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Surviving a Volcanic Supereruption May Have Facilitated Human Dispersal Out of Africa

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AUSTIN, Texas — Researchers working in the Horn of Africa have uncovered evidence showing how Middle Stone Age humans survived in the wake of the eruption of Toba, one of the largest supervolcanoes in history, some 74,000 years ago. The behavioral flexibility of these Middle Stone Age people not only helped them live through the supereruption but may have facilitated the later dispersal of modern humans out of Africa and across the rest of the world.

Excavations at a Middle Stone Age archaeological site, Shinfa-Metema 1, in the lowlands of northwest Ethiopia revealed a population of humans at 74,000 years ago that survived the eruption of the Toba supervolcano.

Modern humans dispersed from Africa multiple times, but the event that led to global expansion occurred less than 100,000 years ago. Some researchers hypothesize that dispersals were restricted to “green corridors” formed during humid intervals when food was abundant and human populations expanded in lockstep with their environments. But a new study in Nature led by scientists at The University of Texas at Austin suggests that humans also may have dispersed during arid intervals along “blue highways” created by seasonal rivers. Researchers also found stone tools that represent the oldest evidence of archery.

The team investigated the Shinfa-Metema 1 site in the lowlands of present-day northwestern Ethiopia along the Shinfa River, a tributary of the Blue Nile River. Based on isotope geochemistry of the teeth of fossil mammals and ostrich eggshells, they concluded that the site was occupied by humans during a time with long dry seasons on par with some of the most seasonally arid habitats in East Africa today. Additional findings suggest that when river flows stopped during dry periods, people adapted by hunting animals that came to the remaining waterholes to drink. As waterholes continued to shrink, it became easier to capture fish without any special equipment, and diets shifted more heavily to fish.

The supereruption occurred during the middle of the time when the site was occupied and is documented by tiny glass shards whose chemistry matches that of Toba. Its climatic effects appear to have produced a longer dry season, causing people in the area to rely even more on fish. The shrinking of the waterholes may also have pushed humans to migrate outward in search of more food.

Projectile points from a Middle Stone Age archaeological site, Shinfa-Metema 1, in the lowlands of northwest Ethiopian dating from the time of the Toba supereruption at 74,000 years ago provide evidence for bow and arrow use prior to the dispersal of modern humans out of Africa. Photograph by Blue Nile Survey Project.

“As people depleted food in and around a given dry season waterhole, they were likely forced to move to new waterholes,” said John Kappelman, a UT anthropology and earth and planetary sciences professor and lead author of the study. “Seasonal rivers thus functioned as ‘pumps’ that siphoned populations out along the channels from one waterhole to another, potentially driving the most recent out-of-Africa dispersal.”

The humans who lived at Shinfa-Metema 1 are unlikely to have been members of the group that left Africa. However, the behavioral flexibility that helped them adapt to challenging climatic conditions such as the Toba supereruption was probably a key trait of Middle Stone Age humans that allowed our species to ultimately disperse from Africa and expand across the globe.

The people living in the Shinfa-Metema 1 site hunted a variety of terrestrial animals, from antelope to monkey, as attested to by cut marks on the bones, and apparently cooked their meals as shown by evidence of controlled fire at the site. The most distinctive stone tools are small, symmetrical triangular points.

“Analyses show that the points are most likely arrowheads that, at 74,000 years in age, represent the oldest evidence of archery,” Kappelman said. “The Ethiopian Heritage Authority has made 3D scans of the points available so that anyone anywhere in the world can download the files and evaluate the hypothesis for themselves.”