Modern humans have a complex history of dispersal from Africa, with the most significant global expansion occurring less than 100,000 years ago. Traditionally, these dispersals were believed to be limited to “green corridors” formed during humid periods when food was plentiful and human populations expanded in sync with their environments. However, a recent study in Nature challenges this notion, suggesting that humans may have also dispersed during arid intervals along “blue highways” created by seasonal rivers.

The research, led by Arizona State University scientists Curtis Marean, Christopher Campisano, and Jayde Hirniak, focused on the Shinfa-Metema 1 site in northwest Ethiopia along the Shinfa River, a tributary of the Blue Nile River. This site showed how early modern humans survived in the aftermath of the Toba supereruption, one of the largest supervolcanoes in history, approximately 74,000 years ago.

The study revealed that the site was occupied during high seasonal aridity, as indicated by stable oxygen isotopes from fossil mammal teeth and ostrich eggshells. Despite these challenging conditions, the inhabitants of Shinfa-Metema 1 demonstrated remarkable adaptive foraging strategies, relying on capturing a diverse range of terrestrial and aquatic animals, including fish, from shrinking waterholes along the seasonal river. This flexible approach to foraging along dry-season waterholes transformed the seasonal rivers into ‘blue highway’ corridors, potentially facilitating out-of-Africa dispersal.

The findings also shed light on the behavioral flexibility of early modern humans, emphasizing their ability to adapt to challenging climatic conditions, including the short-term effects of the Toba supereruption. The evidence of cooking and the discovery of stone tools representing the oldest evidence of archery further highlight the resourcefulness of these early human populations.

This study challenges previous assumptions about human dispersal and underscores the significance of behavioral flexibility in the survival and subsequent global expansion of modern humans. The ability to correlate sites across Africa at a resolution of several weeks, using new cryptotephra methods, represents a groundbreaking implication of this research, offering valuable insights into the complex history of human dispersal.

References

• Arizona State University. (2024, March 20). Toba supereruption may have facilitated dispersal of modern humans out of Africa. Phys.Org; Phys.org. https://phys.org/news/2024-03-toba-supereruption-dispersal-modern-humans.html
• Kappelman, J., Todd, L. C., Davis, C. A., Cerling, T. E., Feseha, M., Getahun, A., Johnsen, R., Kay, M., Kocurek, G. A., Nachman, B. A., Negash, A., Negash, T., O’Brien, K., Pante, M., Ren, M., Smith, E. I., Tabor, N. J., Tewabe, D., Wang, H., … Yanny, S. (2024). Adaptive foraging behaviours in the Horn of Africa during Toba supereruption. Nature, 1–8. https://doi.org/10.1038/s41586-024-07208-3