Late Pleostocene Human Population Bottlenecks. . . (Toba)

Professor Stanley H. Ambrose Department of Anthropology, University Of Illinois, Urbana, USA

The last glacial period was preceded by 1000 years of the coldest temperatures of the Late Pleistocene, apparently caused by the eruption of the Mount Toba volcano. The six year long volcanic winter and 1000-year-long instant Ice Age that followed Mount Toba's eruption may have decimated Modern Man's entire population. Genetic evidence suggests that Human population size fell to about 10,000 adults between 50 and 100 thousand years ago. The survivors from this global catastrophy would have found refuge in isolated tropical pockets, mainly in Equatorial Africa. Populations living in Europe and northern China would have been completely eliminated by the reduction of the summer temperatures by as much as 12 degrees centigrade.

Volcanic winter and instant Ice Age may help resolve the central but unstated paradox of the recent African origin of Humankind: if we are all so recently "Out of Africa", why do we not all look more African?

Because the volcanic winter and instant Ice Age would have reduced populations levels low enough for founder effects, genetic drift and local adaptations to produce rapid changes in the surviving populations, causing the peoples of the world to look so different today. In other words, Toba may have caused Modern Races to differentiate abruptly only 70,000 years ago, rather than gradually over one million years.

The Mount Toba eruption is dated to approximately 71,000 years ago. Volcanic ash from Mount Toba can be traced north-west across India, where a widespread terrestrial marker bed exists of primary and reworked airfall ash, in beds that are commonly 1 to 3, and occasionally 6 meters [18 feet] thick.

Tambora, the largest known historic eruption, displaced 20 cubic kilometres of ash. Mount Toba produced 800 cubic kilometres.* It was therefore forty times larger than the largest eruption of the last two centuries and apparently the second largest known explosive eruption over the last 450 million years.

Mount Toba's eruption is marked by a 6 year period during which the largest amount of volcanic sulphur was deposited in the past 110,000 years. This dramatic event was followed by 1000 years of the lowest ice core oxygen isotope ratios of the last glacial period. In other words, for 1000 years immediately following the eruption, the earth witnessed temperatures colder than during the Last Glacial Maximum at 18-21,000 years ago.

For the volcanic aerosols to be effectively distributed around the earth, the plume from the volcanic eruptions must reach the stratosphere, a height greater than 17 kilometres. Mount Toba's plume probably reached twice this height. Most solar energy falls at low latitudes between the Tropics of Cancer and Capricorn, so eruptions that happen near the Equator cause much more substantial cooling due to the reflection of solar energy. Toba lies 2 degrees north of the Equator, on the Island Sumatra.

The reduction in atmospheric visibility due to volcanic ash and dust particles is relatively short-lived, about three to six months. Longer-term global climatic cooling is caused by the highly reflective sulphuric acid haze, which stays suspended in the upper atmosphere for several years.

Ice core evidence implicates Mount Toba as the cause of coldest millennium of the late Pleistocene. It shows that this eruption injected more sulphur that remained in the atmosphere fo a longer time [six years] than any other volcanic eruption in the last 110,000 years. This may have caused nearly complete deforestation of southeast Asia, and at the same time to have lowered sea surface temperatures by 3 to 3.5 degrees centigrade for several years.

If Tambora caused the " The year without a summer" in 1816, Mount Toba could have been responsible for six years of relentless volcanic winter, thus causing a massive deforestation, a disastrous famine for all living creatures, and a near extinction of Humankind.

The Volcanic Winter/Weak Garden of Eden model proposed in this paper. Population subdivision due to dispersal within African and other continents during the early Late Pleistocene is followed by bottlenecks caused by volcanic winter, resulting from the eruption of Toba, 71 ka. The bottleneck may have lasted either 1000 years, during the hyper-cold stadial period between Dansgaard-Oeschlger events 19 and 20, or 10ka, during oxygen isotope stage 4. Population bottlenecks and releases are both sychronous. More individuals survived in Africa because tropical refugia were largest there, resulting in greater genetic diversity in Africa.

Small and portable, this red ochre stone is engraved with what must be "tally" marks. It is one of two such stones recently found in the Blombos Cave in South Africa and have been dated as being 77,000 years old, making them the oldest form of recorded counting ever found.

The stone is worn which probably indicated that it was constantly handled over a period of time, how long is impossible to tell. It looks as though the stone has been reused at least once before as the lighter marks appear to have been erased rather than worn away naturally.

If the dating is accurate this stone was used 5000 years before the Mount Toba eruption of 71,000 years ago. The evidence from the Toba eruption indicates that the world's population of Modern Man was reduced to a total of around 10,000 adults.

Toba Genetic Evidence

by Stephen Oppenheimer

The Toba explosion 74,000 years ago and the genetic evidence

Perhaps more important than the precision of the dating, the connection between stone tools and Toba volcanic ash in Malaysia puts the first Indians and Pakistanis in the direct path of the greatest natural calamity to befall any humans, ever. The Toba explosion was that disaster, the biggest bang in 2 million years. Carried by the wind, the plume of ash from the volcano fanned out to the north-west and covered the whole of the Indian subcontinent. Even today, a metres-thick ash layer is found throughout the region, and is associated in two Indian locations with Middle and Upper Palaeolithic tools. An important prediction of this conjunction of tools and ash is that a deep and wide genetically sterile furrow would have split East from West; India would eventually recover by re-colonisation from either side. Such a furrow does exist in the genetic map of Asia.

In spite of the proximity of Toba to Perak, the Toba ash plume only grazed the Malay Peninsula. The human occupants of the Kota Tampan site were the unlucky ones – others on the peninsula escaped. Some argue, on the basis of comparing skull morphologies, that the Semang aboriginal ‘Negrito’ hunter-gatherers, who still live in the same part of the dense northern Malaysian rainforest, are descendants of people like Perak Man. The continuity of the Kota Tampan culture as argued by Zuraina Majid provides a link back to the 74,000-year-old tools in the Toba ash.

The Semang are perhaps the best known of the candidate remnants of the old beachcombers. Another relict group possibly left over from the beachcombers in Indo-China and the Malay Peninsula are the so-called Aboriginal Malays, who are physically intermediate between the Semang and Mongoloid populations.

For a film documentary, The Real Eve (Out of Eden in the UK), with with which Stephen Oppenheimer 's book is associated, Discovery Channel helped to fund a genetic survey of the aboriginal groups of the Malay Peninsula which I conducted in collaboration with English geneticist Martin Richards and some Malaysian scientists. This survey was part of a much larger on-going study of East Asian genetics.

The mtDNA results were very exciting: three-quarters of the Semang group (i.e. the ‘Negrito’ types) have their own unique genetic M and N lines with very little admixture from elsewhere, which is consistent with the view that their ancestors may have arrived with the first beachcombers. Their two unique lines trace straight back to the M and N roots (the first two daughters of L3 outside Africa). Their M line is not shared with anyone else in Southeast Asia or East Asia (or anywhere else) and, although it has suffered loss of diversity through recent population decline, it retains sufficient diversity to indicate an approximate age of 60,000 years. Their other unique group on the N side comes from R, N’s genetic daughter. This lack of any specific connection with any other Eurasian population is consistent with the idea that after arriving here so long ago, they have remained genetically isolated in the jungles of the Malay Peninsula.

The colonisation of Australia over 60,000 years ago was part of the same Exodus

Some are still convinced that Australian aboriginals represent an earlier migration out of Africa than that which gave rise to Europeans, Asians, and Native Americans. Yet again our genetic trail tells us otherwise. Several studies of Australian maternal clans have shown that they all belong to our two unique non-African superclans, M and N, and large studies of Y chromosomes show that male Australian lines all belong to the same Out-of-Africa Adam clan as other non-Africans (M168). The same pattern is seen with genetic markers not exclusively transmitted through one parent. In other words, the combined genetic evidence strongly suggests Australians are also descendants of that same single out-of-Africa migration. The logic of this approach, combined with the archaeological dates, places the modern human arrival in the Malay Peninsula before 74,000 years ago and Australia around 65,000 years ago. It is also consistent with the date of exit from Africa predicted on beachcombing grounds.

My date estimates for the trek around the Indian Ocean en route from Africa suggest that the beachcombers could have taken as little as 10,000 years to eat their way down the coastline to Perak and roughly another 10,000 years to reach Australia. Such a time requirement is fulfilled by the difference between leaving Africa around 85,000 years ago and arriving in Australia 65,000 years ago. The former date is consistent with dates estimated for the African L3 cluster expansion using the molecular clock.

A genetic furrow in India resulting from the Toba explosion?

There is an abrupt genetic change to the north and east of India. These changes can be inferred even from physical appearance. In Nepal, Burma, and eastern India we come across the first Mongoloid East Asian faces. These populations generally speak East Asian languages, contrasting strongly with their neighbours who mostly speak Indo-Aryan or Dravidian languages. By the time we get to the east of Burma and to Tibet on the northern side of the Himalayas, the transition to East Asian appearance and ethnolinguistic traditions is complete, as is the rapid and complete change of the mitochondrial sub-clans of M and N. In Tibet, for instance, the ratio of M to N clans has changed from 1:5 to 3:1, and there is no convincing overlap of their sub-clans with India. Instead, Tibet shows 70 per cent of typical East and Southeast Asian M and N sub-clans, with the remainder consisting of as-yet unclassified M types of local origin. The north-eastern part of the Indian subcontinent therefore shows the clearest and deepest east–west boundary. This boundary possibly reflects the deep genetic furrow scored through India by the ash-cloud of the Toba volcano 74,000 years ago.

To the south of the Indian peninsula, the main physical type generally changes towards darker-skinned, curly haired, round-eyed so-called Dravidian peoples. Comparisons of skull shape link the large Tamil population of South India with the Senoi, a Malay Peninsular aboriginal group intermediate between the Semang and Aboriginal Malays (see above).

M born in India, N possibly a little farther west in the Gulf M, who is nearly completely absent from West Eurasia, gives us many reasons to suspect that her birthplace is in India. M achieves her greatest diversity and antiquity in India. Nowhere else does she show such variety and such a high proportion of root and unique primary branch types. The eldest of her many daughters in India, M2, even dates to 73,000 years ago. Although the date for the M2 expansion is not precise, it might reflect a local recovery of the population after the extinction that followed the eruption of Toba 74,000 years ago. M2 is strongly represented in the Chenchu hunter-gatherer Australoid tribal populations of Andhra Pradesh, who have their own unique local M2 variants as well as having common ancestors with M2 types found in the rest of India. Overall, these are strong reasons for placing M’s birth in India rather than further west or even in Africa.

What is perhaps most interesting about the unique Indian flowerings of the M and R clans is a hint that they represent a local recovery from the Toba disaster which occurred 74,000 years ago, after the out-of-Africa trail began. A devastated India could have been re-colonised from the west by R types and from the east more by M types. Possible support for this picture comes from the recent study by Kivisild and colleagues of two tribal populations in the south-eastern state of Andhra Pradesh. One of these populations, the Australoid Chenchu hunter-gatherers, are almost entirely of the M clan and hold most of the major M branches characteristic of and unique to India. The other group, the non-Australoid Koyas, have a similarly rich assortment of Indian type M branches (60 per cent of all lines), but have 31 per cent uniquely Indian R types. The Chenchu and Koya tribal groups thus hold an ancient library of Indian M and R genetic lines which are ancestral to, and include, much of the maternal genetic diversity that is present in the rest of the Indian subcontinent. Neither of these two groups holds any West Eurasian N types. The presence of R types in the Koyas but not in the Australoid Chenchus might fit with some component of a recolonization from the Western side of the Indian subcontinent. As evidence of their ancient and independent development, and in spite of their clearly Indian genetic roots and locality, there were no shared maternal genetic types (i.e. no exact matches) between the two tribal groups.

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