The Toba catastrophe theory holds that 70,000 to 75,000 years ago, a supervolcanic event at Lake Toba, on Sumatra (Indonesia), possibly the largest explosive volcanic eruption within the last twenty-five million years plunged the Earth, which was already in an ice-age, into an even colder spell. This resulted in the world's human population being reduced to 10,000 or even a mere 1,000 breeding pairs, creating a bottleneck in human evolution. The theory was proposed in 1998 by Stanley H. Ambrose of the University of Illinois at Urbana-Champaign.
Within the last five to three million years, after human and other ape lineages diverged from the hominid stem-line, the human line produced a variety of species, including Homo habilis, H. ergaster, H. erectus, H. neanderthalensis, H. sapiens, and possibly H. floresiensis.
According to the Toba catastrophe theory, the consequences of a massive volcanic eruption drove the world's human population to the brink of extinction between 70,000–75,000 years ago when the Toba caldera in Indonesia underwent an eruption of category 8 (or "mega-colossal") on the Volcanic Explosivity Index. This released energy equivalent to about 1 gigaton of TNT (4.2 EJ).
To give an idea of its magnitude, consider that although the eruption took place in Indonesia, it deposited an ash layer approximately 15 centimetres thick over the entire Indian subcontinent; at one site in central India, the Toba ash layer today is up to 6 metres thick and parts of Malaysia were covered with 9 m of ashfall. In addition it has been calculated that one hundred million metric tons of sulphuric acid was ejected into the atmosphere by the event, causing acid rain fallout.
For further comparison, the largest volcanic eruption in historic times, in 1815 at Mount Tambora in Indonesia, ejected the equivalent of around 160 km3 of dense rock and made 1816 the "Year Without a Summer" in the whole northern hemisphere.
In his book "The winds of change: climate, weather, and the destruction of civilizations" (2006), Eugene Linden (2006) writes that "... the "volcanic winter" brought on by the eruption of Mount Toba roughly 71,000 years ago (...) was the biggest eruption in the past 2 million years. Estimates are that the ash and gases thrown into the atmosphere lowered global temperatures between 3 and 5 degrees centigrade for the next six years ..."
According to Alan Robock et al., the Toba incident did not initiate an ice age, but rather exacerbated an ice age that had already been underway. The simulations demonstrated a maximum global cooling down of around 15 °C, approximately 3 years after the eruption . As the saturated adiabatic lapse rate is 4.9 °C/ 1,000 m for temperatures above freezing, this means that the tree line and the snow line were around 3,000 m (9,000 ft) lower at this time. Nevertheless, the climate recovered over a few decades.
Ambrose proposes that this massive environmental change created population bottlenecks in the species that existed at the time; this in turn accelerated differentiation of the isolated human populations, eventually leading to the extinction of all the other human species except for the two branches that became Neanderthals (Homo neanderthalensis) and modern humans (Homo sapiens). More recently several geneticists, including Lynn Jorde and Henry Harpending have proposed that the human race was reduced to approximately five to ten thousand people.
Some geological evidence and computed models support the plausibility of the Toba catastrophe theory. Ashes from this eruption of Lake Toba, located near the equator, should have spread all over the world. While the Greenland ice core data displays an abrupt change around this time, changes in the corresponding Antarctic data are not easily discernible.
Genetic evidence suggests that all humans alive today, despite apparent variety, are descended from a very small population, perhaps between 1,000 to 10,000 breeding pairs about 70,000 years ago.
Gene analysis of some genes shows divergence anywhere from 60,000 to 2 million years ago, but this does not contradict the Toba theory, once again because Toba is not conjectured to be an extreme bottleneck event. The complete picture of gene lineages (including present-day levels of human genetic variation) allows the theory of a Toba-induced human population bottleneck.
Recent work by archaeologist Michael Petraglia suggests that in fact modern humans survived relatively unscathed in at least one settlement in India.
Alan Rogers, a co-author of this study and professor of anthropology at the University of Utah, says: “The record of our past is written in our parasites.” Rogers and others have proposed the bottleneck may have occurred because of a mass die-off of early humans due to a globally catastrophic volcanic eruption. The analysis of louse genes confirmed that the population of Homo sapiens mushroomed after a small band of early humans left Africa sometime between 150,000 and 50,000 years ago.
Recent research states that genetic diversity in the pathogenic bacterium Helicobacter pylori decreases with geographic distance from East Africa, the birthplace of modern humans. Using the genetic diversity data, the researchers have created simulations that indicate the bacteria seems to have spread from East Africa around 58,000 years ago. Their results indicate modern humans were already infected by H. pylori before their migrations out of Africa, and H. pylori remained associated with human hosts since that time.
It is currently not known where human populations were living at the time of the eruption. The most plausible scenario is that all the survivors were populations living in Africa, whose descendants would go on to populate the world. However, recent archeological finds, mentioned above, have suggested that a human population may have survived in India. Dating of early human migrations has given a conclusive answer to this question: the archeological finds are legacies of Homo erectus.
Recent analyses of mitochondrial DNA have set the estimate for the major migration from Africa from 60,000 to 70,000 years ago, whereas the Toba eruption has been dated to 75,500–67,500 years ago. During the subsequent tens of thousands of years, the descendants of these migrants populated Australia, East Asia, Europe and finally the Americas.