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Study offers evidence to support geography's role in shaping human history.

One reason that Eurasian civilizations dominated the globe is because they came from a continent that was broader in an east–west direction than north–south, claimed geographer Jared Diamond in his famous 1997 book Guns, Germs and Steel. Now, a modelling study has found evidence to support this 'continental axis theory'.

Continents that span narrow bands of latitude with little variation in climate contain plants and animals that are adapted to similar climatic regions. That is an advantage, says Diamond, because it means that agricultural innovations are able to diffuse more easily, with culture and ideas following suit. As a result, Diamond's hypothesis predicts, along lines of latitude there will be more cultural homogeneity than along lines of longitude.

To test that prediction, researchers at Stanford University in California used language as a proxy for cultural diversity, and analysed the percentage of historically indigenous languages that remain in use in 147 countries today relative to their shape. They chose countries rather than continents to increase the number of comparisons. For example, the team looked at the difference between Chile, which has a long north–south axis, and Turkey, which has a wider axis running east to west.

The researchers found that if a country had a greater east–west axis than a north–south one, the less likely it was for its indigenous languages to persist. The relationship isn't straightforward, but the model suggests that Mongolia, which is about twice as wide as it is tall, would have 5% fewer indigenous languages than Angola, which is roughly square. Meanwhile, Peru — about twice as tall as it is wide — would be predicted to have 5% more persistent languages than Angola. The result, say the authors, supports Diamond's theory because it indicates that east–west countries have more homogeneous cultures.

“It is a significant relationship that is an observable implication of the Diamond thesis,” says David Laitin, the political scientist who led the work, published today in the Proceedings of the National Academy of Sciences1. Greater cultural diversity is also known to be associated with more unfortunate outcomes such as lower levels of economic growth and higher probabilities of violence, he adds.

While research has previously shown historical empires were more likely to have expanded farther in the east-west direction than north-south2, tests of Diamond's idea have always been limited because of the small number of continents. Raising the sample size by using countries allows a better claim to statistical significance. Using language persistence as a proxy for cultural diversity is controversial, admits Laitin, but he argues it is the best quantifiable way.

The study controls for a number of factors including distance of countries from the equator (closer to the equator more languages have existed historically); the degree of mountainousness (mountains can stop languages spreading); and their age (newer countries could have more languages because there has been less time for homogenisation).

Laitin also dismisses the possibility that the effect could simply have resulted from east-west countries being more interested in state building – more likely, for example, to introduce policies to bring about a single national language. Repeating the analysis in 538 artificially created countries, derived by combining each real country with its neighbours, showed the relationship still held up.

Thomas Currie, an expert in human evolutionary ecology at University College London says the study is a novel way of testing Diamond's hypothesis that does a “thorough job” of controlling for a number of alternative explanations. “The main result seems to be robust. [The study] further supports the idea that human history and cultural evolution are governed by general ecological and biogeographical rules,” he says.

But others, sceptical of the continental axis theory, say the study does little or nothing to strengthen its case. Language is a poor proxy for something as all-encompassing as culture, says John McNeill, a historian at Georgetown University. Many countries are also either so small that the axis-length component of what cultural diversity they have is almost sure to be negligible, or they are so close to square or round that it is hard to imagine a little extra length in one direction or another making much difference. “Unfortunately there aren't many countries shaped like Chile,” he says.

Credit: Horizon International Images Limited/Alamy

References

  1. Laitin, D. D., Moortgat, J. & Robinson, A. L. Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1205338109 (2012).

  2. Turchin, P. et al. J. World-Syst. Res. 12, 219–229 (2006).

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Corbyn, Z. hed. Nature (2012). https://doi.org/10.1038/nature.2012.10808

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