Modern humans first originated in Africa, but the key event that led to their global expansion occurred less than 100,000 years ago. Previous research had suggested these early human dispersals were limited to “green corridors” – regions with abundant food resources during wet climate intervals.

However, a new study published in the journal Nature proposes that humans may have also dispersed during drier periods by using “blue highways” – the seasonal rivers that still flowed.

The research team, led by scientists from Arizona State University, examined a site called Shinfa-Metema 1 located in the lowlands of northwest Ethiopia near the Shinfa River, a tributary of the Blue Nile. They found evidence that this site was occupied during a time period when the devastating Toba supervolcano erupted in Sumatra 74,000 years ago. Tiny fragments of volcanic glass, or cryptotephra, recovered from the archaeological deposits matched the chemical signature of the Toba eruption.

One of the revolutionary implications of this study is that with the new cryptotephra methods developed for our previous work in South Africa, and now applied here in Ethiopia, we can correlate sites across Africa, and perhaps the world, with a time resolution of weeks, said researcher Christopher Campisano.

These microscopic shards of volcanic glass, often less than the width of a human hair, can be used to precisely date and correlate archaeological sites separated by thousands of miles. Searching for cryptotephra in these archaeological sites is like looking for a needle in a haystack, but not even knowing if there is a needle. But having the ability to correlate sites 5,000 miles apart within weeks rather than thousands of years makes it worthwhile, Campisano added.

The research team’s analysis of animal tooth enamel and ostrich eggshell from the site suggests the inhabitants adapted to very dry, seasonal conditions – similar to some of the most arid habitats in East Africa today. As waterholes dried up during the dry season, the people likely shifted to hunting smaller, more easily captured animals and fishing. This increased reliance on aquatic resources may have pushed them to expand their range in search of new food sources.

As the food resources became depleted around a particular waterhole during the dry season, they would have been forced to move to the next, said John Kappelman, the study’s lead author. The seasonal rivers acted like ‘pumps’ that would have diverted populations from one watering hole to the next, potentially driving the most recent dispersal out of Africa.

The stone tools found at Shinfa-Metema 1 include small, symmetrical triangular points that the researchers believe represent the earliest known evidence of bow-and-arrow technology, dating back 74,000 years. This flexibility in behavior, including the ability to adapt to extreme climate changes like the Toba eruption, likely facilitated the eventual spread of modern humans out of Africa and across the globe.

In conclusion, this new evidence suggests that the dispersal of modern humans from their African homeland may have occurred not just during times of abundant resources, but also during periods of environmental stress – with seasonal rivers acting as “blue highways” enabling movement and expansion.

The technological and behavioral adaptations that allowed our ancestors to survive such challenging conditions paved the way for the ultimate global conquest of our species.


Arizona State University | Kappelman, J., Todd, L.C., Davis, C.A. et al. Adaptive foraging behaviours in the Horn of Africa during Toba supereruption. Nature (2024).

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