New research shows that the Toba volcano supereruption, despite its power, was probably not the only or main cause of the genetic bottleneck that characterizes the modern human population.
Jayde N. Hirniak, Ph.D. Candidate in Anthropology, Institute of Human Origins, Arizona State University.
About 74 years ago, one of the most violent ecological events in Earth’s history occurred: the Toba supereruption on the island of Sumatra in Indonesia. It was a gigantic eruption, more than 10,000 times larger than the eruption of Mount St. Helens in 1980. Some 2,800 cubic kilometers of volcanic ash were thrown into the stratosphere, a caldera (giant crater) tens of kilometers long was formed, and the sky probably darkened for years.
It seems like a recipe for a global extinction event: the sun is hidden behind ash clouds, temperatures plummet, acid rain poisons water sources, a thick layer of ash buries vegetation and animals – and human populations living near the volcano are almost completely wiped out.
And yet, Homo sapiens not only survived – but also adapted, innovated, and left behind technological and cultural traces that today provide a glimpse into our impressive survival ability.
In a new article, written by Jayde N. Hirniak, a doctoral student in anthropology at the InstituteInstitute of Human Origins , at Arizona State University, reviews the archaeological, genetic, and geological evidence that attempts to answer the question: How did humans survive the Great Eruption – and what does this say about our past and future?
"The Tova Catastrophe Hypothesis" – Genetic Bottleneck or Historical Exaggeration?
For many years, the “Toba Catastrophe Hypothesis” was prevalent among researchers. According to this hypothesis, the supereruption caused a “volcanic winter” that lasted up to 6 years and a long-term climatic disruption. As a result, it is claimed, the human population declined to a few thousand individuals – perhaps fewer than 10,000 people on the entire planet.
The hypothesis was supported, among other things, by genetic evidence: the genomes of humans alive today point to a “genetic bottleneck” – a past event in which human populations shrank greatly, and genetic diversity declined sharply. This fits the scenario of a global catastrophe, whether as a result of a pandemic, a drought, or – perhaps – a large-scale volcanic eruption.
But today, with the accumulation of data from ice cores, climate records, ancient pollen, and archaeological finds from across Africa and Asia, the picture is becoming more complicated: It is possible that the population decline was caused by other reasons, and that, even if it was a huge event, it was not the "erasure of the world" as previously thought.
Cryptotephra: Searching for tiny volcanic glass among the grains of soil
To understand what really happened 74 years ago, researchers rely on one direct source of information: the ash and rocks ejected in the eruption itself, generally known as tephra.
Each volcanic eruption leaves behind a unique “chemical signature” – for example, a different ratio of iron, magnesium, aluminum and other elements. If tiny volcanic glass is found in a geological layer, its composition can be compared to known samples and identified from which eruption it came.
The problem is that the particles that reach the greatest distances are microscopic – a tiny volcanic glass called cryptotephra.
Hirniak describes the fieldwork as follows:
- At archaeological sites where stone tools, remains of fires, or bones have been found – that is, evidence of human activity – she and others collect soil samples arranged along the cross-section of the layers.
- In the laboratory, microscopic glass grains, sometimes only 50–60 microns in size (about the diameter of a human hair), are separated from the soil using a micromanipulator device that lifts individual grains.
- The chemical composition of the glass was then measured to check whether it was tephra from a good eruption or from another eruption.
The process is reminiscent of “looking for a needle in a haystack” – and sometimes takes many months for a single site. But when a good cryptotaphra is finally found within an inhabited archaeological layer, one can ask: What happened to the people who lived there before, during and after the eruption?
Humans in South Africa and Ethiopia survived and even flourished
Given the size of a good eruption, one would expect that archaeological sites directly affected by volcanic ash would experience population collapse: site abandonment, cessation of activity, or technological decline.
In practice, what we see in many places is actually a pattern of resilience and adaptation.
- At the Pinnacle Point 5-6 site in South Africa, cryptotephra layers originating from Toba were identified within a long sequence of layers containing stone tools from different periods. Analysis of the layers showed that humans lived at the site before, during, and after the eruption. Furthermore, after the ash layer, there is an increase in human activity and even the emergence of technological innovations – evidence of flexibility and adaptability to a changing environment.
- A similar result was found at the Shinfa-Metema 1 site in the Ethiopian lowlands, where cryptotephra from Toba was identified in layers that also contain evidence of human activity. Life in this area during the eruption was likely based on the exploitation of seasonal rivers and shallow pools of water during the dry seasons – a strategy that was critical in conditions of extreme dryness. Around the same time, there is also early evidence of the use of the bow and arrow – a technology that allowed for efficient hunting at longer ranges.
Over the years, similar evidence has been identified at other sites in Indonesia, India, and China. These findings suggest that many people across Africa and Asia not only survived well, but managed to adapt and find new ways of life in the harsh conditions.
This means that the Tova super-outbreak, despite its power, was probably not the only or main cause of the genetic bottleneck that characterizes the modern human population.
What does the past teach us about future disasters?
Tova may not have solved the puzzle of the dramatic decline in human population size in antiquity, but it provides a natural laboratory for understanding how humans cope with catastrophic events.
On the one hand, 74 years ago there were no warning systems, satellite forecasts or monitoring networks. Populations were small, local and directly dependent on environmental resources. Still, humans managed to show behavioral flexibility: changing habitats, moving to new food sources, developing new tools and hunting technologies.
On the other hand, today our situation is very different:
- There are risk monitoring and management programs, such as the USGS Volcanic Hazards Program and the Global Volcanism Program, which monitor volcanic activity in real time using seismographs, gas measurements, satellites, and more.
- It is possible to openly monitor active volcanoes around the world and prepare for evacuation of populations and damage reduction.
Hirniak emphasizes that adaptation is a central part of human identity. By studying past volcanic eruptions – and in particular by combining geology, climate and archaeology – we can understand what conditions were critical to our survival at the time, and from this we can infer how we might cope with future crises – climatic, geological or otherwise.
The message from the distant past, when volcanic ash flooded the sky and the world seemed close to its end, is ambivalent but encouraging: Natural disasters can be catastrophic, but humans are able to survive – and even innovate – even in the most extreme conditions.
The article first appeared on the website The Conversation.
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