Mount Toba: Did a Supervolcano Almost Wipe Out the Human Species?

Seventy-four thousand years ago, the Earth suffered a catastrophic event which, according to the Toba catastrophe theory, led to the extinction of many animals around the globe.

Ash covered the land. The sky was blanketed in darkness. Temperatures plummeted. Plants refused to grow. The few surviving human beings huddled together for warmth as the apocalyptic nightmare dragged on.

Seventy-four thousand years ago, the supervolcano Mount Toba on what is now Sumatra exploded. It was an extinction event that changed the world, and according to some academics, it almost wiped out the human species.

Compared with other animal species, there is a surprising lack of genetic diversity among humans. In fact, there is less genetic diversity between all eight billion of us than there is in a closely related troop of 50 chimpanzees. The reason for this was a mystery for many years until theories were put forward to explain the phenomenon.

The Toba catastrophe theory and the human bottleneck hypothesis posit that around 74,000 years ago, the human species almost went extinct. It resulted from a global catastrophe caused by the eruption of Mount Toba, the remains of which lie on the Indonesian island of Sumatra.

Climate models suggested that the average world temperature dropped by as much as 15 degrees Celsius in the wake of the eruption.

Genetic data contributed to the theory and suggested that the human species went through a genetic bottleneck – a severe reduction in the total gene pool –  suggesting that the species was greatly reduced. Some academics posited that the species was reduced to a few thousand individuals with as few as 1,000 mating pairs in total. This would explain why there is so little genetic diversity among our species.

The theory posits that we survived in small groups scattered in various parts of Africa before repopulating.

Once widely accepted, however, this theory has come under renewed scrutiny over the past few years as evidence paints a more confusing picture as to how humanity coped.

The Eruption Data

The explosivity index on which volcanoes are rated is called the Volcanic Explosivity Index. Like the Richter scale, which measures the power of earthquakes, each number increase represents a factor increase in 10. So, a VEI number of 5, for example, is ten times more powerful than a VEI number of 4.

The VEI number for Toba is estimated to be 8, the highest on the scale. For reference, the eruption of Mount Saint Helens measured 5. Toba was 1,000 times more powerful than Mount Saint Helens and 100 times more powerful than the Krakatoa eruption, which clocked in at VEI 6. Krakatoa was so powerful the sound of the blast curled the Earth four times!

In 1815, the eruption of Mount Tambora, measuring a 7 on the VEI scale, spewed so much ash into the air that 1816 was known as “the year without a summer.” Crops failed the world over, and it snowed in Virginia on July 4! Toba was more powerful than Tambora by a factor of 10.

As Toba exploded, those in the immediate surroundings would have had no chance of survival. Pyroclastic flow billowed from the site, superheated to 1100 degrees Celsius and traveled at 320 kilometers per hour. Anybody who came in contact with it would have died instantly.

Studies show that the total amount of dense-rock equivalent (DRE), the scientific term used when measuring volcanic ejection mass, was between 2,800 km3 (670 cu mi) and 6,000 km3 (1,400 cu mi). All of this tephra (or DRE) was ejected in the space of 9 to 14 days.

This completely dwarfs all volcanic eruptions in history, making the eruptions of Vesuvius and Mount Saint Helens seem tiny and insignificant by comparison.

The exact effects of the eruption are difficult to determine through studying the sedimentary layer. However, there seems to have been a shift in climate that may or may not have been caused by the eruption. It is likely that a climate shift was already taking place and was accelerated by the Toba eruption. Temperatures dropped or were dropping significantly, ushering in the start of an ice age that forced many species to adapt.

If Toba were not directly responsible for the Ice Age, it would certainly have contributed to speeding it up.

What Could the Apocalypse Have Been Like?

Many of the people who lived in Sumatra would have been killed in the initial few days of the explosion. Those unlucky enough to survive the first few days would have had a more drawn-out death as ash whipped through the air and covered the ground in several feet of its toxic mass. In India, 5,000 kilometers (3100 miles) away, six inches of ash covered the ground, and even further away, ash deposits from the Toba eruption have been found in Africa near Lake Malawi as well as on the southern coast of South Africa, 8,900 kilometers (5,530 miles) away.

While the ash may have looked light and fluffy like snow, the truth is that volcanic ash is heavy. It’s thick and toxic. Fine dust breathed in forms cement-like quality in the lungs; those who inhaled the ash would have suffocated, some in lingering torture. It also fell over all the water sources, turning rivers into slushy, ash-filled mud.

If the eruption of Tambora blanketed the sun and turned the entire following year into a miserable winter, Mount Toba would have been exponentially worse, with some academics positing the volcanic winter could have lasted hundreds of years. If this were the case, there would have certainly been an era of darkness and cold, which would have wiped out many plant and animal species and driven many others, including humans, to the brink of extinction.

If there were a mass die-off of plants and animals, humans would have struggled to survive.

Of course, the theory of the catastrophic Toba eruption causing a human bottleneck is debated by academics. Widely accepted during the 1990s and 2000s, the theory has come under intense scrutiny as more discoveries are made pertaining to the event.

What is accepted as fact is that Mount Toba erupted with a power and force that no human being had ever witnessed before. What is debated is how it affects the human species.

The bottleneck theory posits that the volcanic winter lasted as long as ten years and would have caused a mass die-off of plants, leading to mass starvation as food sources dwindled. Evidence also suggests that other animals were affected or coincidentally recovered from drastically reduced populations around this time. The East African chimpanzee, the Bornean orangutan, the central Indian macaque, the tiger, and the cheetah all show signs of having recovered from a bottleneck.

A 2021 study concluded that, according to the assumed amount of sulfur dioxide released into the atmosphere, the temperature drop would have been −3.5 °C (−6.3 °F), and temperatures would have returned to normal within five years of the Toba eruption. This disrupts the theory that a mass die-off of plants around the tropical regions happened.

What is also significant is the survival of other human species. Neanderthals were around until around 35,000 years ago, and the region in which they lived was already frigid. Further temperature drops would have made life far more difficult. But perhaps what’s most intriguing is that current theories suggest Homo floresiensis survived for at least 20,000 years after the eruption of Mount Toba. They lived just 2,600 kilometers (1600 miles) away on the Island of Flores.

Controversy also surrounds human survival on the Asian subcontinent, where significant amounts of ash blanketed the ground. India was directly in the path of the ash cloud and was one of the first substantial landmasses to be affected by the fallout. Opponents of the Toba catastrophe theory point out that there is evidence of abundant habitation in the Jurreru Valley directly before and after the eruption. Proponents of the theory, however, point out that other sites in India, such as the Middle Son Valley to the north of India, show a major population decline. They argue that people survived in the Jurreru Valley and sought refuge there as it was fed by springs – an uncontaminated supply of fresh water.

Similarly, evidence of thriving human populations on the southern coast of Africa may prove that humans weren’t as severely affected as previously thought or that this area may have constituted an area of refuge that was less affected than others.

While some evidence points to the idea that the human species almost went extinct, especially evidence regarding genetics, deep core samples, and climatic models could suggest otherwise. With what we know now, it is difficult to state anything as fact.

Whatever the case, however, Mount Toba’s eruption is the single greatest volcanic explosion ever experienced by humanity, and studying its effects could help us deal with the potential of another event of similar proportions.

What we do know for sure is that there are supervolcanoes that could blow their top without any warning. Whenever that happens, whether tomorrow or a hundred thousand years from now, it will be an extremely dark and terrifying day in human history.