by Timothy Oleson Thursday, February 20, 2014
As agricultural practices spread from the Fertile Crescent across Europe, gradually expanding west and north starting about 8,500 years ago, they brought increased and localized food production to a continent where nomadic hunter-gatherers had long made their living subject to the whims of climate and the environment. With agriculture, long-term settlements developed, fertility rates rose and, thus, populations grew steadily. Or at least that’s been the conventional wisdom.
In recent years, some scientists have questioned just how steady Neolithic population growth really was. And now, researchers analyzing archaeological records gathered from across Europe suggest that many regions actually experienced large population swings during the period.
Booms and busts among animal populations are well known, but scientists have assumed that technology, including agriculture, “buffers out those instabilities” in human populations, says Sean Downey, an ecological anthropologist at the University of Maryland at College Park and a co-author of a new study on the subject published in Nature Communications. But, Downey says, this study documents “dramatic oscillations in human population dynamics” in Neolithic Europe.
He and his colleagues, led by Stephen Shennan of University College London, amassed more than 13,500 radiocarbon dates from previously cataloged Neolithic artifacts, grouping them into 12 subregions — including several each in the British Isles, France, Germany, and southern Scandinavia — based on where they had been found. To study population trends, they analyzed “summed calibrated date probability distributions” (SCDPD), basically treating the number of radiocarbon-dated artifacts collected from a given region as a proxy for that area’s relative population density over time.
It’s a technique that has been criticized and is — admittedly, even to those who use it — fraught with potential sources of error, both in data collection and analysis. To address these issues, Downey and his colleagues first removed duplicate dates taken from single archaeological sites to account for researchers disproportionately sampling some sites more than others. The team then put the remaining data through a rigorous statistical treatment to account for all other known biases, Downey says, including error introduced into the calibration curve used to convert radiocarbon dates to calendar years and population numbers.
“Even when you account for all that, you still have statistically significant boom and bust patterns,” Downey says. In fact, the researchers found such patterns in 10 of the 12 subregions studied, with some areas experiencing multiple collapses between 8,000 and 4,000 years ago. At times, population declines of 30 to 60 percent occurred in individual regions, similar in proportion to the drop experienced when Europe was ravaged by bubonic plague in the 14th century.
After establishing that the booms and busts were real, Downey says, the next question was why the latter happened despite the emergence of agriculture. Sustained periods of cooling have been the most common hypothesis, but support for this idea has largely been qualitative, he says. So, using the same statistical methods they had previously used, they tested for quantitative relationships between their population data and various climate records, including proxies for air temperature and precipitation in the North Atlantic and Northern Europe. Instead of support for the climate hypothesis, they found a “lack of correlation between overall gross changes in climate patterns and the population oscillations,” Downey says, suggesting that other factors were at play.
Outbreaks of disease and conflict and overexploitation of the land in increasingly dense settlements are all possibilities. Mass migrations of people — which could lead to apparent booms and busts as, for example, one region became populated at the expense of others — are another. But, Downey notes, population trends in neighboring regions are “consistently coordinated through time,” seemingly negating such migrations as a likely explanation.
The main advancement of the work is that it has taken “the SCDPD method to the next level,” says Miikka Tallavaara, an archaeologist at the University of Helsinki in Finland who was not involved in the study. The new statistical procedure is important, he says, because faulty radiocarbon calibrations can cause “artificial peaks and troughs” in population densities. It’s also the first time someone has shown “with a large amount of data, how consistent the [boom and bust] pattern is over large areas.”
However, Tallavaara says, it’s too soon to toss out the link between the population collapses and climatic cooling, noting that “with different, more [locally] representative proxies,” the team “might have come to a different conclusion regarding the impact of climate.”
At this point, Downey says, “we don’t really know what caused these collapses.” What is clear is that the busts must have had major implications for early European societies, as well as major effects on the landscape.
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