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  Geotimes - September 2007 - Ancient humans dodged super-eruption?

Ancient humans dodged super-eruption?

Depiction of Mount St. Helens eruption
USGS/Cascades Volcano Observatory
The Toba volcanic eruption 74,000 years ago — much more destructive than the Mount St. Helens eruption depicted here — may have drastically altered Earth’s climate. New research suggests humans were flexible enough to survive these changes.

Seventy-four-thousand years ago, Indonesia’s Mount Toba erupted, spreading ash from the Arabian Sea to the South China Sea. Using data from ice cores, researchers have concluded this eruption — Earth’s largest in the past 2 million years — drastically altered the world’s climate and caused a six-year-long volcanic winter in some parts of the world. Correlating the date of the eruption to evidence of a genetic bottleneck in the modern human population around the same time, some researchers suggested that populations of modern humans (Homo sapiens) that had previously expanded out of Africa were unable to cope with these changes, and thus experienced huge population declines everywhere except in tropical, mainly African, refuges. Now, Michael Petraglia, an archaeologist at the University of Cambridge in the United Kingdom, says his team has found evidence to the contrary. Excavating in Jwalapuram in southeastern India, Petraglia and his crew discovered stone tools that they say show not only that modern humans were living in the area both before and shortly after the eruption, but that they left Africa via a different route than is commonly thought.

Previous thinking about the effects of the Toba eruption — that most modern human populations crashed outside of Africa — was based mainly on conjecture, Petraglia says. Now, he says, “we’re actually finding empirical archaeological evidence correlating stone tools and ash.” Petraglia and his colleagues found stone tools in stratigraphic layers separated by a layer of ash containing the geochemical signature of the Toba eruption. They dated the layer of tools below the ash to roughly 77,000 years ago and the layer of tools above the ash to roughly 74,000 years ago, as they reported July 6 in Science. Analyzing the features of the tools, Petraglia and his team determined that the tools from each layer are very similar, suggesting the same species of Homo made all of the tools. This suggestion — along with the close dates of each layer — led Petraglia and his colleagues to conclude that hominins were resilient enough to continuously occupy this part of India despite climatic changes due to Toba.

“We’re not arguing the Toba super-eruption didn’t have an effect” on our ancestors’ environment, Petraglia says. “What we’re arguing is that … populations certainly survived.” The population in India, he says, represents an early, successful migration of Homo sapiens out of Africa. Because the team did not find any skeletal evidence at their site, they compared their stone tools to tools from Africa and the Middle East to determine what kind of hominin — Homo sapiens, Homo erectus or something else — might have been living in India at this time period. Petraglia’s team found the Indian tools were most similar to modern-human-made Middle Stone Age tools from Africa. This connection, Petraglia says, indicates that the makers of the Indian tools were also Homo sapiens. He speculates that modern humans probably made their way from Africa to India via the Horn of Africa, entering Arabia and heading east through Iran until reaching the Indian subcontinent. Other groups of modern humans probably continued on until they reached Australia sometime between 60,000 and 40,000 years ago, he says.

Stanley Ambrose, a paleoanthropologist at the University of Illinois in Urbana-Champaign, isn’t convinced. “There’s not enough evidence to show cultural traditions that persisted through time,” he says. “They don’t have enough artifacts to determine cultural continuity.”

Furthermore, Ambrose says, in this case, looking at stone tools doesn’t necessarily support the claim that the tools from both statigraphic layers were made by the same species of hominin just because they look somewhat similar to one another. Different hominin species could be using similar tools because “ideas travel faster than genes,” he says. “What you really need to prove this [hypothesis] is something really very simple — human skeletons.” Comparing anatomical features of skeletons of hominins living before and after the eruption would determine if they really belonged to the same species or not.

Sally McBrearty, an archaeologist at the University of Connecticut in Storrs, says that humans could have survived the climatic changes brought on by the Toba eruption. Although skeptical about the evidence linking the Indian tools to the African Middle Stone Age tools, she says she wouldn’t be surprised if modern humans were living in this part of the world at the time. “To be fair,” she says, “this is kind of a murky topic.”

Perhaps part of the reason so much is still unclear about the history of hominins in this region is the lack of attention this region has received from archaeologists and paleoanthropologists in the past, Petraglia says. When it comes to studying modern humans during this time period, there are hundreds of projects elsewhere, but only a handful in India, he says. “We should look at other parts of the world to tell a fuller story about human evolution,” he says.

Erin Wayman

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