The first farmers who swept into Europe 6000 to 7000 years ago may have grown too big for their britches—or animal skins—too fast. A new study of archaeological sites across Western Europe highlights a strikingly consistent pattern in Neolithic farmers' communities: Their populations grew too big, too quickly, and crashed right after they peaked.
"We can see a dramatic history of booms and busts," archaeologist Stephen Shennan of University College London (UCL) reported yesterday in a talk at the 111th meeting of the American Anthropological Association in San Francisco, California.
Researchers have long assumed that as the first farmers settled down in Europe, life was more stable for them than for the nomadic foragers and fishers they had displaced. Cultivated plants and animals were a secure source of food, the reasoning went, allowing the farmers to bear more babies and put down deep roots. An overall picture emerged in which farming populations grew gradually until modern times. "It has been generally assumed that population slowly increased, in line with long-term continental and global trends," Shennan says.
No studies, however, had looked closely at what happened to population growth locally in different regions of Europe after the first farmers arrived. Working with evolutionary geneticist Mark Thomas at UCL, Shennan developed a statistical method to trace the rise and fall of population numbers by using calibrated radiocarbon dates at archaeological sites in Europe. They reasoned that the more dates for Neolithic settlements in a region, the higher the population (after correcting for different numbers of dates from different archaeological sites). Once they used clusters of dates to track patterns of population growth and decline at archaeological sites in Europe, they calibrated their method by studying patterns in the types and dates of pollen found at the sites, which reflect when farmers cleared land of trees to grow crops. The two records of population growth matched, Shennan says.
When they examined their data, they found that populations did indeed increase rapidly in many areas with the onset of farming, as expected. But these levels were not sustained. The new subsistence system, despite its potential for supporting increased populations, did not bring stability. "The characteristic regional pattern indicated by changing population levels is one of instability, of boom and bust," Shennan says.
Why did farmers' numbers rise and fall? Shennan tested whether changes in climate hit the farmers hard by comparing the patterns of population growth and decline with regional fluctuations in climate, as reflected in Greenland ice core samples. But there was no significant match. "So why do we have these boom-bust oscillations? We don't know," Shennan says.
The possibilities include intense competition for food and other resources due to overpopulation; the rise of diseases acquired by living closely with livestock, other humans, and their waste; and increased violence among Neolithic communities.
Shennan's team also suggests that this pattern of boom and bust had a negative impact on social, economic, and cultural life in these communities. One recent study of the skeletons of early farmers in Britain also found a surge in violence right after populations exploded. "Whatever the case, what we've shown is that with this early farming system, these high population levels could not be maintained," Shennan says.
Other researchers at the talk said this was the first time that researchers had shown a pattern of boom and bust in Neolithic populations that was not tied to climate change. "It shows on a grand scale that the replacement of foraging by farming has a huge impact on who we are and what the world was like," says human behavioral ecologist Eric Smith of the University of Washington, Seattle. "If farming from the get-go is associated with unstable populations, that's interesting to know."