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Global spread of humans was influenced by climate change

A multidisciplinary project involving scientists from the universities of Kent, Cambridge and Bristol indicates that the global expansion of humans was influenced by climate change.

By integrating genetics with high resolution climate reconstructions, the team's research, which was published by the journal PNAS on 18 September, has been able to predict the timing and routes taken by modern humans during their expansion out of Africa.

The role of climate change in determining the timing of the expansion of human populations has been long debated. The oldest fossil remains of anatomically modern humans are found in Africa and date back to around 200 thousand years ago, but how and when people expanded from Africa is highly controversial. The newly published research provides the first direct link between climate change and the timing of the expansion out of Africa, as well as the routes taken.

The project involved specialists from a variety of fields. Using reconstructions of climate and sea level changes through time, the scientists were able to explore the effect of these factors on food availability deep into prehistory. Dr Anders Eriksson and Dr Andrea Manica from the University of Cambridge first compared several million different demographic scenarios (e.g. birth rates, local movement rates, link between food availability and population sizes), and were able to identify the scenarios that were most compatible with the geographic patterns of genetic diversity in modern humans. Anthropologists from the University of Kent were then able to compare these scenarios against the dates and localities of known archaeological and fossil finds.

The demographic scenarios chosen by the model revealed a link between food availability and population density in the past, very similar to the link found in present day hunter-gatherers. Based on this link, the model found that climate prevented humans from exiting Africa until a favourable window appeared in North-East Africa approximately 70-55 thousand years ago. The dating of the out-of-Africa exit, as well as the arrival times for other continents identified by the model, were also found to largely agree with archaeological and fossil evidence.

The team of evolutionary anthropologists from Kent included Dr Stephen Lycett (Senior Lecturer in Human Evolution), Dr Noreen von Cramon-Taubadel (Lecturer in Biological Anthropology) and Ms. Lia Betti (PhD Candidate, School of Anthropology and Conservation). Dr Lycett, who led the anthropological team from Kent, said: ‘The rate of human expansion from Africa and across the rest of the globe has always been controversial, but these findings provide important new insights into the role of climate change in this process. The project has required geneticists, climatologists and vegetation modellers working alongside anthropologists from Kent to use data from all of these different fields, and this has been an exciting aspect of this new work.’

Dr Manica, from the University of Cambridge and the senior scientist on the study, added: ‘The idea that we can reconstruct climate, and estimate food availability and finally figure out the demographic changes and movements of our ancestors all over the world is simply amazing. The fact that most of our results are in agreement with archaeological and anthropological evidence, points to the fact that our reconstructions based on genetics are quite realistic.’

Contact: pressoffice@kent.ac.uk

Story published at 12:10pm 18 September 2012