|by Stephen Oppenheimer
Toba Lake in northern Sumatra is the world's largest active volcanic caldera. The volcanic eruption that resulted in Lake Toba (100 x 30 km) 74,000 years ago, is known to have been by far the biggest eruption of the last 2 million years. This mega-bang caused a prolonged world-wide nuclear winter and released ash in a huge plume that spread to the north-west and covered India, Pakistan, and the Gulf region in a blanket 15 metres (315 feet) deep. Toba ash is also found in the Greenland ice-record and submarine cores in the Indian Ocean, allowing a precise date marker. In our story the Toba eruption is the most accurately dated, dramatic, and unambiguous event before the last ice age.
Toba is also regarded by some as having caused worldwide population extinctions as a result of the ‘nuclear winter’ that followed. I have taken this into account in my reconstruction. India bore the brunt of the massive ash fall, and may have suffered mass extinction, since the Toba plume spread north-west across the Indian Ocean from Sumatra. This event may explain why most Indian maternal genetic sub-groups of the two founder lines M & N are not shared elsewhere in Asia and the dates of their re-expansions are paradoxically younger in India than elsewhere in East Asia and Australasia.
Arriving in Asia before Toba
If our ancestors left Africa 85,000 years ago, their descendants would have lived in Asia over 10,000 years before the Toba explosion, and beachcombers around the Indian Ocean would have been in direct line for the greatest volcanic ash fall in the whole of human existence. The Toba eruption is thus a valuable date mark, since the ash covered such a wide area, is accurately dated, and can be identified wherever an undisturbed layer of it is found.
The early archaeological dates for human presence in Australia have been reinforced by an extraordinary reappraisal of the Kota Tampan Palaeolithic culture found in Lenggong Valley, in Perak on the Malay Peninsula, two-thirds of the way from Africa to Australia. This culture first identified by the find of large, curious, and rather crude pebble tools, fashioned on one side only, was thought by archaeologists in the 1960s to be the work of an earlier human species. However, when the geological layers surrounding the tools were reassessed, it became clear they were more recent. Wider interest was sparked in 1975, when Tom Harrison, the colourful curator of the Sarawak Museum, argued that the tools related to the great eruption of the Sumatran volcano Toba.
Malaysian archaeologist Zuraina Majid has explored the remains of this human culture in a wooded valley in Perak State, near Penang. A continuous Palaeolithic tradition known as the Kota Tampan culture goes back tens of thousands of years there. At one site, tools from this tradition lie embedded in volcanic ash from Toba. If the association of the tools with modern humans is confirmed, this means that modern humans got to Southeast Asia before the Toba eruption more than 74,000 years ago. This, in turn, makes the 85,000-year-old exodus more likely. Genetic and other evidence for a human occupation of Australia by 65,000 years ago fits this scenario.
The Toba event specifically blanketed the Indian subcontinent in a deep layer of ash. It is difficult to see how India’s first colonists could have survived this greatest of all disasters. So, we could predict a broad human extinction zone between East and West Asia. Such a deep east-west division, or ‘furrow’, is still seen clearly in the genetic record.
The Genetic Trail
How does such an early date for the exodus fit with the genetic data? This is perhaps the most controversial and exciting part of the story. The short answer is that the genetic dates and tree fit the early exodus well. This also resolves the question about the origins of the Europeans: why it was that Europe was colonized only after 50,000 years ago, yet arose from the same maternal ancestor as the Australians and Asians.
The South Asian region, the first homeland of that single, successful southern exodus, shows the presence of the genetic roots of that expansion not only in the so-called aboriginal peoples around the Indian Ocean, but among the bulk of the modern populations. Among these roots we can detect genetic base camps for the most westerly of the subsequent pioneer treks inland to the vast Eurasian continent. These treks set off, after a pause, for Europe, the Caucasus, and Central Asia. It seems that the vanguard of the beachcombing trail retained a surprising proportion of the original genetic diversity left in the out-of-Africa group and moved rather faster round the shores of the Indian Ocean. So fast, in fact, that they travelled right round to Indonesia and on into Near Oceania, arriving in Australia long before their first cousins made it to Europe.
The exact chronological relationship of the exodus and the subsequent arrival in Southeast Asia to the massive Toba volcanic explosion of 74,000 years ago is critical. First, Toba is one of the most accurately and precisely dated events of the Palaeolithic, and its ashfall acts as a time datum for the whole of southern Asia. Second, the effects of Toba’s direct ashfall followed by the inevitable ‘nuclear winter’ would have been disastrous for any life in its path, and pretty bad farther afield. The presence of tools thought to be made by modern humans found with Toba ash in the Malay Peninsula suggests that the beachcombing vanguard had arrived in the Far East before the eruption. Triangulation of this anchor date with other pieces of evidence supports this scenario.
Other clues include:
New dates for the Liujiang skull in South China, luminescence dates from Australia, the date of the lowest sea level enabling passage to Australia at 65,000 years ago, genetic dates of the expansion of the L3 group at 83,000 years ago, and the onset of significant salinization of the Red Sea dated to 80,000 years ago. The best evidence for early modern humans in Asia should come from real fossils and their dated context. Such work is in process at the site of Liang Bua in Flores.
Now, if Toba really did blow its top after India was first colonized, we would expect a mass extinction event on the Indian Peninsula which affected the eastern side more than the west. This is certainly one interpretation of the paradox of the Indian genetic picture, in which the genetic trail of the beachcombers can be detected, but the bulk of Indian subgroups of M and R are unique to the subcontinent, especially among the tribes of the south-east. This is what we would expect for a recovery from a great disaster. The oldest of these local lines have been dated to around 73,000 years ago.
In the next sections we shall see what those pioneers did on the North and East Asian mainland after they arrived, and how they got to those places. Explicit in all these predictions is the central role of South Asiam (particularly India, Pakistan, and the Gulf) as the fount of all non-African dispersals. As far as the gene tree is concerned, the earliest branches of non-African gene lines are in South and Southeast Asia. The dates of first colonization of East and Central Asia by modern humans are problematic, because of uncertain skull dating and the evidence for a more recent Mongoloid replacement, but if the redating of the Liujiang skull at no less than 68,000 years is correct, South China could have been colonized at the same time as Southeast Asia. If modern humans had reached Southeast Asia before the great Toba volcanic explosion, the sharp genetic break between India and the Far East may be explained by the ash cloud that covered India around 74,000 years ago.