Amplitude of travelling front as inferred from 14C predicts levels of genetic admixture among European early farmers

Positive feedbacks in deep-time transitions of human populations

Abrupt and rapid changes in human societies are among the most exciting population phenomena. Human populations tend to show rapid expansions from low to high population density along with increased social complexity in just a few generations. Such demographic transitions appear as a remarkable feature of Homo sapiens population dynamics, most likely fuelled by the ability to accumulate cultural/technological innovations that actively modify their environment. We are especially interested in establishing if the demographic transitions of pre-historic populations show the same dynamic signature of the Industrial Revolution transition (a positive relationship between population growth rates and size). Our results show that population growth patterns across different pre-historic societies were similar to those observed during the Industrial Revolution in developed western societies. These features, which appear to have been operating during most of our recent demographic history from hunter-gatherers to modern industrial societies, imply that the dynamics of cooperation underlay sudden population transitions in human societies. This article is part of the theme issue 'Evolution and sustainability: gathering the strands for an Anthropocene synthesis'.

The importance of wild resources as a reflection of the resilience and changing nature of early agricultural systems in East Asia and Europe

We examine the changing importance of wild starch rich plant staples, predominantly tree nuts, in early agricultural societies in East Asia and Europe, focusing on Korea, Japan, and Britain. A comparative review highlights variations in the importance of wild plant staples compared to domesticated crops. The Korean Middle to Late Chulmun periods (c. 3,500–1,500 BC) was characterized by a high reliance on nuts alongside millet. This declines with the transition to rice agriculture, but remains significant during the Mumun period (c. 1,500–300 BC). In Japan, the arrival of rice and millets in the Yayoi Period (c. 1,000 BC−250 AD) saw continued evidence for high levels of reliance on wild resources, which declines only in the Kofun and early historical periods. In Early Neolithic Britain (c. 4,000–3,300 BC) cereal agriculture is accompanied by high evidence for wild plant foods. But during the Middle to Late Neolithic (3,300–c. 2,400/2,200 BC) cereals were abandoned on the mainland with hazelnuts becoming a prominent plant staple. Agriculture returned in the second half of the 3rd millennium BC, followed by a strong decline in wild plant food use during the Middle to Late Bronze Age (1,700–700 BC). Such patterns have previously been attributed to the slow adoption of farming by indigenous peoples, with a continued reliance on wild resources. In light of evidence demonstrating that the dispersal of agriculture was largely driven by a mixture of demic-diffusion and introgression of hunter-gatherers into agricultural groups, a reinterpretation of the role of wild foods is needed. It is argued that the relative importance of wild plant staples provides an indicator of the stability and dependability of agricultural and social systems. A heavy reliance on wild foods in early agricultural societies is tied to the slow adaptation of domesticated crops to new environments, where agricultural and social landscapes are yet to be firmly established, and social systems that could mitigate for poor harvests and storage were often absent. The retained lengthy persistence of wild plant staples in East Asian subsistence systems compared to the British Isles likely reflects differences in the ecological and labor demands for rice compared to Western Asiatic cereals.

p3k14c, a synthetic global database of archaeological radiocarbon dates

Archaeologists increasingly use large radiocarbon databases to model prehistoric human demography (also termed paleo-demography). Numerous independent projects, funded over the past decade, have assembled such databases from multiple regions of the world. These data provide unprecedented potential for comparative research on human population ecology and the evolution of social-ecological systems across the Earth. However, these databases have been developed using different sample selection criteria, which has resulted in interoperability issues for global-scale, comparative paleo-demographic research and integration with paleoclimate and paleoenvironmental data. We present a synthetic, global-scale archaeological radiocarbon database composed of 180,070 radiocarbon dates that have been cleaned according to a standardized sample selection criteria. This database increases the reusability of archaeological radiocarbon data and streamlines quality control assessments for various types of paleo-demographic research. As part of an assessment of data quality, we conduct two analyses of sampling bias in the global database at multiple scales. This database is ideal for paleo-demographic research focused on dates-as-data, bayesian modeling, or summed probability distribution methodologies.

Barbarigenesis and the collapse of complex societies: Rome and after

"Barbarism" is perhaps best understood as a recurring syndrome among peripheral societies in response to the threats and opportunities presented by more developed neighbors. This article develops a mathematical model of barbarigenesis-the formation of "barbarian" societies adjacent to more complex societies-and its consequences, and applies the model to the case of Europe in the first millennium CE. A starting point is a game (developed by Hirshleifer) in which two players allocate their resources either to producing wealth or to fighting over wealth. The paradoxical result is that a richer and potentially more powerful player may lose out to a poorer player, because the opportunity cost of fighting is greater for the former. In a more elaborate spatial model with many players, the outcome is a wealth-power mismatch: central regions have comparatively more wealth than power, peripheral regions have comparatively more power than wealth. In a model of historical dynamics, a wealth-power mismatch generates a long-lasting decline in social complexity, sweeping from more to less developed regions, until wealth and power come to be more closely aligned. This article reviews how well this model fits the historical record of late Antiquity and the early Middle Ages in Europe both quantitatively and qualitatively. The article also considers some of the history left out of the model, and why the model doesn't apply to the modern world.

A Bayesian approach for fitting and comparing demographic growth models of radiocarbon dates: A case study on the Jomon-Yayoi transition in Kyushu (Japan)

Large sets of radiocarbon dates are increasingly used as proxies for inferring past population dynamics and the last few years, in particular, saw an increase in the development of new statistical techniques to overcome some of the key challenges imposed by this kind of data. These include: 1) null hypothesis significance testing approaches based on Monte-Carlo simulations or mark permutations; 2) non-parametric Bayesian modelling approaches, and 3) the use of more traditional techniques such as correlation, regression, and AIC-based model comparison directly on the summed probability distribution of radiocarbon dates (SPD). While the range of opportunities offered by these solutions is unquestionably appealing, they often do not consider the uncertainty and the biases arising from calibration effects or sampling error. Here we introduce a novel Bayesian approach and nimbleCarbon, an R package that offers model fitting and comparison for population growth models based on the temporal frequency data of radiocarbon dates. We evaluate the robustness of the proposed approach on a range of simulated scenarios and illustrate its application on a case study focused on the demographic impact of the introduction of wet-rice farming in prehistoric Japan during the 1st millennium BCE.

The Neolithic Demographic Transition in the Central Balkans: population dynamics reconstruction based on new radiocarbon evidence

In this paper, we test the hypothesis of the Neolithic Demographic Transition in the Central Balkan Early Neolithic (6250-5300 BC) by applying the method of summed calibrated probability distributions to the set of more than 200 new radiocarbon dates from Serbia. The results suggest that there was an increase in population size after the first farmers arrived to the study area around 6250 BC. This increase lasted for approximately 250 years and was followed by a decrease in the population size proxy after 6000 BC, reaching its minimum around 5800 BC. This was followed by another episode of growth until 5600 BC when population size proxy rapidly declined, reaching the minimum again around 5500 BC. The reconstructed intrinsic growth rate value indicates that the first episode of growth might have been fuelled both by high fertility and migrations, potentially related to the effects of the 8.2 ky event. The second episode of population growth after 5800 BC was probably owing to the high fertility alone. It remains unclear what caused the episodes of population decrease. This article is part of the theme issue 'Cross-disciplinary approaches to prehistoric demography'.

Dispersals as demographic processes: testing and describing the spread of the Neolithic in the Balkans

Although population history and dispersal are back at the forefront of the archaeological agenda, they are often studied in relative isolation. This contribution aims at combining both dimensions, as population dispersal is, by definition, a demographic process. Using a case study drawn from the Early Neolithic of South-Eastern Europe, we use radiocarbon dates to jointly investigate changes in speed and population size linked to the new food production economy and demonstrate that the spread of farming in this region corresponds to a density-dependent dispersal process. The implications of this characterization are evaluated in the discussion. This article is part of the theme issue 'Cross-disciplinary approaches to prehistoric demography'.

Climate shaped how Neolithic farmers and European hunter-gatherers interacted after a major slowdown from 6,100 BCE to 4,500 BCE

The Neolithic transition in Europe was driven by the rapid dispersal of Near Eastern farmers who, over a period of 3,500 years, brought food production to the furthest corners of the continent. However, this wave of expansion was far from homogeneous, and climatic factors may have driven a marked slowdown observed at higher latitudes. Here, we test this hypothesis by assembling a large database of archaeological dates of first arrival of farming to quantify the expansion dynamics. We identify four axes of expansion and observe a slowdown along three axes when crossing the same climatic threshold. This threshold reflects the quality of the growing season, suggesting that Near Eastern crops might have struggled under more challenging climatic conditions. This same threshold also predicts the mixing of farmers and hunter-gatherers as estimated from ancient DNA, suggesting that unreliable yields in these regions might have favoured the contact between the two groups.

A three-population wave-of-advance model for the European early Neolithic

Ancient DNA studies have shown that early farming spread through most of Europe by the range expansion of farmers of Anatolian origin rather than by the conversion to farming of the local hunter-gatherers, and have confirmed that these hunter-gatherers continued to coexist with the incoming farmers. In this short report, I extend a previous three-population wave-of-advance model to accommodate these new findings, and derive the conditions supportive of such a scenario in terms of the relative magnitudes of the parameters. The revised model predicts that the conversion rate must, not surprisingly, be low, but also that the hunter-gatherers must compete more strongly with the converted farmers than with the alien farmers. Moreover, competition with the hunter-gatherers diminishes the speed of the wave-of advance of the farmers. In addition, I briefly consider how the wave-of-advance approach may contribute to interpreting the results of archaeological studies using the summed probability distribution of radiocarbon dates.

Sparse Radiocarbon Data Confound Culture-Climate Links in Late Pre-Columbian Amazonia

It has recently been argued that pre-Columbian societies in the greater Amazon basin during the Late Holocene were subject to “adaptive cycling”. In this model, cultures practicing “intensive” land use practices, such as raised field agriculture, were vulnerable to perturbations in hydroclimate, whereas “extensive” land use patterns, such as polyculture agroforestry, are viewed as more resilient to climate change. On the basis of radiocarbon data, the relative rise and fall of late pre-Columbian cultures and their inferred patterns of land use in six regions are highlighted to exemplify this model. This paper re-examines the radiocarbon evidence marshalled in favour of adaptive cycling, demonstrating that alleged temporal patterning in these data are overwhelmingly likely due to a combination of sampling effects, lack of statistical controls, and unacknowledged uncertainties that are inherent to radiocarbon dating. The outcome of this combination of factors seriously limits the possibility of cross-referencing archaeological data with palaeo-ecological and -climatological data without controlling for these effects, undermining the central archaeological pillar in support of adaptive cycling in Amazonia. This paper illustrates examples of such mitigation measures and provides the code to replicate them. Suggestions for how to overcome the serious limitations identified in the Late Holocene radiocarbon record of Amazonia are presented in the context of ongoing debates on inferring climatic causation in archaeological and historical datasets.

Widespread population decline in South America correlates with mid-Holocene climate change

Quantifying the impacts of climate change on prehistoric demography is crucial for understanding the adaptive pathways taken by human populations. Archaeologists across South America have pointed to patterns of regional abandonment during the Middle Holocene (8200 to 4200 cal BP) as evidence of sensitivity to shifts in hydroclimate over this period. We develop a unified approach to investigate demography and climate in South America and aim to clarify the extent to which evidence of local anthropic responses can be generalised to large-scale trends. We achieve this by integrating archaeological radiocarbon data and palaeoclimatic time series to show that population decline occurred coeval with the transition to the initial mid-Holocene across South America. Through the analysis of radiocarbon dates with Monte Carlo methods, we find multiple, sustained phases of downturn associated to periods of high climatic variability. A likely driver of the duration and severity of demographic turnover is the frequency of exceptional climatic events, rather than the absolute magnitude of change. Unpredictable levels of tropical precipitation had sustained negative impacts on pre-Columbian populations lasting until at least 6000 cal BP, after which recovery is evident. Our results support the inference that a demographic regime shift in the second half of the Middle Holocene were coeval with cultural practices surrounding Neotropical plant management and early cultivation, possibly acting as buffers when the wild resource base was in flux.

Palaeodemographic modelling supports a population bottleneck during the Pleistocene-Holocene transition in Iberia

Demographic change lies at the core of debates on genetic inheritance and resilience to climate change of prehistoric hunter-gatherers. Here we analyze the radiocarbon record of Iberia to reconstruct long-term changes in population levels and test different models of demographic growth during the Last Glacial-Interglacial transition. Our best fitting demographic model is composed of three phases. First, we document a regime of exponential population increase during the Late Glacial warming period (c.16.6-12.9 kya). Second, we identify a phase of sustained population contraction and stagnation, beginning with the cold episode of the Younger Dryas and continuing through the first half of the Early Holocene (12.9-10.2 kya). Finally, we report a third phase of density-dependent logistic growth (10.2-8 kya), with rapid population increase followed by stabilization. Our results support a population bottleneck hypothesis during the Last Glacial-Interglacial transition, providing a demographic context to interpret major shifts of prehistoric genetic groups in south-west Europe.

The archaeological record provides large ensembles of radiocarbon dates which can be used to infer long-term changes in human demography. Here, the authors analyse the radiocarbon record of the Iberian peninsula, finding support for a bottleneck during the Last Glacial-Interglacial transition