Journal Club

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In Mesopotamia, early cities may have faltered before climate-driven collapse

Surveys of archaeological sites such as this one, in northern Syria, reveal regional population trends. Image credit: Andrea Ricci.

Surveys of archaeological sites such as this one, in northern Syria, reveal regional population trends. Image credit: Andrea Ricci.

Some of the earliest cities, now buried in the soil of Syria, Iraq, Turkey, and Lebanon, are thought to have collapsed because of rapid climatic change. Drought along with lower temperatures descended on these settlements some 4,200 years ago, forcing residents to adapt, move away, or starve. A recent study in PLOS One suggests these cities may also have been overpopulated before the drought. “It was unsustainable in its own right,” says lead author Dan Lawrence, a landscape archaeologist at Durham University in the United Kingdom. “Climate might have pushed it over the cliff, but it was already teetering on the edge.”

For the study—among the most ambitious and large-scale analyses of population trends in the region during that era—Lawrence and his coauthors combined data from 18 archaeological surveys, including their own work and that of other research groups, into a database representing 1,157 settlement sites. They assumed that time periods with more settlements on the landscape were more densely populated, and that time periods with more carbon-14 data points—from charcoal or seeds, for example—had been more populous than those with fewer carbon-14 records.

Based on these proxies of population change over time, the authors found a population spike followed by subsequent decline about 4,200 years ago, suggesting early cities may have been overpopulated beyond what local resources would allow. Their results agree with previous findings indicating a population boom followed by collapse.

Big data studies covering many sites over thousands of years are still rare in the field, making this recent work “an exciting new approach,” says Near Eastern archaeologist Glenn Schwartz at Johns Hopkins University in Baltimore, MD. That the results agree with past studies makes the work that much more encouraging, Schwartz says.

Archaeologist Jesse Casana at Dartmouth College in Hanover, NH lauds the study for assessing long-term trends in populations and in urbanization at regional scales. “In many ways the promise of regional archaeology has been to provide these kinds of insights,” Casana says. “But so often we struggle with integrating disparate datasets produced over decades by archaeologists with very different methodologies and agendas.”

Schwartz did question, however, whether the number of carbon-14 data points is an accurate proxy for changes in population size. “I’m not sold,” Schwartz says. After all, these data are collected by archaeologists, only some of whom are especially interested in radiocarbon and take many dates from their sites. Site selection, too, can be a product of specific research goals, political pressures, or trends in the field, rather than a truly random sample of how much evidence is out there, he notes.

Methodological caveats aside, researchers are still grappling with what climate change would have meant for city settlements in the early Bronze Age. “I don’t think anyone’s gotten to grips with what it means for individuals in this society,” Lawrence says. Preserved textual sources say some of these sites had “crazy numbers of sheep, like half a million for one city,” he notes. If that was the case, it’s easy to imagine the sparsely-vegetated landscape being quickly denuded for pasture, and then overgrazed.

How ancient societies responded to climate change could inform modern resilience, Lawrence says. For example, understanding whether ancient small villages or dense urban centers adapted more nimbly could guide today’s climate adaptation strategies. “We live in the most complex society that’s ever existed,” he says. “Is complexity what saves you because you can redistribute resources and adapt more easily?” Future work, Lawrence says, might assess how climate precipitated other societal changes in the past. Later in the Bronze Age, for example, another drought and cooling event is associated with urban collapse across the Mediterranean. Perhaps those populations were similarly fragile.

Other recent papers recommended by Journal Club panelists:

Singlehood in contemporary Japan: Rating, dating, and waiting for a good match

Ancient genomes reveal hybridisation between extinct short-faced bears and the extant spectacled bear (Tremarctos ornatus)

Fitness variation across subtle environmental perturbations reveals local modularity and global pleiotropy of adaptation

Synthetic lateral inhibition in periodic pattern forming microbial colonies

Enhanced problem-solving ability as an adaptation to urban environments in house mice

Natural variation in plant telomere length is associated with flowering time

Clinically relevant mutations in core metabolic genes confer antibiotic resistance

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