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Hunter-Gatherers Show Human Populations Are Hardwired for Density

The views expressed are those of the author and are not necessarily those of Scientific American.


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High density living seems like a particularly modern phenomenon. After all, the first subway didn’t run until 1863 and the first skyscraper wasn’t built until 1885. While cities have existed for thousands of years—some with population densities that rival today’s major metropolises—most of humanity has lived at relatively low densities until recently, close to the land and the resources it provided. Before farming, nearly everyone was directly involved in the day-to-day hunting and gathering of food, which required living at even lower densities. It would seem as though our current proclivity for high density living runs counter to our biological underpinnings, that density has been thrust upon us by the demands of modern life.

It’s easy to arrive at that conclusion, in part because density is a hot topic these days. More than 50 percent of the world’s population now lives in cities—a fact repeated so often it’s almost a litany. But reciting that phrase doesn’t reveal the subtle effects implied by the drastic demographic shift. People migrating from the countryside face untold challenges wrought by density. Cities are complex places, fraught with crime, diseases, and pollution. Yet cities are also places of great dynamism, creativity, and productivity. Clearly, the benefits outweigh the drawbacks or else cities would have dissolved back into the landscape.

The benefits of living close to other people are evident even to hunter-gatherers. Though their societies have changed over the millennia, studying characteristics of present-day hunter-gatherers can let us peer into the past. That’s what was done by three anthropologists—Marcus Hamilton, Bruce Milne, and Robert Walker—and one ecologist—Jim Brown. In the process, they seem to have discovered a fundamental law that drives human agglomeration. Though their survey of 339 present-day hunter-gatherer societies doesn’t explicitly mention cities, it does show that as populations grow, people tend to live closer together—much closer together. For every doubling of population, the home ranges of hunter-gatherer groups increased by only 70 percent.

The way home ranges scale with population follows a mathematical relationship known as a power law. Graphs of power laws bend like a graceful limbo dancer—sharply at the base and more gradually thereafter—toward one axis or another, depending on the nature of the relationship. They only straighten when plotted against logarithmic axes—the kind that step from 1 to 10 to 100 and so on. One variable, known as the scaling exponent, is responsible for these attributes.

Fig. 1 Hunter-gatherer home ranges scale to the three-fourths power. Above are representations of three populations and the size of their home range according to this relationship.

To see how scaling exponents apply in the case of hunter-gatherer territories, let’s look at the range of possible values and what each would mean in terms of density. If the exponent were equal to one, then home ranges would scale linearly with population size—10 people would occupy 10 square miles and 100 people would occupy 100 square miles. If the exponent were 1.2, then a group of 100 would occupy 250 square miles. And if the exponent were 0.75, a group of 100 people will only occupy 32 square miles. This last one is what Hamilton and his co-authors found.

Their result is the average of 339 societies, and there’s a bit of heterogeneity within that statistic. Not every group has a perfectly “average” way of hunting and gathering. Some hunt more, some gather more. Some find food on land, others in the water. Where and how hunter-gatherers get their food has a large impact on how densely they live, causing the density exponent to deviate slightly or greatly from three-quarters. For instance, groups which derive more than 40 percent of their food from hunting require larger territories because prey is not always evenly distributed or easily found. Their home ranges scale to the nine-tenths power, indicating sparser living. Gatherers require less space—their home ranges’ scale at the 0.64 power—largely due to plants’ sedentary lifestyles.

Hunter-gatherer societies which draw food from the water lived more compactly, too. The home range of aquatic foragers was consistently smaller across the range of population sizes—their exponent was 0.78 versus terrestrial foragers’ 0.79. Hamilton and his colleagues suspect this is because food from rivers, lakes, and ocean shores is more abundant and predictable than comparable terrestrial ecosystems.

But no matter what types of food are consumed, the overall trend remains the same. Every additional person requires less land than the previous one. That’s an important statement. Not only does it say we’re hardwired for density, it also says a group becomes 15 percent more efficient at extracting resources from the land every time their population doubles. Each successive doubling in turn frees up 15 percent more resources to be directed towards something other than hunting and gathering. In other words, complex societies didn’t just evolve as a way to cope with high-density—they evolved in part because of high density.

References:

Hamilton, Marcus J., Bruce T. Milne, Robert S. Walker, and James H. Brown. 2007. Nonlinear scaling of space use in human hunter-gatherers. Proceedings of the National Academy of Sciences 104(11): 4765-4769. DOI: 10.1073/pnas.0611197104

Image: Algerien by Patrick Gruban on Flickr.

 

Tim De Chant About the Author: Tim De Chant is the creator of Per Square Mile, a blog about density. He has written for the Ars Technica, Chicago Tribune, The Atlantic Cities, Grist.org, and others. You can find him on Twitter at @tdechant and on the web at www.de-chant.com/tim. Follow on Twitter @tdechant.

The views expressed are those of the author and are not necessarily those of Scientific American.






Comments 7 Comments

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  1. 1. kclancy 10:34 am 08/16/2011

    This is very interesting stuff from some colleagues I respect, so thanks for covering the paper.

    However, can we PLEASE move away from using “hardwired?” This finding doesn’t support anything being hardwired. It supports density being an ancestral condition. This is really cool, cool enough that we don’t need to talk about it being immutably hardwired into our brains to make it any cooler.

    In any case, again, I appreciate the coverage of this neat anthro paper.

    Link to this
  2. 2. neuromusic 1:31 pm 08/16/2011

    +1 @kclancy

    This study describes a mathematical relationship between population and resource use and it says *absolutely nothing* about population density being hardwired.

    The authors of the actual paper highlight that future work will be necessary to determine where this mathematical relationship between population and resource use came from. They propose studies of other social animals, as well as non-hunter-gatherer societies.

    If you want to use the term “hardwired” you better be ready to show a causal relationship between the genetics and the phenomenon of interest (and NO epigenetic effects).

    Link to this
  3. 3. mskele 4:16 pm 08/16/2011

    Ok, there’s a correlation, but why are some populations stable and others grow? The authors give us a hint when they point out that that these coefficients vary according to what kinds of foods are exploited. It appears to me that the causal variable here is the carrying capacity of the land, on which both the population size and range are dependent. All of this interacts with a third variable, technology, which changes carrying capacity and allows greater population. I don’t see where a drive for lots of close neighbors enters into the thing.

    Link to this
  4. 4. CherryBombSim 1:59 am 08/17/2011

    What they have shown is that if food supply is more abundant and predictable, hunter-gatherer population densities will be higher. Wow.

    I think maybe the power-law relationship says more about the range of habitats available to hunter-gatherers than about their “propensity” to cluster together, but I have to think about that some more.

    Link to this
  5. 5. jgrosay 8:45 am 08/17/2011

    Is there a limit for high density to be an advantage and start causing problems ?. Some animal populations engage in destructive and aggressive behavior when in a limited space or crowded place.

    Link to this
  6. 6. TimDeChant 9:36 am 08/17/2011

    @CherryBombSim—the authors also accounted for ecosystem temperature (a proxy for productivity) and population density still emerged as a significant variable. So while food abundance is one factor, it’s not the only one.

    Link to this
  7. 7. charlesnsiegel 3:12 pm 08/18/2011

    I am in favor of high urban densities, but I am afraid that this study reached absurd conclusions.

    When humans evolved as hunter-gatherers, they lived at low densities, typically about 1 person per square mile, because they needed a large range to gather enough food for each person. This is when we were hard-wired by evolution.

    In a study of existing hunter-gatherer societies, it is not surprising that density gets higher as population increases. These societies generally have a limited amount of free land around them, so they cannot expand their territories enough to keep their densities down to the optimum level for gathering food, as people were able to do as our species evolved.

    Nevertheless, these existing hunter-gatherer societies live at much lower densities than urban societies. The densities in the study vary from .56 to .316 square kilometers per person. The fact that density increases to 3 per square kilometer as population increases certainly does not prove that humans are hardwired for urban densities.

    My own theory is that early human societies spread out to hunt and gather during most of the year, but periodically came together in larger groups. Coming together in this way was adaptive, because it created a larger gene pool for mating, which makes it less likely for children to have genetic defects. People who were attracted to these dense temporary settlements were more likely to have healthy children, so the genes that attract us to these sorts of settlements spread through the gene pool, hard-wiring us to like density.

    Any theory about why we are hard-wired for density has to involve this sort of evolutionary advantage. The theory in this article does not do this, so it is completely inadequate as evolutionary psychology.

    Link to this

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