What makes us move?
Over several million years, the human species has evolved, surviving as hunter-gatherers, living in small groups and adapting to different environments. Today, humans have spread to almost every part of the planet. But why do we move?
For prehistoric times, the archaeological record mainly provides insights into long-term developments, not into individual decision-making processes, and rarely into the historical contexts of dispersals. However, similar to today, reasons for dispersal were likely diverse: Economic and ecological motivation, knowledge and technical skills, as well as social factors, must be considered.
When Anatomically modern humans (AMH) spread across Europe, global climatic conditions were different from today. A prevailing cooler and drier climate of the last Ice Age was repeatedly interrupted by warmer interglacial periods, with some changes occurring abruptly and others gradually. Despite dramatic climate changes that repeatedly drove groups to extinction, the human population survived the changing environmental conditions on a transcontinental scale.
How to model past human dispersal?
Long-term dispersals of human populations were highly complex processes of advance, retreat, abandonment and resettlement. Some human movements may have been rapid, while others took place over several generations and were barely recognised by individual actors. The processes at work thus operated on different temporal and spatial scales, requiring us to use models to reconstruct past potential movements.
We developed the so-called Human Existence Potential (HEP) to reduce complexity and at the same time take into account as many factors as possible in the „Our Way Model“ (OWM) model. The HEP combines information from archaeological and palaeoclimate information, the latter derived from data simulated by the Global Climate Model. Using archaeological data on the spread of AMH in Europe as a case study, HEP is trained to recognise the climatic suitability of the past environments for the AMH.
This first step in our efforts to model dispersal processes produces static maps that predict areas with high or low potential for AMH presence. An interesting observation for us during this phase was that sometimes predictions contradict the actual distribution of archaeological sites: Why did areas with no sites have a high HEP? Are the sites yet to be discovered? Did people avoid these areas for reasons other than those considered in the HEP? Such discrepancies require further investigation.
Regarding the development of our model, we needed to describe dynamic dispersal processes over long periods of time. Therefore, we had to adapt the HEP to the constantly changing climatic conditions of the period studied. A general but robust solution to this challenge was to interpolate/extrapolate the already estimated HEP using oxygen isotope data from Greenland ice cores (GICC05) as a proxy for global temperature changes. This allows HEP to be simulated in 20-year time steps, providing us with a dynamic model.
Results from “Our Way Model” (OWM)
The model now provides a detailed reconstruction of the dispersal processes of the first AMH into Europe, combining large climate with archaeological datasets. The OWM story of the AMH’s way into Europe can be divided into four phases:
At first a phase of relatively slow westward expansion from the Levant to the Balkans is followed by a second phase of rapid expansion of AMH into western Europe. Although interrupted by brief setbacks, AMH populations now rapidly reached an estimated number of 60.000 people across Europe and spread across the full extent of the known distribution of archaeological sites during this period.
The subsequent third phase is characterised by a decline in human population, both in terms of its size and density as well as the area occupied by the population. However, AMH survived in several areas that they had just started to occupy in the previous phase. In the fourth phase, when HEP conditions improved again, the population size rose immediately. Regional increases in population density and further advances into previously unsettled areas of Great Britain and the Iberian Peninsula are interesting outputs of the model, broadly in line with the archaeological evidence.
The entire process took place across large distances, and over several thousands of years. A detailed look at the HEP maps indicates that at the end of this process, parts of the human population were better adapted to cold climatic conditions than others, allowing them to push the boundaries of previously settled environments.
Arrived in Europe … and our way forward
For archaeologists, exciting results from the OWM relate to the speed of simulated dispersal and the estimated data on population size and density.
For the dispersal processes of AMH, it is interesting to see which factors in the model impede or even reverse them. In most cases, factors such as low population density or local conditions – like the lack of favourable retreat areas or the existence of geographical barriers, e.g., rough terrain – can be identified. How these factors interact and contribute to overall long-term trends at the subcontinental scale is something that regional studies are hardly able to encompass – certainly a major advantage of this type of approach. However, further research must test the underlying assumptions made in the model, e.g., what conditions were favourable or unfavourable for prehistoric populations.
For the demographic data, the model provides regionally differentiated population densities. Such estimates of regional and sub-continental developments can be compared with other sources of archaeological evidence, as we have suggested in the discussion, to see how humans not only dispersed but continued to interact over large areas.
At present, the OWM regulates most of the dispersal dynamics by the HEP, which is related to climate change as captured in the training dataset of archaeological sites. However, the model did not consider the role of cultural evolution on the human dispersal process. Meanwhile, a new project on Human and Earth System Coupled Research (HESCOR) has been established at the University of Cologne, and the integration of more aspects of Human-Earth system interactions into the OWM is one of the main foci of HESCOR.
Modelling Human Dispersal in the Past – “Our Way Model”
