The mighty volcano that may have altered the course of human evolution.
What is Toba?
Toba today is a beautiful lake on the island of Sumatra in Indonesia. It claims the title of having the largest island within an island, Samosir. It is popular tourist destination and attracts many who are seeking gorgeous vistas and relaxation.
However, it's highly unusual for lakes the size of Toba to form on islands. So your next question might be:
But What Was Toba?
Lake Toba, NASA Landsat7 image, courtesy of The Smithsonian Institution
Toba was a supervolcano that erupted approximately 73,500 years ago. It is considered by geologists to have been the largest volcanic eruption within the past 20 million years. The eruption lasted two weeks. Core samples evidence ash layer deposits of 3 meters in some parts of India and 10-12 centimeters in the Indian Ocean. The island of Samosir formed as a "resurgent dome" to cap off the last explosions of Toba's supereruption.1
What is a Supervolcano?
A supervolcano is like a regular volcano in that it is a geologic phenomenon in which the buildup of magma causes an eruption through the earth's crust of hot molten rock and volatile gases. However, a supervolcano's eruption is on a considerably larger scale.
All volcanoes are measured on what is called the Volcanic Explosivity Index (VEI). Eruptions are categorized on a range of 0 to 8, with a score of 8 being reserved for only the most cataclysmic of eruptions. Each gradation in the range represents an eruption that is ten times as powerful than the previous categorical gradation. The largest eruption in recorded human history, Tambora, received a VEI score of 7. Toba received a score of 8.
Volume comparison of volcanic expulsion.
(Image from de Boer, 2002.)
Supervolcanoes don't look like regular volcanoes. They do not form familiar conical shapes like Mount St. Helens or Mount Fuji. Instead, they form large crater-like depressions in the earth called calderas. Rather than breaking the surface via periodic eruptions like other volcanoes, magma from the earth's core concentrates just beneath the crust, building up an unbelievable amount of pressure for tens, even hundreds of thousands of years. Once this pressure becomes too much for the brittle layer of crust containing it, the magma breaks through, expelling a titanic volume of ash and gases way up into the upper atmosphere. After the super-eruption, the earth's crust collapses and forms the caldera depression.
Eruptions with the magnitude of a supervolcano occur infrequently, on average once every 50,000 years.2 This is pretty fortunate since the volume of volcanic materials expelled is many times greater than any volcano man has experienced in human history. Because of their scale, super-eruptions have the potential to severely alter the world's climate which has some devastating effects on world agriculture, vegetation, animal life and humans.
What were the effects on global climate?
The debris kicked up by a supervolcano is essentially enough to create an atmospheric veil that absorbs and reflects the sun's rays back into space. This would in turn serve to cool the earth as it blocks solar radiation from reaching the surface of the planet. Scientists term the climatic aftereffects of a supereruption a Volcanic Winter.
Toba occurred during a global transition from a relatively warm, humid climate to that of a colder one marked by increased ice and snow accumulation (Oxygen-Isotope Stage 5a-4 Boundary). During this period, sea levels and sea-surface temperatures were dropping considerably, but only gradually. Toba may have been just the impetus needed to accelerate the shift.
The supereruption of Toba released 2,800 cubic kilometers of magma and anywhere from between1,000 to 10,000 metric tons of volcanic ash , dust and sulfuric aerosols into the atmosphere.2 As stated previously, these volcanic materials serve to create a veil surrounding the earth, blocking the sun's radiation from reaching (and heating) the Earth's surface. Rampino asserts that aerosol loading of just 1,000 metric tons would have been enough to cause a global cooling of 3° to 5° Celsius, with regional drops of up to 15°C.2
(Diagram courtesy of NASA)
It follows then that the climatic aftermath of Toba effectively devastated the world's ecosystem from the bottom of the foodchain upwards. Since plant life relies on photosynthesis, any decrease in sunlight would have a damaging effect on the world's vegetation. Fauna adapted to relatively tropical, warm climates would essentially die off in a matter of weeks. Even hardier vegetation in the temperate regions would begin to suffer. Studies have shown that while hardy vegetation fares relatively well during sub-normal temperature and sunlight dips in the winter, during the warmer growing season, any sustained dip over the course of several years could prove fatal.3 Additionally, oceanic concentrations of algae and phytoplankton, which also survive by nature of photosynthesis, would suffer, consequently destroying much of the sea's fragile ecosystem. Given that scientists believed the atmospheric effects of Toba lasted for 6 years, once could imagine quite a bit of Earth's plant life suffered in the aftermath.
What are the main theories for the origin of modern humans?
There are two main theories: the first is the multiregional hypothesis, which says that modern humans evolved generally at the same time in several regions, and gene flow between groups accounts for them not evolving into different species. The second, the Out of Africa or replacement hypothesis, says that modern humans evolved in Africa and then moved out to other parts of the world, replacing hominids that were already there. DNA studies support the Out of Africa hypothesis, and it is now more widely accepted.4
There is a hypothesis, the “weak Garden of Eden” model, which is a version of the Out of Africa theory. It suggests that humans dispersed from Africa about 100,000 years ago, suffered from a population bottleneck, and then recovered. The timing of Toba roughly matches the timing of the population bottleneck.4 Genetic evidence backs this up: since Africa, situated primarily on the equator, had the largest areas where humans could survive the volcanic winter, it retained a larger human population. A larger bottleneck in Africa explains why African populations have a higher genetic diversity; populations passing through smaller bottlenecks in other parts of the world would have less genetic diversity, and this is indeed seen in the DNA evidence.5
What were the particular effects of Toba on human evolution?
At the time Toba erupted, 74000 years ago, humans had already spread out of Africa into other regions such as southern Asia and the Middle East. They had not yet reached the Americas, Europe, or Australia, although it has been argued that about 70,000 years ago, as a result of the ice age caused by Toba, sea level dropped enough that humans could island-hop their way over to Australia.6 Toba erupted at a bad time for human populations: the ice expanses in northern Eurasia were expanding, making humans retreat southward, and migrating groups were spreading out of Africa.1
The Toba-induced volcanic winter hit these small bands hard, reducing their numbers drastically; in what is known as a population bottleneck, the worldwide human population shrank to as small as 10,000 people or even less (estimates vary).5 The temperature drop and climatic effects around the world hit populations in higher latitudes the hardest; humans most likely to survive were those who lived in tropical regions, where the temperature would be warmer. The largest of these “tropical refugia” was in Africa, near the equator; this is backed up by the fact that the most genetic diversity in humans is found in Africa.5 The genetic evidence also tells us that there was an extinction event in India around the time of the Toba eruption. This can be explained if we remember that the ash cloud produced by Toba would have hit the area around the Indian Ocean the hardest; humans in that area were wiped out, and then the region was recolonized from the east and west.6
What happened when human populations recovered from the bottlenecks?
Stanley Ambrose explains it best:
“[S]maller, isolated African populations that did survive Toba’s volcanic winter in refugia within and/or outside Africa may have experienced the equivalent of the Founder Effect. When populations are reduced to very small sizes, most genetic diversity is lost. A small, random subset of pre-existing genetic diversity is retained in each isolated population. If founder populations remain small and isolated for many generations, then genetic drift leads to the random loss of additional alleles and fixation of others, reducing genetic diversity and increasing between-population difference even further. Additional differentiation can occur through adaptations to local environments.”5
So these bottlenecks could account for several things: how little diversity there is in the human genetic code as a whole; why there is more genetic diversity in African populations; and how those superficial physical characteristics which differ between geographic populations, which we call “race,” arose.5, 4
(Human evolution phylogenetic tree, from the Human Origins Program at the Smithsonian Institution)
The human evolution phylogenetic tree can be put together in several different ways, and there is no one tree that is universally accepted. This is one version. The Toba eruption, 74,000 years ago, falls in the far right-hand side of this picture.
de Boer, Jelle Zeilinga, and Donald Theodore Sanders. Volcanoes in Human History: The Far-Reaching Effects of Major Eruptions. Princeton and Oxford: Princeton University Press, 2002.
Rampino, Michael R.. "Supereruptions as a Threat to Civilizations on Earth-like Planets." Icarus 156(2002): 562-569.
Engvild, Kjeld C.. "A review of the risks of sudden global cooling and its effects on agriculture." Agricultural and Forest Meteorology 115(2003): 127-137.up 4
Rampino, M.R., and Ambrose, S. (2000) “Volcanic winter in the Garden of Eden: The Toba super-eruption and Late Pleistocene human population crash,” in Volcanic Disasters in Human History, Geological Society of America Special Paper 345: 71-82.
Wesleyan University Professor Emeritus Jelle Zeilinga de Boer discusses Toba and its effects on climate and humanity in a series of mp3s.
Holly Wood and Annalisa Bolin are not experts. We are undergraduate students taking an introductory level course in Environmental Science. This site is a course project and while we hope you find the site informative and entertaining, we highly recommend you look towards the actual experts we cite rather than just this site.
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