The transition from the Middle to Upper Paleolithic, occurring approximately 50,000 to 38,000 years ago, was a pivotal period characterized by the decline and extinction of Neanderthals alongside the emergence and expansion of anatomically modern humans, known as Homo sapiens. Paleoanthropologists at the University of Cologne have created a high-resolution model of population dynamics to reconstruct this significant transition on the Iberian Peninsula. Their ensemble simulations investigated Neanderthal survival, the arrival of modern humans, and the potential for interbreeding.

During this critical transition from the Middle Paleolithic to the Upper Paleolithic, Neanderthal populations experienced a steady decline across Europe, particularly in the Iberian Peninsula, leading to their eventual extinction.

Simultaneously, anatomically modern humans spread throughout Europe, marking a significant shift in human history.

This era was further defined by dramatic climate fluctuations, featuring alternating cold and warm periods. Rapid warming events occurred within centuries, contrasting with gradual cooling phases, known as the Dansgaard-Eschger phenomenon, which were punctuated by severe cooling caused by iceberg releases into the North Atlantic (Heinrich phenomenon).

The precise timing of Neanderthal extinction and the arrival of modern humans remains uncertain, leaving open the possibility of interactions between the two species.

Genetic analyses of archaeological bones compared to modern populations indicate admixture events in eastern Europe during the early phases of modern human migration.

Given the uncertainty of these dates, it remains possible that the two populations on the Iberian Peninsula may have intermixed at a later time, though this has yet to be substantiated.

In this innovative study, Professor Yaping Hsiao and his colleagues from the University of Cologne utilized numerical models to exploratory simulate potential encounters between Neanderthals and modern humans on the Iberian Peninsula.

These models considered ongoing climate changes while simulating the populations of both groups, along with their interactions and connectivity.

“By running the model multiple times with varying parameters, we can assess the plausibility of different scenarios, such as the early extinction of Neanderthals, small at-risk populations, or prolonged survival leading to admixture,” explained Professor Hsiao.

“However, for the majority of the simulations, the two groups did not converge.”

Across all scenarios, the populations exhibited high sensitivity to climate change.

Mixing between the two species was plausible if both populations could maintain stability over an extended period.

At a low probability (1%), a small fraction of the total population—approximately 2-6%—could possess genes from both groups by the end of the simulation.

This admixture likely occurred in the northwestern region of the Iberian Peninsula, where modern humans may have arrived early enough to interact with still-surviving Neanderthal populations.

“By integrating climate, demography, and cultural factors, our dynamic model provides a comprehensive framework that enhances our interpretation of the archaeological and genomic records,” stated Professor Gerd Christian Weniger from the University of Cologne.

For further details, refer to a paper that will be published in the online journal PLoS ONE.

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Y. Xiao et al. 2025. Pathways at the Crossroads of Iberia: Dynamic Modeling of the Middle to Upper Paleolithic Transition. PLoS ONE 20 (12): e0339184; doi: 10.1371/journal.pone.0339184