Prehistoric Effects of the YTT
As with most prehistoric civilizations, the history of prehistoric India is rather murky, due to it being, well, prior to recorded history. Still, historians and archaeologists have managed to piece together a picture, however fractured, of what India was like before recorded history through a variety of factors, a prominent one being the study of ruins and artefacts.
When studying human migrations and movement, anthropologists have a variety of methods at their disposal. One method for doing so is archaeogenetics, where ancient genetic material is studied using genetic analysis methods, usually in comparison to modern genetic material. Haplogroups, such as those of mitochondrial DNA, are genetic groups that are inherited entirely from one parent. Therefore, haplogroups of descendants are only separated from those of their ancestors by mutations, which are rather less drastic in their effects than groups formed through recombination and happen at a fairly consistent rate. Thus, you can get a good picture of human migrations by comparing the haplogroups of different populations to see how much they have diverged.
By studying human tissue and other evidence from sites such as Border Cave, Klasies River Mouth, and Omo, archaeologists have collectively determined that anatomically modern humans first originated in Africa during the Middle Paleolithic, 150–130 thousand years ago (ka). Using similar methods, they have found that humans first began to leave Africa about 120–90 ka. However, the precise dating of initial immigration into India has been long disputed, and only recently has definitive evidence been unearthed about it. Thus began the first age of Indian history–the Paleolithic period, beginning with the Middle paleolithic.
The use of haplogroup records becomes somewhat inaccurate when looking for the precise dates of early migrations, since predictions are more variable the farther back in history you go. Indian human tissue from before the Lower Paleolithic is very difficult to find and study, and therefore archaeologists must rely on products of human manufacturing and a great amount of inference. Using haplogroup records, anthropologists have concluded that the Indian subcontinent was first permanently populated by anatomically modern humans anywhere from 75,000 to 50,000 years ago, a significant range. However, recent evidence has narrowed down the time period greatly, so that we can precisely pinpoint the range of time in which the first human settlers arrived on the Indian subcontinent.
The youngest Toba supereruption, which happened in what is now Lake Toba in Indonesia, is speculated to have been one of the most devastating volcanic eruptions in history, drastically affecting the climate worldwide and blanketing a significant portion of Asia (including India) in ash. Until recently, it was thought that the eruption had a significant impact on the human population, causing numbers to dwindle into the few thousands thanks to a subsequent “volcanic winter’’, where the climate cooled drastically.
However, historians have long disputed the temporal relation of the settlement of India to the youngest Toba Catastrophe, a volcanic supereruption dated to approximately 74,000 years ago. Although there is very little evidence on early human excursions out of Africa, relatively unchanging stone tools at the Dhaba site of the Middle Son River Valley of central India were recently found at multiple data points spanning 79,600 and 65,200 years ago, meaning that this region was continuously populated throughout before and after the Toba supereruption. Still, while this evidence is inconclusive by itself, as macrolithic tools are not exclusive to anatomically modern humans, those of the Dhaba plain closely mimic those of the African and Australian Middle Paleolithic period. Therefore, these tools are most likely the product of humans dispersed from Africa, and these humans lived before and survived Toba.
The research into the relationship between the Youngest Toba Tuff (YTT) and human migration into the Indian subcontinent has wide-ranging implications. First, it gives a more precise date to the human migrations, as early as 79,000 years ago. Secondly, it shows that these settlers continuously populated the subcontinent in the aftermath of the eruption. This means that the first settlers gained a foothold and managed to survive on the subcontinent. Lastly, the evidence gives credence to the theory that the Toba eruption had less of an effect on humanity as previously thought. The fact that a significant human population stayed on the subcontinent in the aftermath of the eruption shows that the eruption did not kill off most of humanity, and that its effects may have been very limited.
In addition to this indirect evidence casting doubt on the “volcanic winter” theory, direct evidence has also been found. When compared to the Tambora eruption of 1816 in modern Indonesia, which triggered an immediate global cooling of 0.53 °C, and which had a Volcanic Explosivity Index (VEI) of 7–compared to Toba’s VEI of 8–scientists extrapolated that Toba must have caused at least as much cooling. Computer modeling has estimated this glaciation at anywhere from 4 °C to 17 °C. Evidence to support this theory has been found in the form of 6-year long records of sulfate (one of the factors of glaciation) spikes in both Greenland and Antarctica, dated to 74.2 ± 1.7 ka, although no accompanying volcanic material has been found. While sediments containing cryptotephra (volcanic glass shards), which would be created by an eruption paired with glaciation, have been found in the South China Sea, North India, and Bengal, tephra (rocks and sediment released from a volcanic eruption) in the Arabian Sea shows little evidence for global cooling. Still, if it occurred, such a period of glaciation would have drastic effects upon the population, including that of India.
Although there have long been many conflicting viewpoints upon whether global glaciation did occur, the lack of undisturbed rock records containing ash from the YTT has long obstructed conclusiveness. These rock cores, if well-preserved, would certainly contain ecological markers regarding fluctuations in climate. From 2005 to 2015, archaeologists examined two sedimentary cores precisely matching this description from Lake Malawi, in East Africa. However, the cores only recorded natural fluctuations in the climate at the time, meaning that the eruption likely had relatively little impact on the location’s climate at the time. Further, the cryptotephra found within these cores would only be sufficient for a maximum of a 1.5 °C glaciation, not nearly enough for a lasting effect on global climate longer than a decade. These observations are further supported by the fact that the two cores were extracted from locations 100 km apart; therefore, room for outliers is greatly diminished. In addition, recent evidence shows that the YTT eruption released far fewer aerosols than previously thought, lending credence to this theory. Models using a more accurate aerosol concentration of 3 times Pinatubo discovered a maximum glaciation of 0.8 ± 0.3 °C show that cooling was not significant. Even a model by Robock et al, with an SO2 concentration of 900 times Pinatubo, did not trigger a significant cooling period. Although most evidence points to a subdecadal, notable period of cooling, the projected 1,000-year glaciation did not occur.
The Weak Garden of Eden theory for the origin of humans postulates that humans originated singly in Africa, as a subpopulation of other hominids, slowly dispersed across the world in multiple migrations, and formed isolated communities by 100 ka. It also proposes that humans encountered separate population bottlenecks after their dispersal, limiting their genetic diversity. This theory arises from the present unnatural lack of genetic diversity in humans. Using an estimated mtDNA mutation rate to project current genetic diversity, anthropologists find that true human mtDNA diversity is lower than projected, although higher in Africa, at dispersals dated before 100 ka. Therefore, since consensus has been reached that ancient populations dispersed around 100 ka, humans must have encountered multiple population bottlenecks to limit their genetic diversity more than would be expected. These bottlenecks are projected to have occurred 100–50 ka, near the YTT. Solely due to the assumed contemporaneity of these two events, bottlenecks were attributed to the volcanic winter and global cooling caused by Toba. Africa’s genetic diversity is shakily explained by the refuge provided by tropical forests.
Directly and indirectly, this hypothesis also accounts for population declines of many other organisms. A drastic volcanic aerosol release would have lowered light levels, and along with global cooling, this would kill a majority of above-ground plants. A modeled increase in deadwood and decreased precipitation would also cause more wildfires. Generally, there would be a great ecological disaster, across populations, and this is called the Toba Catastrophe hypothesis.
However, the theory of a bottleneck and its conflation with glaciation from the YTT is rather shaky, and has recently experienced vast setbacks. First, the mounting certainty against the existence of a volcanic winter and glaciation following the YTT prevents it from being a limiting factor of human populations. In Lake Malawi, which is downwind and proximal to Toba, although phytoliths (infillings and casts of plant cells of opal and silica) were found to have increased concentrations following Toba, signifying plant death, Middle Paleolithic human sites have been found nearby both before and after Toba. These sites also have remains of edible plants on both sides of Toba. Also, as genomic studies become more accurate, the genetic bottleneck becomes more precisely dated at 50 ka, far after Toba. Although genetic bottlenecks have not been documented in many other species, this is largely due to the lack of specific genetic analysis of species other than humans. The bulk of recent evidence concludes that Toba did not importantly affect human evolution, overturning long-held views on its effects.
