Climate and human dispersal

A brief summary of the key findings of my research on the effects of climate on human dispersal, a collaborative effort with several colleagues from different universities. For human skeletal adaptation to different climates, see this page.

Climate shaped early human dispersal out of Africa

Humans evolved in Africa for most of their existence as a species, but rapidly expanded into other continents around 80-60 kya (thousand years ago). Climate payed a major role in facilitating the expansion out of Africa, as North-East Africa, the Arabian Peninsula and the Levant became less arid during certain periods creating an ecological corridor between Africa and Eurasia for humans and other fauna. Climate fluctuations in the late Pleistocene also facilitated the colonisation of Australia (thanks to low sea levels) and the Americas (retreat of ice after the last glacial maximum). In the figure you can see the most likely time of opening of the ecological corridors to different continents (blue histograms), and compare it with key archaeological finds (red arrows). We now have archaeological evidence of earlier arrival in South-East Asia, that fits better with our model, but we got Europe pretty wrong. Climate was clearly not the main factor in delaying the colonisation of Europe; maybe Neanderthals were pushing back? The open-access paper can be found here.

Median arrival times as predicted by the model. Histograms show the distribution of times for key areas of the world. Red arrows show key archaeological findings commonly referred to as the earliest evidence for Homo sapiens in those areas

Climate and the arrival of Neolithic farmers in Europe

Around 9,000 years ago, Neolithic farmers from the Levant started expanding into Europe, and rapidly colonised most of the continent largely displacing local hunter-gatherer populations. Archaeologists have wondered for some time why the expansion of farming, that was so successful in Southern and Central Europe, suffered a sudden slowdown in the Northern and North-Eastern regions of the continent. One suggestion was that hunter-gatherer communities were larger and occupied the land more densely near the coast of the Northern coast, pushing back more effectively against the invading farmers. We show that it was climate that slowed down the Neolithic expansion, as Levantine crops started failing more often when farmers encountered lower temperatures at higher latitudes (i.e. fewer days suited for crop growth, called growing degree days). When growing degree days decreased, farmers started interacting more with local hunter-gatherers, probably because they needed to supplement their crops with foraged food. We found evidence of increased admixture between the two groups in ancient DNA extracted from skeletal remains. So climate determined the speed of farmers’ expansion, the viability of their way of life and the change in the genetic make-up of Europe. You can find the paper here.

Main axes of expansion of farmers into Europe. The occurrence of a slowdown is highlighted by black thick lines.

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