Human Migrations from Africa

One of the great ongoing debates in human paleontology is how we evolved out of Africa. Traditionally, two polar opposite pathways from Africa have been discussed: the Out of Africa model and the Multiregional Hypothesis. The Out of Africa model says that we anatomically modern humans (AMH) evolved in Africa and then spread out through the world. The Multiregional Hypothesis says we spread as Homo erectus (or another archaic form) throughout the world and then evolved to AMH over and over again. Scientists have walked away from these two extreme poles, because there is evidence for both, that there were fully AMH who evolved only in Africa but that changes and adaptations took place elsewhere after we left.

Isolation by Distance Theory of Human Population. Click to see larger image and caption.
Photo by Philipp Gunz, University of Vienna & Max Planck Institute for Evolutionary Anthropology Leipzig.

Solving the exact puzzle of the exit pathways and timing from Africa is difficult, because a) the fossil record is so darn patchy and b) ancient DNA hasn't survived adequately for us to parse it. But a research team, led by Philipp Gunz of the University of Vienna & Max Planck Institute for Evolutionary Anthropology in Leipzig, has come up with a new strategy, and a new theory, which I'm going to call Isolation by Distance. The report was published in the March 23rd issue of the journal Proceedings of the National Academy of Sciences.

The researchers measured the morphology of crania—the shapes of skull caps—from early anatomically modern humans between about 200,000—60,000 years ago, including Upper Paleolithic humans from Dolni Vestonice, Mladec, Prednosti and Zhoukoudian Upper Cave; early AMH from Omo Kibish and Skuhl caves; Neanderthals from Chapelle aux Saintes and Atapuerca, and Archaic Homo from Ngangdong, Sangiran and Zhoukoudian lower cave.

They chose to look at skull shapes, primarily because that's what has survived all these long millennia. Neanderthal and Archaic Homo skulls cluster tightly; but anatomically modern humans, interestingly enough, overlap and vary. The researchers suggest that this variation and overlap results from the dispersal of human groups in Africa first and again later, leaving Africa by different routes, at different times.

Although the researchers do not detail it, I'm going to be interested to see what they make of the South African Howiesons Poort/Stillbay data, which would seem clearly to be a set of "isolated" sites.

Theories of Human Migration

• Isolation by Distance
• Multiregional Hypothesis
• Out of Africa Model
• Southern Dispersal Route

Gunz, Philipp, et al. 2009 Early modern human diversity suggests subdivided population structure and a complex out-of-Africa scenario. Proceedings of the National Academy of Sciences Early Edition.