent behavior, right? How do we take advantage of this knowledge of our evolutionary history to reduce violence?

Marcos: Exactly. Our study indicates phylogenetic (or evolutionary) roots, but this cannot be naïvely interpreted as meaning a “genetic component” or be utilised to support any genetic determinism. Our historical data indicate that the levels of lethal violence have changed dramatically in the last 20,000 years, suggesting that these changes are due to cultural, sociological or ecological factors and not to genetic factors. If we can understand how these factors have influenced the level of lethal violence, we can get useful insights of how to reduce conflict, or how to solve conflicts in a non-lethal way. Certainly, a lot of work remains to be done in this respect and this will be a very multidisciplinary task.

Greg: In fact, it seems that humans would have better success than other mammals at modulating their intraspecies violence throughout history. Did you also track changes in intraspecies violence in other mammals?

Marcos: We do not know whether humans are more or less successful than other mammals. Long-term data for other mammals are very rare or absent. But we can speculate that lethal violence in other mammals can also be affected by ecological, demographic or other factors and, thus, fluctuate in time.

“If we can understand how [sociological and other] factors have influenced the level of lethal violence, we can get useful insights of how to reduce conflict, or how to solve conflicts in a non-lethal way.”

Greg: Given the findings of your research, what’s next? Do you have any follow-up studies planned? How do you envision researchers outside of ecology and evolution building upon your discoveries?

Marcos: Yes, this study opens many avenues for future research. First of all, the scientific community needs more long-term studies about sources of mortality in mammals. In addition, factors influencing the levels of lethal violence need to be studied in depth. Our study combined many sources of death that could be studied separately. Finally, collaboration is needed between sociologists, philosophers, psychologists and evolutionary biologists to explore the consequences of our findings and to produce guidelines for conflict resolution.

José María Gómez is an evolutionary biologist who studies the evolution of ecological interactions, particularly plant-animal interactions. Miguel Verdú is an evolutionary ecologist whose research focuses on the evolution of plant interactions. Adela González-Megías is an evolutionary ecologist who studies interactions between organisms (particularly insects) and their distribution. Marcos Méndez is a plant ecologist with very general interests in evolutionary ecology. The majority of his research is in the sexual expression of plants, in plant-pollinator interactions and in how plants allocate resources to reproduction. Their paper “The phylogenetic roots of human lethal violence” is available in Nature.