Did a major epidemic of plague trigger a prolonged collapse in Europe’s population in late neolithic times – from around 5,600 to 4,000 years ago?
In Europe, the neolithic is part of the stone age, spanning the time from the introduction of agriculture by migrant groups from Anatolia, up until the bronze age.
Scientists now know that prehistoric plague infected neolithic farmers in Europe.
What hasn’t been clear until now is whether these early strains of the plague bacterium were even deadly. New evidence shows that they were, but other factors still don’t line up to support the evidence for a late neolithic epidemic.
Plague DNA found in human remains from over 4,000 years ago is genetically quite different to the plague strains which caused the Black Death in Europe. Prehistoric plague strains lack a gene that allows the bacteria to effectively hijack fleas, turning them into bubonic plague delivery systems.
They also have ancestral forms of other genes that are known to be important in promoting virulence. Detections of prehistoric plague cases were also quite scattered across archaeological contexts, without evidence of mass mortality accompanying outbreaks – until very recently.
All this has meant that researchers have hotly debated whether these infections caused by the plague bacterium Yersinia pestis would have been a death sentence in prehistory, or something more like a stomach bug that only occasionally causes severe complications, like plague’s ancestor, Yersinia pseudotuberculosis.
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Nonetheless, the detection of many cases of plague in Europe at around the same time as a major inferred population slump – the late neolithic demographic decline – has led some to implicate these plague outbreaks as the cause of around 500 years of prolonged population decline.
New results published in Nature show extensive plague outbreaks among prehistoric hunter-gatherers 5,000km east of Europe, at Lake Baikal in southern Siberia. The findings clearly show that early plague strains could indeed cause mass death.
Baffling deaths
The two outbreaks at Lake Baikal took place around 5,500 years ago and 5,300 years ago. The largest of the hunter-gatherer cemeteries analysed in the study, called Ust’Ida I, had previously baffled archaeologists.
Radiocarbon dating showed that the deaths occurred at the same time and that there was an unusually high proportion of dead children and adolescents. However there was no clear indication of a cause (such as mass violence).
Scientists retrieved plague DNA from the skeletons and carried out genetic analysis of the individuals buried in the cemeteries. The latter analysis revealed that small family groups were affected, which is indicative of human-to-human transmission of the disease.
The genetic findings emphasise the human impact of these outbreaks: young siblings died of plague infection and were buried in shared graves, with parents buried close to their children.
As far as we know, these hunter-gatherers were isolated from contemporary neolithic cultures in Asia, and certainly had no means of contact with late neolithic farmers in Europe. One interpretation, given in that study, is therefore that plague independently spilled over from wild animal “disease reservoirs” in both Europe and at Lake Baikal. Catching the disease from a wild animal still happens very frequently today (both in parts of Asia and in the US.
Kelvin Wilson.
Discovering that the first evidence of deadly mass outbreaks of prehistoric plague comes from isolated hunter-gatherer communities is important because it challenges our assumptions about disease in the past. For one thing, it shows that plague infections by themselves were not a unique factor in the late neolithic decline. For an epidemic to have happened, other factors would have to be involved. People travelling around more would have spread the disease, and higher population densities would have maintained it in populations.
Yet while population densities were certainly higher in the neolithic, we know that overall mobility actually fell among neolithic farmers compared with the hunter-gatherer populations that preceded them, at an individual level (based on ancient genome data). It’s also puzzling that we don’t have similar evidence of mass mortality from plague in Europe yet, despite vastly more sampling for ancient DNA having been undertaken here than in Asia.
A drop in density?
The most obvious hole in the evidence for a late neolithic plague epidemic is that the dates of the plague cases detected so far don’t match the timing of the late neolithic decline. Based on thousands of radiocarbon dates, the modelled population density in the late neolithic follows a boom-then-bust trajectory in north west Europe, with a peak around 5,600 years ago, followed by a series of sharp declines.
If plague were the cause of this, then we would expect to find the most cases soon after 5,600 years ago, when population collapse is at its most dramatic. Instead, we still only have evidence of plague cases from around 400 years after this date.
Adapted from Figure 1 published by Colledge et al., 2019.
Before plague was proposed, the main explanation for the late neolithic population decline across Europe was that it resulted from a decline in agricultural production associated with climatic deterioration. Researchers analysed data on the distribution of cereals and weeds across north west Europe during a period of boom and bust following the arrival of farming. They found a correlation between population decline and decreasing cereal production.
For different parts of the British Isles, the same pattern emerged in more detail, and its beginning coincided with a shift to cooler, wetter conditions. The fact that there has been no evidence of plague in Britain and Ireland at this time seems like further evidence against plague as an explanation.
However, a very recently discovered case of plague from Orkney, in Scotland, dated to between 4,961-4,833 years ago, might change that.
The population decline from 5,600 years ago is also not the only one archaeologists have found – earlier instances from central and south east Europe suggest that these could have been part of a more cyclical pattern of boom and bust across the neolithic.
Finally, another explanation for the late neolithic decline could also be that we’re interpreting the data for this incorrectly. A possibility, suggested by archaeologist Amy Bogaard, is that it could be evidence of prolonged population dispersal, rather than an absolute decline in numbers: people being forced to move elsewhere, into lower population densities, due to too much strain on resources.
There are also many other reasons why we should be cautious about inferring demographic processes based on substantial datasets of radiocarbon dates.
Right now, we think that a lot more evidence is still needed to support the idea that a plague epidemic lies behind a late neolithic decline in population, or population density.
