Positive feedbacks in deep-time transitions of human populations

Spatiotemporal distribution of the North American Indigenous population prior to European contact

We examine spatiotemporal trends in the pre-European-contact Indigenous population of North America using radiocarbon (14C) dates of the past 2000 y. At a continental scale, the Indigenous population of the past ~14,000 y peaked at ~1150 CE and then declined until a brief recovery shortly before 1500 CE, after which 14C probability declines precipitously. After testing, we reject the hypothesis that the 1150 CE peak and decline is a result of 14C sampling issues. We then examine the 14C record of the past 2000 y in each of 18 watersheds where we find peaks ranging from ~800 to 770 CE to after European contact, with the majority, in the interior of the continent, declining ~1080 to 1300 CE. Although all Indigenous populations declined after European contact, that of a large portion of the country (the Great Lakes, New England, the Mid-Atlantic, the Central Plains, the Northwest, and California) did not decline until after contact.

Multicentennial cycles in continental demography synchronous with solar activity and climate stability

Human population dynamics and their drivers are not well understood, especially over the long term and on large scales. Here, we estimate demographic growth trajectories from 9 to 3 ka BP across the entire globe by employing summed probability distributions of radiocarbon dates. Our reconstruction reveals multicentennial growth cycles on all six inhabited continents, which exhibited matching dominant frequencies and phase relations. These growth oscillations were often also synchronised with multicentennial variations in solar activity. The growth cycle for Europe, reconstructed based on  >91,000 radiocarbon dates, was backed by archaeology-derived settlement data and showed only a weak correlation with mean climate states, but a strong correlation with the stability of these states. We therefore suggest a link between multicentennial variations in solar activity and climate stability. This stability provided more favourable conditions for human subsistence success, and seems to have induced synchrony between regional growth cycles worldwide.

A model of long-term population growth with an application to Central West Argentina

We propose an Ideal Specialization Model to help explain the diversity of population growth trajectories exhibited across archaeological regions over thousands of years. The model provides a general set of expectations useful for guiding empirical research, and we provide a concrete example by conducting a preliminary evaluation of three expectations in Central West Argentina. We use kernel density estimates of archaeological radiocarbon, estimates of paleoclimate, and human bone stable isotopes from archaeological remains to evaluate three expectations drawn from the model's dynamics. Based on our results, we suggest that innovations in the production of food and social organization drove demographic transitions and population expansion in the region. The consistency of population expansion in the region positively associates with changes in diet and, potentially, innovations in settlement and social integration.

Landscape of fear: indirect effects of conflict can account for large-scale population declines in non-state societies

The impact of inter-group conflict on population dynamics has long been debated, especially for prehistoric and non-state societies. In this work, we consider that beyond direct battle casualties, conflicts can also create a 'landscape of fear' in which many non-combatants near theatres of conflict abandon their homes and migrate away. This process causes population decline in the abandoned regions and increased stress on local resources in better-protected areas that are targeted by refugees. By applying analytical and computational modelling, we demonstrate that these indirect effects of conflict are sufficient to produce substantial, long-term population boom-and-bust patterns in non-state societies, such as the case of Mid-Holocene Europe. We also demonstrate that greater availability of defensible locations act to protect and maintain the supply of combatants, increasing the permanence of the landscape of fear and the likelihood of endemic warfare.

Phase synchronization between culture and climate forcing

Over the history of humankind, cultural innovations have helped improve survival and adaptation to environmental stress. This has led to an overall increase in human population size, which in turn further contributed to cumulative cultural learning. During the Anthropocene, or arguably even earlier, this positive sociodemographic feedback has caused a strong decline in important resources that, coupled with projected future transgression of planetary boundaries, may potentially reverse the long-term trend in population growth. Here, we present a simple consumer/resource model that captures the coupled dynamics of stochastic cultural learning and transmission, population growth and resource depletion in a changing environment. The idealized stochastic mathematical model simulates boom/bust cycles between low-population subsistence, high-density resource exploitation and subsequent population decline. For slow resource recovery time scales and in the absence of climate forcing, the model predicts a long-term global population collapse. Including a simplified periodic climate forcing, we find that cultural innovation and population growth can couple with climatic forcing via nonlinear phase synchronization. We discuss the relevance of this finding in the context of cultural innovation, the anthropological record and long-term future resilience of our own predatory species.

The long-term expansion and recession of human populations

Over the last 12,000 y, human populations have expanded and transformed critical earth systems. Yet, a key unresolved question in the environmental and social sciences remains: Why did human populations grow and, sometimes, decline in the first place? Our research builds on 20 y of archaeological research studying the deep time dynamics of human populations to propose an explanation for the long-term growth and stability of human populations. Innovations in the productive capacity of populations fuels exponential-like growth over thousands of years; however, innovations saturate over time and, often, may leave populations vulnerable to large recessions in their well-being and population density. Empirically, we find a trade-off between changes in land use that increase the production and consumption of carbohydrates, driving repeated waves of population growth over thousands of years, and the susceptibility of populations to large recessions due to a lag in the impact of humans on resources. These results shed light on the long-term drivers of human population growth and decline.