The causes of the great migration, when modern humans went from being restricted to a single continent to conquering the world, remain unknown. However, a new record of climate changes in the Middle East and Mediterranean region provides some clues.
Today the Earth reaches its closest approach to the Sun in early January, but 125,000 years ago this occurred during the northern summer instead, making Northern Hemisphere summers hotter and winters cooler. In contrast, orbital elongation currently damps the north's seasonal variation down while reinforcing the Southern Hemisphere's changes.
Professor John Kutzbach of the University of Wisconsin-Madison said in a statement: "There were stronger summer rains in the Sahara and stronger winter rains in the Mediterranean" at the time. The same thing happened about 20,000 and 40,000 years later, confirmed with pollen records from old lake beds, isotopes in limestone caves, and marine sedimentary deposits. "We don't really know why people move, but if the presence of more vegetation is helpful, these are the times that would have been advantageous to them," he said.
In the Proceedings of the National Academy of Sciences, Kutzbach provides the most detailed timeline yet on temperature and rainfall changes in Africa, Arabia, and the Mediterranean Basin. He proposes it was one of these cyclical climatic changes that drove the great migration.
The theory makes intuitive sense. While the Sahara was as dry as it is today it would have posed a formidable barrier, and the Middle East may not have been particularly attractive to the hunter-gatherers of the day, but at other times the promise of new hunting grounds would lead explorers on. These cycles also explain evidence Homo Sapiens left Africa much earlier, but fail to consolidate their presence in nearby regions.
Between 70,000 and 15,000 years ago the most intense part of the ice age reduced the cycle's amplitude, softening the expansion and retreat of the deserts.
Kutzbach has been studying ancient climate cycles for a long time. More than 40 years ago he was the first person to put together records of strong and weak periods in Africa's monsoons with Milankovitch cycles in the Earth's orbit. He describes his work since then as a slow process of refining the models using increased computing power and extra data.
The work on human migration patterns brings Kutzbach full circle. The magnificent cave art at Lascoux made him ponder how close the makers were to the edge of the ice sheet, which in turn got Kutzbach interested in glaciology, and then paleoclimates, leading to all his subsequent work.
For all the unqualified commentators who will excitedly announce that these orbital-driven climate changes prove our current rising temperatures are natural, Kutzbach's work shows the exact opposite. Not only are the changes he has tracked 10-100 times slower than what we are experiencing, but orbital forces are currently (marginally) cooling, not warming, the planet.