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Climate Drivers of Human Migration

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Homo sapiens left Africa around 100 thousand years ago in a series of astronomically-paced migration waves and arrived for the first time in southern Europe around 80 thousand years ago, according to a new study published in the journal Nature. These results by a researcher team from the University of Hawaii challenge single exit models that assume an exodus out of Africa around sixty thousand years ago.

The wobble of earth’s axes with a period of about 20 thousand years and its corresponding changes in climate seasonality are known to have caused massive shifts in vegetation in tropical/subtropical regions. Such shifts opened up green corridors between Africa, the Sinai and the Arabian Peninsula, enabling Homo sapiens to leave Northeastern Africa and embark onto their grand journey into Asia, Europe, Australia and eventually the Americas. When this happened and whether climate shifts really orchestrated the human migration has been a matter of intense debate.

Researchers from the University of Hawaii have used one of the first integrated climate/human migration computer models in an attempt to quantitatively recreate the grand journey of Homo sapiens over the past 125 thousand years and determine the role of climate in human dispersal. The model simulates ice-ages, abrupt climate change and captures the arrival times of Homo sapiens in the Levant, Arabian Peninsula, Southern China and Australia in excellent agreement with paleo climate reconstructions, fossil and archaeological evidence.

“One of the surprising results of our study is that the scenario that agrees best with all the Asian data is one that also simulates a very early arrival of Homo sapiens in Europe around 80-90 thousand years ago, pre-dating the oldest fossil evidence by about 40 thousand years ago” says Axel Timmermann, lead author of the study and Professor at the University of Hawaii. The green migration gateway that opened up between Africa and Eurasia 110-95 thousand years ago would have also promoted a low-density migration into Southern Europe and possibly an early interbreeding with Neanderthals” explains Tobias Friedrich, co-author of the study.

This study documents that about every 20 thousand years warmer and wetter northern Hemisphere summers boosted the migration and exchange between Africa and Eurasia. “In our model simulation we see complex pattern of human dispersal out of Africa and back flow into Africa, that challenges the more unidirectional away-from-Africa perspective that is still very prevalent in anthropology and some genetic studies.” says Axel Timmermann.

Another important question that the new University of Hawaii study addresses for the first time with a computer model is whether the abrupt glacial climate transitions, known as Dansgaard-Oeschger events, impacted human migration patterns. Comparing simulations of the human migration model with and without this characteristic mode of climate variability, the researchers find only regional impacts on simulated human density in regions extending from northern Africa to Europe. “According to our results, the global-scale migration patterns were not affected by past abrupt climate change events”.