The Frontal Cortex

Population Density

What led to the birth of human civilization? How did a naked ape manage to invent complex cultural forms such as language and art? One possibility is that something happened inside the mind, that a cortical switch was flipped and homo sapiens was suddenly able to paint on cave walls. But that doesn’t seem to be the case, as UCL anthropologist Ruth Mace explains in a recent Science article:

Traits such as the creation of abstract art, improvements in stone and other tools, long-distance “trading,” and the manufacture of musical instruments mark the emergence of modern humans who behaved much as we do (see the figure). These material expressions of the modern condition emerged much later than did anatomically modern humans. Some aspects of behavioral modernity first appeared in southern Africa, possibly as early as 90,000 years ago, only to disappear again and reappear in Eurasia ~45,000 years ago. The timing of these events makes a biological change in cognitive capacity a somewhat unlikely explanation.

What, then, led to the birth of “modern human behavior”? A new paper by Adam Powell, et. al. argues that the crucial shift was increasing population density. The main evidence for this effect comes from genetic studies of the late Pleistocene, which demonstrate that “densities in early Upper Paleolithic Europe were similar to those in sub-Saharan Africa when modern behavior first appeared.”

The larger implication is that the birth of human culture was triggered by a new kind of connectedness. For the first time, humans lived in dense clusters, and occasionally interacted with other clusters, which allowed their fragile innovations to persist and propagate. The end result was a positive feedback loop of new ideas.

While it’s very nice to have some statistical evidence for this idea (even if I can’t pretend to understand the “Bayesian coalescent inference” method used by the scientists to calculate the population densities in the late Pleistocene), it’s worth pointing out that the density explanation isn’t particularly new. In The Economy of Cities, Jane Jacobs forcefully argued against the “dogma of agricultural primacy,” which assumed that farmers and agricultural innovations made civilization possible. Jacobs argued that the dogma was exactly backwards, and that it was the density of urbanesque clusters which generated the innovations that made farming possible. As Jacobs writes: “It was not agriculture then, for all its importance, that was the salient invention…Rather it was the fact of sustained, interdependent, creative city economies that made possible many new kinds of work.” After all, you can’t learn how to grow food until you’ve got a system for transmitting knowledge, which is why population density is so essential.


  1. #1 davidavid
    July 2, 2009

    So, out of population density agriculture was born. But how exactly did humans achieve such settled density in the first place? By hunting & gathering?

    This is simply a chicken-before-egg argument, which is always fun to have since you can never really prove your position or ever be proven wrong.

  2. #2 DR
    July 2, 2009

    The ability to paint on a cave wall, to use your example, Jonah, is quite different from the inclination to do so. Inclination toward such behaviors would surely vary with population and prevailing culture. But what of the “cortical switch” that enables these types of behavior in the first place? If it’s plausible to suppose that dense populations created (selected for) individuals with imaginative ability, isn’t it more plausible to suppose that individuals with imaginative ability might have survived in greater numbers and found themselves in larger groups? If I had to guess, I’d date the cortical switch prior to the formation of increasingly dense human populations, and in any event, I wouldn’t much trust any evidence of it based on anatomical remains. Toulouse-Lautrec could have painted a hell of a cave.

  3. #3 NB
    July 2, 2009

    Excuse this brief caveman rant:

    So, if my Muk clan meet your Uug clan, Muk know where meat is and have pointy sticks. Uug know where good plants and have baskets. Muk and Uug try to make noise with mouth, cant do good at it. Uug use berries draw picture. Muk use blood draw picture (possibly the beginning of culture, not necessarily modern human). Now Muk and Uug friends and survive and share/trade ‘ideas’ (furthering of culture). Make big clan. Meet more big clans. Make city and or farms. Continue to share/trade. Lots of people in the same area(density). Muk and Uug decide that everyone needs to communicate in one way so everyone can share/trade more efficiently. Maybe new clan figures out how to farm and raise animals, or build city. Now big city with agriculture has what we call ‘modern culture’. Food, language, ideas, tools etc…

    I think its a really great idea, that is the cortical switch that occurred that made us modern. I would like to assume that due to our basic need to survive; our cities, farms, art, tools, music and what not, our essential culture grew simultaneously with us, and we with it. Just where modern man began to develop is a great question. What makes us ‘modern man’? Are we modern because we are civilized and practice certain culturally accepting behaviors when immersed within our own culture? What about being in a different culture? Is modern man the human that can act accordingly regardless of culture thus adapting and increasing his chances for survival? Is it our technology
    (tools)and medical practices that make us modern?

    Apologies again for the cave man talk, but I am very curious as to what makes modern man? My hope would be one that makes peaceful agreements in order for the good for all and the survival of the species as a whole regardless of culture. I’m afraid though if this is true, we have yet to become this modern man we speak of becoming. I look forward to some feedback. Positive and negative are welcomed. Thanks for your time.

    Undergrad Wayne State B.S. Bio, B.A. Phil

  4. #4 Art
    July 2, 2009

    As population density goes up you start to be able to suppress predators simply because of the strength in numbers and the ability to set up watchmen and guards. This allows creation of safe zones where the weak and young are relatively safe and where people can concentrate on other things. This ability to concentrate allows specialization and expertise.

    It can be assumed nearly everyone could cover the bases of being able to hunt, manufacture workable tools and weapons, gather food and assemble a simple shelter. But, as with all things, some people are more skilled and so over time you get specialization as people want o use the slightly better spear and a talented spear maker can spend more of his time making spears and less time on other skills. With a safe zone and specialization that allows all skills to be covered within a community of specialists and not the individual you get barter and trade.

    Specialization and trade allow creation of what amounts to guilds as people within a specialty seek to capitalize on their skills by keeping specialized knowledge secret and transmitting this knowledge to the next generation within the specialty. Specialists also advance the technology of their field faster as they focus their mental resources and activities on one subject instead of spread over a wider area of concern. Specialists that start making marginally better spears eventually are able to make increasingly improved models. This working both ways in that the spears the specialist get better and the spears made by non-specialists get worse as they begin to depend on the specialists for most or all of their spears.

    This process could considerable predate agriculture. Of course at some level of population density the ability to grow crops and stay put would be useful if not vital. A population that has to move, hunter-gatherers and herdsmen, will spend more time traveling and less time on their specialty. A substantial, but not necessarily crippling, loss of efficiency.

  5. #5 Thomas Schroeder
    July 3, 2009

    I think cooperation is what started it all (see Comment #3 for a entertaining explanation of how this might of happened or Comment #4 for another). People came together and realized through cooperation things could be often be done better and easier for all. The extra benefit of cooperation is man now had extra time and peace of mind to explore other endeavors, such as the arts and leisure. Of course, language and art (aka drawings to explain things) were first utilitarian, but easily became cultural activities.

    Our neocortex is the cortical switch, in my opinion. It gave us the ability (amongst many other things) to realize cooperation was the way to go (although people didn’t directly realize this then and we still haven’t fully realized it… which maybe explains why we’re still not considered modern man yet in the minds of people like Commenter #3, Phil). Sure, there are other creatures without neocortices that work together, more instinctively I might speculate, but this is different than choosing to cooperate.

    In response to Comment #1, here’s my thinking: the egg came first. Before the egg was pre-chickens. From the egg of the pre-chickens (through natural selection, if you will), came (and survived up until this point) the chicken.

  6. #6 Neil B ♪
    July 6, 2009

    OK, Ruth Mace could have a point. However,

    1. Density means in clumps and wouldn’t be an excuse for lots of people everywhere,

    2. That “high density” was relative to a relatively small total human population – that was much smaller than ours today.

    None of this is an excuse for continued growth in human population. Like most optimality curves, that for population reaches a hump, and then it’s worse. We humped decades ago IMHO.


  7. #7 Julie Simon Lakehomer
    July 7, 2009

    I’m with Art (#4). A similar phenomenon occurred with the evolution of a digestive tract with two openings instead of one. Suddenly an organism didn’t have to be eating all the time. It could eat and then do something else; and lots of something elses eventually developed.

  8. #8 B Berry
    July 12, 2009

    If I may speak broadly:

    It’s like independent or small groups of cells ‘deciding’ to form tissues. Do civilizations then form into organ systems and then into fully conscious entities. And then do those entities start over with other entities (life on other planets) and form into some sort of cooperative structure. That’s too much to hope for probably.

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  10. #10 Jerry
    July 28, 2009

    Reading the article’s summary (the article itself is gated), the point you made in your penultimate paragraph–“The larger implication is that the birth of human culture was triggered by a new kind of connectedness.”–is not mentioned. While this explanation seems intuitive, is there work that connects technological/cultural progress with human interconnectedness and richness of communication? Seems like it would be a fertile field, surveying the last century.

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    This is simply a chicken-before-egg argument, which is always fun to have since you can never really prove your position or ever be proven wrong.
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