Tübingen, 22 August 2024 – Large-scale study of fossil human teeth from Ice Age Europe shows that climate change significantly affected the demography of prehistoric humans Using the largest dataset of human fossils from Ice Age Europe to date and a new method based on a machine learning algorithm, an international research team shows how prehistoric hunter-gatherers dealt with climate change between 47,000 and 7,000 years ago. Population size fell sharply during the coldest period, and in the west the Ice Age Europeans were even on the brink of extinction.

Around 45,000 years ago, the first modern humans migrated to Europe during the last ice age, marking the beginning of the so-called ‘Upper Palaeolithic’. These early groups colonised the European continent continuously – even during the so-called ‘Last Glacial Maximum’ around 25,000 years ago, when glaciers covered a large part of northern and central Europe. ‘Archaeologists have long been discussing the influence of climatic changes and the associated new environmental conditions on the demography of the hunter-gatherers of that time. Due to the few fossils available and their often inadequate molecular preservation for the analysis of ancient DNA, it is very difficult to draw conclusions about how climatic factors affected migration, population growth, decline and extinction,’ explains lead author of the study Dr Hannes Rathmann from the Senckenberg Centre for Human Evolution and Palaeoenvironment at the University of Tübingen.

Rathmann and a research team from Italy, the USA and Germany therefore chose a new approach to clarify this issue: Instead of analysing the few scattered prehistoric individuals for which ancient DNA is available, they examined their teeth. ‘Teeth are the hardest tissue in the human body and therefore the most common fossil skeletal elements found by archaeologists. In this way, we were able to collect an unprecedented dataset that significantly surpasses previous ones in size. Our newly compiled collection includes dental data from 450 prehistoric humans from all over Europe, covering the period between 47,000 and 7,000 years ago,’ explains Rathmann. The researchers concentrated on ‘morphological’ tooth characteristics – small variations within the dentition, such as the number and shape of the crown cusps, ridge and groove patterns on the occlusal surface or the presence or absence of wisdom teeth.

‘These traits are heritable, which means that we can use them to trace genetic relationships among Ice Age humans without the need for well-preserved ancient DNA,’ explains Rathmann. As these features can be observed with the naked eye, the team also examined hundreds of published photographs of fossils. ‘Analysing historical photographs for dental features was particularly exciting as it allowed us to include important fossils that unfortunately no longer exist, such as those that were lost or destroyed during the Second World War,’ says Rathmann.

The results of the study show that around 47,000 to 28,000 years ago – during the ‘Middle Pleniglacial’ – the populations in Western and Eastern Europe were genetically well connected. ‘This finding is consistent with our previous knowledge from archaeological studies, which identified widespread similarities in stone tools, hunting weapons and even portable art from the different regions,’ explains co-author Dr Judith Beier from the DFG Center for Advanced Studies “Words, Bones, Genes, Tools” at the University of Tübingen. During this period, Europe was largely characterised by open steppe landscapes that could support large herds of mammals – the main source of food for hunter-gatherers. These conditions probably favoured the networking of populations.

In the subsequent period, the ‘Late Pleniglacial’ between 28,000 and 14,700 years ago, the researchers found no genetic links between Western and Eastern Europe. In addition, the analyses show that both regions experienced a significant reduction in population size, which led to a loss of genetic diversity. ‘This drastic demographic change was probably caused by massive climate changes: temperatures fell to the lowest values of the entire Upper Palaeolithic during this period and culminated in the Last Glacial Maximum, a time when the ice sheets reached their maximum extent and covered most of northern and central Europe,’ the Tübingen scientist explains and adds:

‘The deteriorating climate caused a shift in vegetation from a steppe to a predominantly tundra landscape, which affected the habitats of the prey animals and consequently the hunter-gatherers that depended on them,’ explains Rathmann. ‘Our results support the long-held theory that populations were not only driven southwards by advancing ice sheets, but also dispersed into largely isolated refugia with more favourable environmental conditions,’ adds Beier. Another remarkable discovery of the study is the realisation that populations in Western Europe died out at the transition from the Middle to the Late Pleniglacial and were replaced by a new population that migrated from Eastern Europe.

After the Late Pleniglacial, temperatures rose steadily again, glaciers retreated and steppe and forest vegetation returned, allowing the first recolonisation of previously abandoned areas. During this period, the research team observed that the previously isolated and greatly reduced populations in Western and Eastern Europe increased in number again and migration between the regions resumed.

‘Our new method – based on a machine learning algorithm we call Pheno-ABC – has enabled us for the first time to reconstruct complex prehistoric demographic events using morphological data. As far as we know, this has never been achieved before,’ enthuses co-first author Dr Maria Teresa Vizzari from the University of Ferrara, who played a key role in the development of the algorithm. The new analytical tool makes it possible to identify the most likely demographic scenario among many tested ones. According to the researchers, the Pheno-ABC method could revolutionise the analysis of fossil skeletal morphology in the future.

‘Our study provided important insights into the demographic history of Ice Age Europeans and highlights the profound impact of climate and environmental change on the lives of prehistoric people. We urgently need to learn from our past if we are to tackle the complex environmental problems of the future,’ concludes Rathmann.

Originalpublication:

Rathmann et al. Human population dynamics in Upper Paleolithic Europe inferred from fossil dental phenotypes. Science Advances 10 (2024): DOI: 10.1126/sciadv.adn8129 https://www.science.org/doi/10.112 6/sciadv.adn8129