How and where modern humans lived when they were on the cusp of their dispersal out of Africa to populate the rest of the world has long been a focus of studies of human evolution. We studied the remains of an early human population in the lowlands of northwestern Ethiopia that lived along a trunk tributary of the Blue Nile River, one of the hypothesized dispersal routes. The Middle Stone Age archaeological site, Shinfa Metama 1, with Youngest Toba Tuff cryptotephra dated to around 74,000 years age,  provides early and rare evidence of intensive riverine-based foraging aided by the likely adoption of the bow and arrow. The diet included a wide range of terrestrial and aquatic animals. Stable oxygen isotopes from fossil mammal teeth and ostrich eggshell show that the site was occupied during a period of high seasonal aridity. The unusual abundance of fish suggests that capture occurred in the ever smaller and shallower waterholes of a seasonal river during a long dry season, revealing flexible adaptations to challenging climatic conditions during the Middle Stone Age. Adaptive foraging along dry-season waterholes would have transformed seasonal rivers into ‘blue highway’ corridors, potentially facilitating an out-of-Africa dispersal and suggesting that the event was not restricted to times of humid climates. The behavioral flexibility required to survive seasonally arid conditions in general, and the apparent short-term effects of the Toba supereruption in particular were probably key to the most recent dispersal and subsequent worldwide expansion of modern humans.

John Kappelman is Professor Emeritus at the University of Texas at Austin. He holds a BS degree in Geology and Geophysics from Yale University and a PhD in Anthropology and Earth and Planetary Sciences from Harvard University.