A sophisticated simulation has traced the fateful collision of two human species across ancient Iberia, showing that Neanderthals were already in decline when modern humans arrived, and their chances of interbreeding were remarkably slim. Researchers at the University of Cologne have developed a sophisticated dynamic model that reconstructs the population movements, settlement patterns, and potential interactions between Neanderthals and anatomically modern humans during a pivotal period between 50,000 and 38,000 years ago. The study challenges long-held assumptions about how these two groups coexisted and whether significant mixing occurred on the Iberian Peninsula.
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Neanderthals Confined to Coastal Refuges
The research team, led by Professor Yaping Shao from the Institute of Geophysics and Meteorology and Professor Gerd-Christian Weniger from the Department of Prehistoric Archaeology, created what they call the OW-CABM (Our Way Constrained Agent-Based Model). This innovative approach combines climate data, archaeological evidence, and population dynamics to simulate how Neanderthals moved across the landscape.
The model reveals that by the time anatomically modern humans arrived in northern Iberia around 42,000 years ago, Neanderthal populations were already severely diminished and largely confined to coastal refugia. Rather than thriving across the entire peninsula, these ancient humans had retreated to favorable pockets where resources remained accessible.
The simulations incorporated data from 99 Middle Paleolithic sites across Iberia associated with Neanderthals and 66 Aurignacian sites linked to the first modern humans in the region. By running the model thousands of times with varying parameters, researchers could assess the probability of different scenarios.
"By linking climate, demography, and culture, our dynamic model offers a broader explanatory framework that can be used to better interpret archaeological and genomic data," Professor Weniger explained in the study published in PLOS One.
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Map showing classic Neanderthal fossil sites across Europe and the Middle East. (Maximilian Dörrbecker (Chumwa)/CC BY-SA 2.5)
Climate Shocks Accelerated Neanderthal Decline
The period during which these two groups potentially overlapped was marked by dramatic climate fluctuations. The study focused particularly on Heinrich Event 5, a period of extreme cold caused by massive iceberg discharges into the North Atlantic Ocean. These climate disasters alternated with warmer interstadial periods, creating an environmental roller coaster that challenged survival for both populations. The model demonstrates that Neanderthal populations were highly sensitive to these climatic shifts, which likely accelerated their path toward extinction through demographic collapse.
When anatomically modern humans pushed into Cantabria from southern France, they encountered an environment where Neanderthal presence was already fragmented. The simulations ran thousands of scenarios with different population sizes and survival rates. In most runs, the two groups never actually met. Professor Shao noted that:
"repeated runs of the model with different parameters allow for an assessment of the plausibility of different scenarios: an early extinction of the Neanderthals, a small population size with a high risk of extinction, or a prolonged survival that would allow mixing."
Only One Percent Chance of Genetic Mixing
Perhaps the most striking finding concerns the possibility of interbreeding between the two populations. While genetic evidence confirms that modern humans and Neanderthals interbred in eastern Europe during earlier migration phases, the question of whether such mixing occurred on the Iberian Peninsula has remained contentious. The new simulations suggest that genetic mixing was theoretically possible but highly improbable. In scenarios where both populations survived long enough to overlap spatially, mixing occurred with only a 1 percent probability. When it did happen, the admixed population represented just 2 to 6 percent of the total population.
The model indicates that if any interbreeding took place in Iberia, it would most likely have occurred in the northwest of the peninsula, where modern humans could have arrived early enough to encounter surviving Neanderthal groups before complete demographic collapse. This finding has significant implications for understanding sites like Lapa do Picareiro in Portugal, where claims of early modern human occupation between 41,100 and 38,100 years ago have sparked debate among archaeologists.
The research team plans to refine their model further by incorporating animal populations as potential prey and using machine learning algorithms to improve predictive accuracy. This dynamic modeling approach offers insights that static archaeological and genomic data cannot provide alone. By simulating the complex interplay of climate, culture, and demography, researchers can now test competing theories about this crucial transition period in human prehistory. The study demonstrates that the replacement of Neanderthals by modern humans was not a simple tale of one group overwhelming another, but rather a complex process shaped by environmental stress, population dynamics, and the unpredictable nature of rare encounters across a fragmented landscape.
Top image: Neanderthal and modern human encounter in the Iberian Peninsula. Source: AI Generated
By Gary Manners
References
Shao, Y., Klein, K., Wegener, C., & Weniger, G. 2025. Pathways at the Iberian crossroads: Dynamic modeling of the Middle-Upper Paleolithic Transition. Available at: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0339184
University of Cologne. 2025. Study models the transition from Neanderthals to modern humans in Europe. Available at: https://www.uni-koeln.de/en/university/news/news/news-detail/study-models-the-transition-from-neanderthals-to-modern-humans-in-europe
