The Toba eruption in Indonesia 74,000 years ago was so massive that its debris, including glass shards, likely fell atop a site inhabited by humans 5,592 miles away.

Credit: pixabay

During a volcanic winter, ash as well as sulfuric acid, combine with water to obscure the sun and increase the reflection of solar radiation. The result is that summer never arrives, temperatures drop, crops can fail, and people — those lucky enough to have survived the initial volcanic eruption — may experience famine and undergo mass migrations.

This all happened in 1815 following the eruption of Mount Tambora in Indonesia. The event was rated a 7 out of 8 on the Volcanic Explosivity Index (VEI).

Geologists believe that an eruption 74,000 years ago of Toba, also in Indonesia, was 100 times more powerful than the Mount Tambora event. Approximately sixty VEI 8 eruptions have been identified in the geological record, with Toba's rated as being among the most explosive.

Evidence for the rock, gas, pieces of glass, and other debris propelled into the atmosphere by the supervolcano have been found in numerous places across the globe.

Some of the glass fragments, known as cryptotephra, have just been unearthed in South Africa. The discovery, reported in the journal Nature, marks the first time that such deposits have been identified so far from their source volcano. In this case, the distance was over 5,592 miles away.

"It is important to realize that the glass shards are particles and won't stay in the atmosphere for very long, so it can be safely deduced that their deposition occurred soon after the eruption," co-author Zenobia Jacobs of the University of Wollongong told Seeker.
 

Credit: Erich Fisher

Jacobs and her colleagues explained that the cryptotephra have a characteristic hook shape that can be seen when magnified. The shape is produced when glass fractures across a bubble as a volcano erupts.

The timing and location of the newly discovered shards are of interest because humans were living at the South African site when Mount Toba erupted.

Senior author Curtis Marean told Seeker that the cryptotephra were found in a sand dune geological layer associated with extensive human artifacts at two archaeological sites on the southern coast of South Africa: Vleesbaai and Pinnacle Point.

Marean is the project director of excavations at Pinnacle Point, a site that he has been studying for nearly two decades. He is also the associate director of the Institute of Human Origins at Arizona State University and is an honorary professor at the Nelson Mandela University's Center for Coastal Paleoscience in South Africa.

He said the people at both Vleesbaai and Pinnacle Point had access to a rock shelter. Sea levels at the time had dropped, forming a plain in front of the rocky outcropping.

"They lived under this rock shelter, looking out on this plain with the coast in the distance," Marean said. "They hunted animals like zebra and wildebeest and collected plant foods on that plain. There were likely fresh water sources available as seeps."

"Their technology was stone and wood and bone and leather," he continued, "but relative to the rest of the world, they were ahead of the game with a new, advanced projectile technology called microlithic technology."

The innovation enabled the early South Africans to create composite tool components as part of advanced weapons that could be thrown over greater distances.

Human life therefore seemed to be thriving at these coastal South African locations when Mount Toba erupted.
It is hard to imagine what the people witnessed just after the explosive event.

"The length and severity of the Toba volcanic winter are hotly debated," Marean said. "This can only be answered with climate modeling and high-resolution climate and environmental records."

What is now known is that the early modern humans living at Vleesbaai and Pinnacle Point prospered despite the natural disaster that likely had a global impact. The determination adds important insight to the Toba catastrophe theory.

This theory goes back to the early 1990s, when biologist Michael Rampino of New York University, volcanologist Stephen Self of the University of Hawaii at Manoa, and science journalist Ann Gibbons all posited that the Mount Toba eruption could have caused a sharp reduction in the human population, known as a population bottleneck. The theory has since been developed by Stanley Ambrose of the University of Illinois at Urbana-Champaign.

Marean said that he found Ambrose's work on the subject "to be exciting and worth investigating ... Then my colleague Panagiotis Karkanas saw a shard under his microscope and we were off on the chase."

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Lead author Eugene Smith of the University of Nevada Las Vegas told Seeker that analysis of the cryptotephra "took a lot of very careful work over a period of nearly two years." The analysis occurred not only at his lab, but also at Guillaume Girard's lab at Michigan State University.

It is possible that the rock shelter sufficiently shielded the early South Africans from the potentially dangerous volcanic debris. If the volcanic winter extended to this region, it is even possible that this population was one of the few that survived nature's onslaught. The people could have survived the devastating time due to their distance from the eruption and the ample potable water and food sources at the coastline, the authors suggest.

As for what happened in Indonesia and surrounding areas, the evidence is unclear. Jacobs said, "There is currently no fossil remains of Denisovans or Neanderthals from Indonesia or anywhere in southeast Asia at any time." She added that Toba's debris has not been found on the Indonesian island of Flores, known to be inhabited by Homo floresiensis, aka the hobbit humans, so named because of their relatively diminutive size.

She explained that the massive Toba ash cloud did not blow to the east, but rather moved northeastwards. Significant amounts of the deposits have therefore been found in the South China Sea, as well as across India.
She added, however, that "it is hard to imagine that a volcanic eruption of that size did not have any effects on the tectonic plates running through Indonesia and triggered other smaller volcanoes that may have had variable effects."
 

Credit: Erich Fisher

Remains for Denisovans, Neanderthals, and anatomically modern humans dating to well before the Toba eruption have been found in other parts of Eurasia as well as North Africa. For example, earlier this year it was announced that a Homo sapiens jawbone dating to 177,000–194,000 years ago was found in Israel.

"Israel is on the doorstep of Africa and, under some climate conditions, is more African than it is Asian," Marean said. "Finding early modern humans in Israel does not surprise me — this does not indicate a migration, but rather a range expansion."

The massive Toba eruption could have temporarily slowed or even stopped the "range expansion," given the potential enormity of its impact. If entire hominid populations were reduced or even eliminated as a result of the event, surviving groups well to the north as well as to the south might have wound up contributing more to today's human gene pool.

It is known that a major out-of-Africa migration took place around 60,000 years ago, long after the cataclysmic volcanic eruption, which could happen again. Geophysicists believe that a magma reservoir is once more growing under what is now Lake Toba. Models suggest that a large Toba eruption is unlikely to happen for thousands of years, however.

The authors hope that future studies of archaeological sites elsewhere in Africa may help to determine whether other human populations thrived during the time of the prehistoric Toba event. If not, then the southern coast of South Africa might represent a refuge: the place where some of the only human survivors of a likely prolonged volcanic winter could have lived.