Cultural evolution and population density

Many popularisers of science have made a claim something like this:

Human history at last took off around 50000 years ago, at the time of what I have termed our Great Leap Forward. [Jared Diamond, Guns, Germs and Steel, p39]

and others have asserted that this is due to a change in our “wetware”. But a new paper in this week’s Science argues that the evidence suggests that we were effectively “completely modern” by 200kya to 160kya, and that the reason why there is a cultural explosion 50kya and elsewhere much later (in Australia, 25kya) has to do with population density.

The argument is this: Suppose you have a cultural technique, a spear head, for example, that takes a certain amount of knowledge to make. There’s an error rate in transmitting it to later generations, so by accident, knowledge can be lost at a more or less gradual rate in time. This is how, for example, the pre-European Tasmanians had lost the making of fire. The knowledge got to the point where it was not effectively being passed on.

But if the population density is high enough, and there is sufficient contact between populations, if one population loses a technique, it can be reimported from an adjacent group with whom one might trade, arrange exogamous marriages, and so on. Isolated populations will lack the ability to “store” cultural information “offsite”, to use a computer analogy. Any losses in knowledge will have to be made up by a more or less constant rate of innovation (where “noise” in the transmission sometimes tricks on a better way of doing it, rather than a worse way). Since that, too, is a rare event, the overall problem is that information will be lost. In high density populations, however, good tricks, as Dennett calls them, will be spread rapidly and maintained by displacement of less effective ideas. The metapopulation (the population of all populations in contact) acts as a backup and dynamic storage of culture. The good trick is likely to be retained somewhere, and can spread back when lost in other populations.

Coincidentally, this is the point I made recently when I noted that old school taxonomy is being maintained among less well off scientific communities that can’t afford the high end molecular technologies that are supplanting it in the “western” economies. When we need those techniques, we can reimport them from India and Mexico and Columbia. Of course, by then, the need may have abated, if the populations of organisms being described have gone extinct in the meantime, but I digress.

If you graph cultural change in terms of innovations in the archeological record (which is like the cultural equivalent of the fossil record), it is likely you will get some sort of curve where little happens in the first few tens of thousands of years and then suddenly it explodes. But the crucial bit of information is that population densities exploded then – so even if there was a completely stable rate of innovation, innovations are retained in higher density populations so that incremental cultural improvements can be made (e.g., to hunting technologies).

The authors note that there was an earlier explosion in technology which petered out, around 70kya to 90kya. The explanation is that population densities were insufficient at some point to retain these techniques. Consider a period of desertification after a period of lush environments in southern Africa at that time. You would lose all sorts of information as populations thinned out. Imagine what would happen to modern technology if you suddenly had only the ability to support 10% of the population. You’d lose machinists, industrial chemists, mining technologies, and all the key professions and trades on which our hi-tech society subsists. It would take a very few generations for us to have to revert to medieval technologies, even if we had the books (because not all knowledge is stored in books, as any panel beater can tell you, and books are not immortal).

There’s a lot of cautionary morals to be drawn from this, I think. Our present society is so predicated on technologies that rely on populational stability that even a slight perturbation could drive it into a downward spiral. It is inordinately fragile in its cultural transmissions. A single solar flare could knock out all out IT, for example. What do we do then, if all our books have been lost to “digitisation”?

Something to think about.