More than 2,000 years ago, a man died defending his family. For Mark Allen, it was a haunting reminder of how the struggle for resources can drive humanity to some of their darkest impulses.
The professor of anthropology at California State Polytechnic University, Pomona was reading through old accounts of the hasty salvage archaeology operation conducted on a site in an Oakland, California, suburb as it was dug up to make way for a paint factory in the 1920s. Archaeologists described hundreds of burials, but the story that most stood out to Allen was that of a middle-aged man whose bones were pierced at least four times by obsidian blades. The skeleton still had an obsidian blade in his hand. The bodies of three children and another man lay around him, also pierced.
"There's a lot of important information about this site, but one of those pieces is this guy who went down fighting trying to protect his family and failed," Allen says. The story in part inspired him to examine the causes of lethal violence among hunter-gatherers over the past two millennia in central California. Last month, he published a study on the cause of violent death in the Proceedings of the National Academy of Sciences.
"This study suggests that a key predictor in why humans would resort to violence is resource scarcity," Allen says.
The vexing question of how human violence originated has long provoked debate among anthropologists. Researchers are divided over questions like whether warfare was always with us or whether humans were initially peaceful, becoming more violent only as they became more organized. A 10,000-year old massacre recently discovered in Africa, for instance, reopened the dispute over how bellicose our hunter-gatherer ancestors truly were.
A survey of violence in the mammal world published in Nature in September found rates of violence in humans compared closely to evolutionary cousins like monkeys, lemurs and apes. Yet while humans may be only average-murdery among primates, that still makes them pretty violent toward each other: Primates are by far the most murderous mammalian group. The study suggested that we have a predilection for killing that has somewhat slackened as we became more organized.
To approach this longstanding problem, Allen and his coauthors needed a lot of data. They found it in the form of a massive database compiled from records of indigenous burials in central California excavated since 1975. Of the roughly 16,000 burials recorded over the past 5,000 years in the database, around 7.4 percent of the males and 4.5 percent of the females showed evidence of injuries from things like sharp blades, spears or arrow tips.
This was key: sharp-force trauma wounds, the researchers believed, were the best indicator of human-on-human violence. (Blunt force trauma could have been caused by falling, or even could have happened to the bones after burial.) Death by pointy objects, it seems, “is common throughout California's history," says Allen, who also wrote about the Oakland site in a chapter of the book Contemporary Issues in California Archaeology.
In the mid-1700s, California was one of the densest parts of North America above the Mexican border. The region boasted an estimated 300,000 people, compared to a total of 1 million across what is now Canada and the U.S. Many were concentrated in central California, partly due to the fact that it was a very productive environment, Allen says.
Groups of hunter-gatherers subsisted on salmon and birds, deer and other animals attracted to the water, and extracted flour from an abundant supply of acorns to last them through the winter. "[The area] could support a large population of hunter-gatherers," Allen says, "but that doesn't mean it was paradise."
To figure out the relative productivity of the area, the researchers turned to modern-day NASA satellite maps showing biomass, or the complete mass of biological material in a given area. Biomass, they write in the paper, demonstrates the abundance of food as well as the material available for hunter-gatherers to make tools. By comparing biomass and drought data to the burial evidence, Allen and team found that sharp-force trauma violence broke out more often when resources were low.
"That supports a long-standing hypothesis that resources scarcity would be the main predictor of the origins of violence and warfare in small-scale groups," he says.
Yet other researchers caution that merely measuring biomass is not a specific enough way to indicate available food. Net productivity in biomass might not always be the same thing as the availability of resources; it can also include, say, tree trunks, microbes and poisonous frogs. In the Amazon jungle, for example, there is a high amount of biomass, but there isn't always much for humans to eat.
The study “kind of raises my eyebrows a lot," says Raymond Hames, a professor of anthropology at the University of Nebraska who was not involved in the study.
Allen and team also examined the social and political complexity of the groups. They divided the burials into 19 different cultural groups with different levels of social complexity, based on other evidence found by the remains and Spanish accounts from missionary period. Organization levels varied from societies that had a single leader with informal councils of elders to those with powerful theocratic secret societies rather than secular leaders.
Yet none of these factors seemed to have any more influence on the number of people who'd been injured by piercing objects than resource availability. "[Groups with more command and control] don’t show more evidence of violence," Allen says.
While Hames appreciates the study's attention to causes of variation in violence rates, he questions the assumptions that political complexity remained consistent over the time period. "What guarantee do we have that that data reflects political complexity of 1,000 years ago?" he says.
Michael Blake, the head of the anthropology department at the University of British Columbia, questions the study's sample size, saying it isn't large enough to discount the possibility that political complexity may play a part leveraging violence. Blake points out that, while 19 different cultural groups were examined, the majority of these sit in the middle range of social organization, with only a couple outliers on the low or high ranges.
“I think it’s a really great idea as far as it goes,” Blake says. The solution, he adds, would be to examine a wider range of societies along the Pacific coast to see if the results still rang true.
For Lawrence Keeley, an anthropologist and author of the book War Before Civilization, any attempt ot examine the relationship between war and political or social complexity is futile. He says Allen's study crushes the concept of the myth of the pacified past, but is concerned that the argument that resource scarcity drives violence levels is too simplistic. "Humans are not passive subjects of their environments, but planners and anticipators," he points out.