© Sipa Press/Rex FeaturesThe landscape after Pinatubo's eruption may give a glimpse of what early humans experienced
Toba is a supervolcano on the Indonesian island of Sumatra. It has blown its top many times but this eruption, 74,000 years ago, was exceptional. Releasing 2500 cubic kilometres of magma - nearly twice the volume of mount Everest - the eruption was more than 5000 times as large as the 1980 eruption of mount St Helens in the US, making it the largest eruption on Earth in the last 2 million years.
The disaster is particularly significant since it occurred at a crucial period in human prehistory - when Neanderthals and other hominins roamed much of Asia and Europe, and around the time our direct ancestors, Homo sapiens, were first leaving Africa to ultimately conquer the world. Yet with no recent eruptions for easy comparison, the full extent of its fallout and impact on early humans has been shrouded in mystery.
Now dramatic finds from archaeological digs in India, presented in February at a conference at the University of Oxford, are finally clarifying the picture of the eruption and its effects, and how it shaped human evolution and migration. Further results from the digs may even rewrite the timing and route that modern humans took out of Africa.
The new work portrays a somewhat different view of the eruption from the most popular current theory. Previous computer models of the eruption had suggested the event was truly cataclysmic - very nearly a doomsday for early humankind. With calculations based on the assumption that Toba belched out 100 times more aerosols than the 1991 eruption of mount Pinatubo in the Philippines, and scaling the environmental effects accordingly, the models suggested global temperatures dropped by about 10 °C following the blast. This supports the idea of a decade-long "volcanic winter" and widespread catastrophe (Journal of Geophysical Research - Atmospheres, vol 114, p D10107).
To make matters worse, the aerosols would have blocked out life-giving sunlight and absorbed water vapour in the atmosphere, causing a dryer global climate for the next few years. This would have resulted in a rapid decline in tree cover and a concomitant expansion of grasslands, leading to the extinction of many mammals and nearly wiping our ancestors out (Palaeogeography, Palaeoclimatology, Palaeoecology, vol 284, p 295). The few primitive humans that did survive the eruption would have had to act fast, quickly adapting their way of life to suit the new conditions, travelling further to find food and cooperating with neighbouring populations in the battle for survival.
Indeed, the event may have drastically altered the path of evolution for our own species, Homo sapiens. Modern humans, who were still thought to be living in Africa, would have been whittled down to just a few thousand breeding pairs scattered in dispersed refugia - creating a so-called "genetic bottleneck" in evolution. As the separate colonies developed independently of one another, they would have sown the seeds for the genetic differences between races once these separate groups eventually left Africa.
Yet this theory has drawn some criticism since it was first put forward 17 years ago, with scholars such as Hans Graf, an atmospheric scientist at the University of Cambridge, believing that the climate change following the explosion has been wildly overestimated.
For Graf, the crux of the argument concerns the precise cooling effect of the sulphur dioxide released by the explosion. During smaller eruptions, like that of Mount Pinatubo, most of the released sulphur dioxide reacts with hydroxide (OH) ions from water molecules in the atmosphere to form particles of sulphate - a highly reflective substance that bounces sunlight back into space before it can warm the Earth.
Previous estimates had placed Toba's sulphur dioxide production at 100 times that of Mount Pinatubo's output. Graf thinks this figure is misguided: recent chemical analyses of Toba's fossilised magma suggests it should be roughly half that. "We think Toba was more of a giant in ash production, not sulphur," says Graf.
© New Scientist
Taking all of this into account, Graf and his colleagues suggest a new estimate of global cooling of just 2.5 °C, which lasted for just a few years. According to this model, the effects were also highly regional. In places like India the average temperatures may only have fallen by about 1 °C - not such a dramatic climate shift.
This new view is highly contentious. Alan Robock from Rutgers University in New Brunswick, New Jersey, who came up with the original simulations, stands by his original predictions. "Our model showed that extra water would be lofted into the stratosphere because of warming at the top of the troposphere [the lowest layer in the atmosphere], so water would not be a limiting factor," he says. "We simulate a decade or two of very cold, dry, dark conditions, which would have been difficult for humans to adapt to."
Yet recent archaeological and geological work in India seems to support Graf's claims, suggesting the environmental impact of the super-eruption was much less than previously imagined. Firstly, had there been a sudden deforestation event caused by the cooling and drying of the atmosphere, topsoil no longer anchored by trees would be expected to wash down into valleys, where it would quickly accumulate. "We don't find a rapid influx of soil arriving on top of the ash layers," says Peter Ditchfield of the University of Oxford.
To build further evidence, Ditchfield analysed the ratio of different carbon isotopes - which are each absorbed at different rates by different plants - in ancient plant remains in the Jwalapuram region of southern India and the Middle Son river valley in central northern India, both of which are around 2000 kilometres from Toba. He saw only a slight increase in the carbon-13 isotope after the Toba eruption, which suggests there was just a small increase in grassland environments at this time. "Woodlands weren't obliterated by Toba. We see a diverse range of habitats persisting after the eruption, which would have provided a diverse range of game and hunting opportunities," he says.
Nevertheless, hominin species living at the time of the eruption would undoubtedly have faced tough conditions. The blanket of ash, for example, would have been quickly washed into the freshwater supplies: Ditchfield found deposits up to 3 metres deep on the valley floors where rivers would once have flowed. And there is no doubt that in the years immediately following the eruption the early humans would have had to adjust to colder temperatures, probably having to economise significantly as food resources dwindled.
Flight to the refugia
"We are not saying that it wasn't difficult for humans after Toba," says Mike Petraglia at the University of Oxford, who has led the investigations. "We are just saying that we don't think it was a catastrophic change."
The remains of hominin species living at the time of the eruption would shed some light on life during this difficult period, but sadly no skeletons have been preserved in the sediments. "The moist tropical environment is not good for preserving bone," explains Petraglia.
Nevertheless, the tools that they left behind provide a window into their owners' lives. Petraglia and his team have investigated a number of sites at Jwalapuram. One has been particularly fruitful. Labelled Jwalapuram 22, it was probably a hunter-gatherer camp. It has yielded more than 1800 tools, including stone flakes, scrapers, points - the everyday tools for cutting and scraping - and the stone "cores" left over following tool manufacture. "The surface is just littered with stone tools, and then buried by the Toba ash," says Michael Haslam, also at the University of Oxford.
© New Scientist
Again, that's not to say the eruption was an easy ride for the hominins living in India. Jwalapuram and the Middle Son valley may have been special cases - refugia in which hominin populations sheltered when the times got tough. Jwalapuram, for example, is an ancient type of Indian geological formation known as a Purana basin, which contains highly fertile soil and abundant freshwater springs protected from contamination by the surface ash. Such circumstances could have buffered some of the effects of the eruption. "There are springs popping out everywhere in these basins, they contain plentiful rocks for making stone tools and their vegetation is generally resilient to environmental changes," explains Ravi Korisettar of Karnatack University in Dharwad, India. Still, the findings present a challenge to the traditional view of Toba as a devastating catastrophe for hominins alive at the time.
Stanley Ambrose at the University of Illinois at Urbana-Champaign is a leading proponent of the catastrophe theory. He says he has observed strong evidence for technological change in south and east Africa following the eruption, which may have resulted from a need to adapt to pressured conditions, and reckons India should be no different. He says that flood erosion may have released some of the tools from the older sediment at the Indian sites, re-depositing them in younger sediments and creating the illusion of continuity. Not so, says Haslam. While he admits the artefacts do show some signs of abrasion, he reckons they shifted only a short distance through the layers of sediment - not enough to skew the dating significantly.
Answering such queries is of the utmost importance, since the new, less-devastating picture of the Toba eruption painted by recent research could have wide ramifications for theories of human evolution and migration. Just how wide depends on which species of human produced the tools found in India. According to the traditional view, modern humans did not arrive in this part of Asia until 60,000 years ago or later, at least 14,000 years after the Toba eruption. Before this time, modern humans are thought to have been confined to Africa, barring one failed dispersal to the Levant - the eastern part of the Mediterranean - about 125,000 years ago.
If you follow this line of reasoning, the tools must therefore be the product of a more primitive species, perhaps the descendents of Homo erectus, which first occupied India 700,000 years ago or more. The new evidence would still be significant should that be the case, since the survival of these species would suggest the eruption may not have had a drastic impact on the Homo sapiens populations in Africa either, throwing the "genetic bottleneck" theory of human evolution into doubt.
Petraglia and his colleagues have much grander claims, however. They report evidence that the tools in India were indeed made by Homo sapiens - a finding that, if true, would rewrite the textbooks on human migration from Africa.
Previous evidence would just about allow an earlier migration. Fossil evidence from this period of human prehistory is pretty thin in the ground, so scientists have turned to genetics to retrace our ancestors' footsteps. By analysing the differences in mitochondrial DNA between modern aboriginal populations from different parts of the world, and considering how long it would have taken for these variations to emerge, Martin Richards at the University of Leeds, UK, Stephen Oppenheimer at the University of Oxford and colleagues have calculated that the earliest date for a migration out of Africa would have been 71,000 years ago. That is some 3000 years after Toba blew (The American Journal of Human Genetics, vol 84, p 740). Many others go even further, believing the likely date of exit to have been just 60,000 years ago. However, the large uncertainties associated with these techniques give a small probability that a pre-Toba migration may have been possible.
Indian invasion
In fact, there are other reasons to challenge the established theory of a late dispersal. Some Australian artefacts suggest modern humans made it to that part of the world 60,000 to 50,000 years ago, thousands of years earlier than current theory would allow (Nature, vol 345, p 153). And at Kota Tampan, in the Lenggong valley in Malaysia, archaeologists have uncovered a stone-tool culture spanning from 74,000 years to 4000 years ago. Since the tools don't change at all during this period, it would seem they were all produced by the same species of human. A 10,000-year-old Homo sapiens skeleton found at Kota Tampan suggests that this species was the modern human.
The tools discovered in India would seem to provide further tentative evidence for a pre-Toba migration. Firstly, Clarkson has spotted a subtle change in the way the tools in India were manufactured about 80,000 years ago (6000 years before the eruption) that may be evidence of a Homo sapiens invasion at this time. "They start to make better use of the stone and strike multiple flakes off in a more radial pattern, often from only one side of the core," he explains.
Comparing the patterning he sees on the Indian cores with more than 800 stone cores belonging to both modern humans and other hominins at sites all over the world, Clarkson finds that the Indian cores most resemble the cores made by modern humans in south Africa, south-east Asia and Australia. Meanwhile, the older Indian cores, with a flatter and more circular shape and often worked on both sides, have more in common with cores made by Neanderthals and other non-modern humans. "Tool-making is a skill and it takes a close apprenticeship to learn these methods. This was a cultural behaviour that was taught and passed down the generations," he says.
Further evidence for modern humans ousting non-modern hominins in India at this time comes from the tools themselves, with heavy hand-axes being abandoned for more lightweight tools. Petraglia and his colleagues have also uncovered three possible projectile points from beneath the ash at Jwalapuram 22, two of which are shaped carefully at the blunt end, apparently to enable them to be hafted onto a spear - a tool generally associated with Homo sapiens.
Ambrose is dismissive of the suggestion of modern humans arriving so early in India. "It is completely conceivable that Neanderthals could have made these hafted projectile weapons," he says. But others are more supportive of the idea. Chris Stringer from the Natural History Museum in London thinks it is possible that these tools are evidence of modern human activity, although he doesn't feel the evidence is conclusive. Robin Dennell, an archaeologist at the University of Sheffield, UK, is thoroughly impressed by the research. "Clarkson's work is excellent. I'm prepared to argue that Homo sapiens was in India pre-Toba," he says.
If modern humans really did live in India at this time, what route did they take from Africa to Asia? There is some good evidence that modern humans first attempted to leave Africa across treacherous desert regions into the Levant 125,000 years ago. Archaeologists had previously assumed the venture ultimately failed due to the adverse conditions, and that it was only much later, when humans tried coastal routes, that they succeeded. Petraglia, however, thinks that the wide range of new evidence in India challenges this view. "We're suggesting that perhaps this wasn't a failed dispersal," says Petraglia. "Maybe these people got out across Arabia and over to India after all."
"There could easily have been a core population of Homo sapiens in southern Arabia by 100,000 years ago," agrees Dennell, "and that population could then have been the source of populations that subsequently dispersed eastwards across southern Asia."
Evidence in the Thar desert in Rajasthan, northern India, seems to support this idea, with remains suggesting that modern humans were adept at crossing desert regions, hopping from oasis to oasis. "In the Thar desert we find fossilised sand dunes. After cutting them open like cabbages, we have found stone tools inside," says Hema Achyuthan, from Anna University in Chennai, India. Unfortunately, the tools are hard to date precisely, but they do show strong similarities to those found in Jwalapuram and the Middle Son valley.
But here's the killer question: if modern humans did migrate to Asia so early in prehistory, why isn't their journey reflected in modern mitochondrial DNA? Sacha Jones, at the University of Cambridge, thinks she has a solution that may just reinstate Toba's importance in human evolution.
She suggests a double dispersal from Africa, with the first migrants arriving in India pre-Toba around 80,000 years ago and bringing the new tool technology Clarkson observed. Later, more than 10,000 years after the Toba eruption of 74,000 years ago, a second wave of migrants arrived that sounded the death knell for the early pioneers. "This explains the pre-Toba tools we find in India, but it also fits with the genetic data," says Jones. "If the older population were wiped out then no genetic signature would remain."
If this theory reflects the reality, the role of Toba's eruption in human evolution may have been highly significant after all, weakening the first wave of migrants and pushing them into luscious refugia. The later, competitive migrants would have then elbowed their distant cousins off their bountiful land, finishing them off in the process.
It's a tempting proposition that promises to tie up the loose ends of the other theories - though much more archaeological evidence is needed to shore it up. As the digs in India continue, all eyes will be searching for those elusive human remains, perhaps even a skeleton cocooned in the ash, that could settle the score once and for all.
Today's supervolcanoes
The term "supervolcano" refers to any volcano capable of throwing out at least 300 cubic kilometres of magma during an eruption. At least one of these beasts explodes every 100,000 years or so, the geological record suggests. One of the most recent was the Toba eruption, 74,000 years ago. A medium-sized super-eruption, releasing 1000 cubic kilometres of magma, would wreak the same devastation as a 1-kilometre-wide asteroid smashing into the Earth. The bad news is that such a super-eruption is five to 10 times more likely than an asteroid strike, according to a 2005 report by the Geological Society of London.
Previous super-eruptions have been linked to mass extinction events, such as the Permian mass extinction 250 million years ago, which wiped out more than 90 per cent of marine species and was associated with an eruption at the Siberian Traps. The eruption of the Deccan Traps in India, together with a meteorite impact, might even have finished off the dinosaurs.
There is no doubt that Earth will experience more super-eruptions. "It is not a question of 'if' - it is a question of 'when'," says Bill McGuire, director of the Aon Benfield Hazard Research Centre at University College London.
Possible contenders for the next eruption include Yellowstone volcano in Wyoming, the Phlegrean fields volcano west of Naples, Italy, and Lake Taupo in New Zealand. However, there are many other areas where a supervolcano could one day pop up, including Indonesia, the Philippines, several Central American countries, Japan, the Kamchatka peninsula in eastern Russia, and even Europe (the area around Kos and Nisyros in the Aegean Sea might be a supervolcano).
As the recent research into the impact of the Toba eruption reveals, super-eruptions may not necessarily be as catastrophic as we fear. That said, a super-eruption would almost certainly devastate our civilisation. Unlike the humans living when Toba erupted, we depend on globalised trade and food production, with much reliance on air travel and space-borne communications, all of which would be severely knocked by a super-eruption.
Using similar considerations as they did for the Toba model, Hans Graf and his team at the University of Cambridge predict a Yellowstone super-eruption would cause the global temperature to drop by at least 1 °C. Several centimetres of ash would blanket all of North America. Oceans would become even more acidified and plant growth across the globe would be disrupted for several years.
The Geological Society of London's working group went one step further in 2005 when they described the impact of a generic super-eruption. "An area the size of North America or Europe could be devastated, and pronounced deterioration of global climate would be expected for a few years following the eruption. Such events could result in the ruin of world agriculture, severe disruption of food supplies, and mass starvation. The effects could be sufficiently severe to threaten the fabric of civilisation," they conclude. So fingers crossed, everybody.
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