Chapter Ten: The Bottleneck

A catastrophic disaster centered somewhere in South Asia may well have produced one or more major population bottlenecks, which could have had important consequences for the future of the human race. As we’ve learned, Stephen Oppenheimer has a particular event in mind, the gigantic explosion of Mt. Toba, ca. 74,000 years ago. Let's take a closer look at this hypothesis. Here, according to Oppenheimer's Journey of Man website, is how the homo sapiens world would have looked circa 75,000 years ago, prior to the Toba eruption:

(See Figure 9.2, Chapter Nine)

Here's how he thinks things looked just 1000 years later, ca 74,000 years ago. The grey puff you see is the fallout from the explosion of Mount Toba – the red dot is where Toba is located, in northwest Sumatra:

Figure 10.1 Toba Aftermath (The Journey of Mankind)

One of the many things that makes Toba potentially useful as evidence is that we have a pretty good idea when it erupted and the extent of the damage it would have caused, thanks to the considerable amount of volcanic ash it dumped, which can be identified and dated. The remnants of Toba ash were found largely to the west and north of the volcano, thus the prevailing winds must have been northwest at the time.

Oppenheimer’s The Real Eve was published in 2003. A paper from the following year (Metspalu et al. 2004) (Most of the extant mtDNA boundaries in South and Southwest Asia were likely shaped during the initial settlement of Eurasia by anatomically modern humans) provides striking support for some of the same discontinuities noted by Oppenheimer, confirming, among other things, the strange bias in the distribution of the M haplogroups (see previous chapter): “Our results indicate that the frequency distribution of haplogroup M varies across different Indian regions by a significant cline towards the south and the east . . .”

The very interesting distributions of some of the most important M sub-haplogroups are mapped in their Figure 1:
 

Figure 10.2 Distribution of M Haplogroups in India (Metspalu et al. 2004)

Note especially the distributions of M6, M6b and M2b, all found, for the most part, in the east and south, where Oppenheimer noted the prevalence of haplogroup M. There is an equally interesting presence of M6 and M6b in the northernmost reaches of the Indus valley. The pattern is clear from the map at the upper left, where the heaviest distribution of M6 is found in two different places, not only the south and east of India, but also far to the northwest, in the Punjab-Kashmir region shared with Pakistan.

There are two things about this region that make it especially interesting: its location places it at a greater distance from Toba than any other part of South Asia, and it is the only region between Africa and East Asia where tone languages are commonly found. This would have been a likely spot where a branch of the Out of Africa migrants, who could have broken from the main group to travel north along the banks of the Indus, might have been able to survive the effects of Toba with their African traditions more or less intact. If tone language was part of their HMC inheritance, then that could explain the prevalence of tone language in this area today.

The maps presented in Figure 11 of the same paper are especially significant:

Figure 10.3 "The segregation of West Eurasian, East Eurasian and South Asian mtDNA pools" (Metspalu et al. 2004)

From the caption:

The segregation of West Eurasian, East Eurasian and South Asian mtDNA pools. Partial map of Eurasia illustrating the spatial frequency distribution of mtDNA haplogroups native to West Eurasia (panel A), South Asia (panel B) and East Eurasia (panel C). . . ¹

The three so-called “isofrequency” maps, based on some of the oldest known Asiatic lineages, some estimated to date from over 70,000 years ago, paint a remarkable picture of human history in this part of the world, from possibly only a few thousand years after the Out of Africa migration began, to the present, with the most significant later migrations presented as clines.

They remind me of another map that some of you may recognize, produced by a satellite known as the Cosmic Background Explorer (COBE):

Figure 10.4 Cosmic microwave background (COBE)

Here we see an image produced by truly ancient (ca 14 billions years old) microwave signals, emanating from every corner of the universe, based on events thought to have taken place ca 380,000 years after the Big Bang. The maps of Figure 11 are strangely analogous in that they too give an idea of the distant past by mapping evidence available in the present. Instead of the echo of ancient microwave signals still detectable in space after billions of years, we have the echo of ancient mutations, still detectable in our DNA after thousands of years. But the pictures are very different, in the first case relatively uniform, in the other radically disjunct. In fact the genetic maps are described as “The segregation of West Eurasian, East Eurasian and South Asian mtDNA pools.”

If the Out of Africa migration was a smooth progression along the “southern route,” from the Horn of Africa to Southeast Asia and beyond, then this should be reflected in the genetic markers as a steady west-east cline. Unless the traces of the original migration, like those of the Big Bang, were obliterated by what happened at a later time. What happened during the course of the Big Bang, according to a theory now widely accepted, was “cosmic inflation.” What happened during the course of the great migration is unknown -- but it must have been something big, because it had a huge effect.

Here's how the authors explain the strange discontinuities so clearly illustrated in their maps:

We found that haplogroup M frequency drops abruptly from about 60% in India to about 5% in Iran, marking the western border of the haplogroup M distribution. A similarly sharp border cuts the distribution of Indian-specific mtDNA haplogroups to the east and to the north of the subcontinent. We therefore propose that the initial mtDNA pool established upon the peopling of South Asia has not been replaced but has rather been reshaped in situ by major demographic episodes in the past and garnished by relatively minor events of gene flow both from the West and the East during more recent chapters of the demographic history in the region (my emphasis).

There is a significant difference between the discontinuity dividing the Middle East and the South Asian Peninsula and that between the Peninsula and its neighbors to the east and north. The ancient haplogroup M is hardly found at all to the west of Pakistan, while it is found in abundance in India, Southeast Asia and East Asia generally. Since the mainland of the Arabian Peninsula and neighboring Iran would have been uninhabitable desert at the time, we can safely assume either that no colonies were left in these regions during the Great Migration (GM) or that whatever colonies might have been left never survived. Thus the discontinuity so evident in Maps A and B can be attributed to the presence of a natural barrier. What we see in Map A must therefore be the result of migrations dating from a much later period.

The very strange discontinuity illustrated in Maps B and C cannot be so easily accounted for. In this case, the same ancient root, haplogroup M, underlies the genetic picture for both regions; representing, no doubt, a faint echo of the original east-west migration (and in all likelihood confirming the southern route). But the M's found in India are not the same as the M's found to the east, southeast and north. Which is why Map B has a different color than Map C -- based on the fact that a very different set of haplogroups, M included, are “native to” each region. 


How can this be? Unlike the region to the west of Pakistan, there is no natural barrier that might hinder the colonization of territories bordering on, or beyond, India's eastern boundary. The authors wisely explain “that the initial mtDNA pool established upon the peopling of South Asia,” i.e., the great migration following the Out of Africa exodus, “has not been replaced.” This is evident by the pervasiveness of certain ancient haplogroups, not only M but also N and R, in India. But their attempt to explain the discontinuities that so clearly overlay and obscure the traces of this early migration, on the basis of an mtDNA pool “reshaped in situ by major demographic episodes in the past,” while reasonable, is inadequate.

First of all, the clear demarcation we see between India and its closest neighbor to the east, Myanmar, is unlikely to have been produced by a series of different, unrelated, events or migrations. It's hard to believe it could have been produced by more than one. And clearly it was not produced by a migration, since a migration would have resulted in continuity, not discontinuity. So it could only have been produced by a disruptive event of major proportions.

To facilitate comparison, I've blown up Oppenheimer’s map, representing the fallout from the Toba explosion, and placed it just above Map C, from Figure 11 (see above). You’ll recall that the red dot on the upper map represents the location of Toba, in northwest Sumatra. The northernmost tip of Sumatra can just barely be seen at the bottom of the lower map.

Figure 10.5 Detail of Fig. 10.1

Figure 10.6 Detail of Fig. 10.3

The match between the ash cloud and the negative space formed by the haplogroup distribution (in shades of orange) is remarkable. Note that the outer edges of the Toba cloud perfectly match the distribution cline to its northeast. Since there are no significant natural borders along the coastal route, and no other readily apparent explanation for the genetic segregation of the two areas, it's certainly tempting to attribute the pattern we see in the lower map to the event represented in the upper.

Toba would not only explain the discontinuity between India and points east, so evident on the genetic maps, but also the gap I've been stressing, involving cultural practices found in both Africa and greater Southeast Asia, but almost completely absent from the Middle East, Pakistan and India. African-related cultural survivals can indeed be found in exactly those areas to the east and northeast of Toba that would have been upwind from the eruption and thus relatively unaffected.

The Toba event might also explain the strange distribution of both M and N-related haplogroups in South Asia. As Oppenheimer noted, and Metspalu et al confirmed, there is a distinct northwest to southeast cline in the distribution of M, with most instances by far to be found along the eastern and southern coasts. N-derived haplogroups, on the other hand, are relatively rare in this region, though common in the west and northwest of India -- and also farther east, beyond the border with Myanmar. 


Oppenheimer associates this puzzling distribution with the Toba event, suggesting that the prolonged ash cloud could have devastated all or most of India, especially both M and N related populations in the east and south, closest to the volcano. He hypothesizes that this area could then have been repopulated by M dominated groups immigrating from the east, who might then have spread, in a cline, to the rest of the subcontinent, while N-related groups to the west could have repopulated India from that region. This could have left India populated by more recent M and N hapolotypes than those found farther east, a pattern noted by both Cordaux et al and Soares et al, as reported above.

What makes the Toba hypothesis particularly convincing is the fact that it seems to fit our gap so well. As we’ve seen, the plume of ash perfectly covers a large portion of the gap. And the regions to the northeast, east and south, which would have been upwind from the direction of the plume, would have been largely, though not completely, spared. Which fits beautifully with the distribution of the African musical signature, both vocal and instrumental, scattered among so many indigenous peoples of Southeast Asia, southern China, Indonesia, Taiwan, the Philippines, and Melanesia. Those largely spared could have managed to preserve a significant part of their African traditions, while those caught in the gap could have lost most or at least some of them – assuming they or their descendants survived at all.

The Archaeological Evidence

There is no controversy regarding the eruption of Toba itself. It definitely happened, it was huge, and can be dated to within a few thousand years. Everything else is controversial and there is a lot of misinformation and confusion out there. The first person to associate it with human evolution was archaeologist Stanley Ambrose and it is his interpretation that's usually quoted. According to Ambrose, modern humans were all living in Africa when Toba exploded, but the explosion was so huge that it had a dramatic effect on their development, even at so great a distance. For Ambrose, the trauma of Toba was a major factor in prompting humans to become more “advanced,” a development that in his view led directly to the Out of Africa adventure.

Ambrose was also the first to suggest that Toba could have been responsible for human differentiation, producing the various “races” and other signs of major biological and cultural diversity. However, if all humans were confined to Africa when Toba hit, as Ambrose assumes, it's impossible to see how any differences produced by that event could have evolved into the worldwide distinctions so evident today, of which Africa is a relatively homogeneous part. Toba can explain the large-scale differentiation patterns we now see only if humans had already left Africa and had occupied most or all of the south Asiatic coast by the time it erupted. However, Ambrose, for reasons that continue to puzzle me, insists that this is not possible and that all the archaeological evidence points to an African exodus after the Toba eruption, not before (personal communication).

It was only Stephen Oppenheimer who saw the necessity of the Out of Africa migration preceding the Toba event, because otherwise it could not have had the necessary effect. And in this sense it's possible to turn things around, so that instead of timing the great migration on the basis of (shaky and incomplete) archaeological assumptions, we can use the timing of the Toba eruption itself to much more precisely estimate the date of the fateful exodus, which, if Oppenheimer is right, would have to have preceded it by at least a thousand years or so.

Currently, most archaeologists and geneticists seem to agree, based on certain fossil finds in Australia, as well as estimates of genetic “coalescence,” that the most likely date for the great migration is somewhere between 60,000 and 50,000 years ago, thousands of years after Toba. And on that basis, Toba has been discounted as a factor in human evolution. For many, the last nail in the coffin was provided by archaeologist Michael Petraglia, who, after years of digging and probing, found some very interesting stone tools both below and above Toba ash (Middle Paleolithic Assemblages from the Indian Subcontinent Before and After the Toba Super-Eruption, Petraglia et al, 2007).

In a blog post entitled At Last, the death of the Toba bottleneck, paleoanthropologist and long time Toba skeptic John Hawks gleefully reported the findings thus: “This week's paper by Petraglia and colleagues (2007) appears to have sunk the Toba bottleneck entirely. Very simply, they found a Toba ash horizon in India, and found very similar archaeology both below and above the eruption.” Petraglia's results were widely reported in the media in much the same terms, as though the mere fact that more or less the same type of tools were found above as below the ash meant that the effects of the Toba eruption could be discounted.

What was all but ignored in such reports was a far more significant finding:

these pre- and post-Toba industries suggest closer affinities to African Middle Stone Age traditions (such as Howieson's Poort) than to contemporaneous Eurasian Middle Paleolithic ones that are typically based on discoidal and Levallois techniques. . . This interpretation would be consistent with a southern route of dispersal of modern humans from the Horn of Africa; the latter, however, will remain speculative until other Middle Paleolithic sites in the Indian subcontinent and Arabian Peninsula are excavated and dated (my emphasis).

In other words, what Petraglia found that went almost unnoticed at first was evidence that the Out of Africa migrants may have been in southern India at the time of the Toba eruption after all. This was a finding of major importance, the first archaeological evidence consistent with the presence of modern humans in Asia at such an early date, but it got lost in all the hoopla surrounding the apparent debunking of the Toba “myth.”

Petraglia went out of his way to clarify the meaning of his results in a subsequent interview (Modern Humans Lived in India Earlier Than Thought, Study Finds, by Chris Dolmetsch, Bloomberg):

“This is some of the earliest evidence for the spread of modern humans out of Africa towards Australia,'' Petraglia said in a telephone interview from New York.

The study says the relics, made of limestone, quartzite, chert and other minerals, are likely from a variety of stone tools from the Indian Middle Paleolithic era that lasted from about 150,000 to 38,000 B.C.

Yet the characteristics of the artifacts are more typical of the African Middle Stone Age that ended about 40,000 years ago than they are of younger artifacts found elsewhere in Europe and Asia, the study says. That finding suggests that modern humans had migrated out of Africa and were already in southern India when the Toba Tuff eruption blanketed the region in ash. [my emphasis]

“It will be very much debated,” Petraglia said. “There are people that are wedded to their theories and won't like it at all, and there are others who will welcome our study because this part of the world is very understudied.”

Oh and by the way, the artifacts in question were found “under a 2.5 meter (8.4-foot) thick ash deposit . . .” It’s hard to see how the effects of an accumulation of over 8 feet of ash on human survival can be discounted.

In another interview (National Geographic) Petraglia makes this clear:

“The fact that we have this ash is just icing on the cake, because it tells us that if it's modern humans, then they were able to persist through a major eruptive event,” he said. “But they would have had a very, very difficult time.”

What Petraglia's findings suggest is that the Toba blast was not sufficient to have had much of an effect on Africa, as Ambrose has argued, or Europe either -- but it would certainly have had a very significant effect on any modern humans living in South Asia shortly after the out of Africa excursion, and could for that reason have had lasting consequences for subsequent human history.

Climatological Evidence

Additional evidence on Toba has surfaced recently, in the form of two climatological studies. In the first, Did the Toba volcanic eruption of ∼74 ka B.P. produce widespread glaciation?, May 2009, Alan Robock et al conducted

six additional climate model simulations with two different climate models, . . . in two different versions, to investigate additional mechanisms that may have enhanced and extended the forcing and response from such a large supervolcanic eruption.

While “none of the runs initiates glaciation” and, in all cases, their simulations revealed that “the climate recovers over a few decades”, nevertheless,

the “volcanic winter” following a supervolcano eruption of the size of Toba today would have devastating consequences for humanity and global ecosystems. These simulations support the theory that the Toba eruption indeed may have contributed to a genetic bottleneck. (my emphasis)

A second study, conducted by Stanley Ambrose and Martin Williams, was recently (Nov. 2009) reported in Science News:

Ambrose and his colleagues pursued two lines of research: They analyzed pollen from a marine core in the Bay of Bengal that included a layer of ash from the Toba eruption, and they looked at carbon isotope ratios in fossil soil carbonates taken from directly above and below the Toba ash in three locations in central India.

The investigators concluded that there was

“incontrovertible evidence” that the volcanic super-eruption of Toba on the island of Sumatra about 73,000 years ago deforested much of central India, some 3,000 miles from the epicenter . . . The bright ash reflected sunlight off the landscape, and volcanic sulfur aerosols impeded solar radiation for six years, initiating an “Instant Ice Age” that -- according to evidence in ice cores taken in Greenland -- lasted about 1,800 years.

When we combine such reports of Toba-induced devastation with Petraglia's findings, strongly suggesting the presence of modern humans in South Asia at the time, the possibility of major population bottlenecks downwind from the volcano seems strong indeed. While stone artifacts were found both above and below the Toba ash, indicating that at least some humans survived, we can assume that any survivors would have been struggling very hard in an environment radically different from the one that first greeted them. And while the presence of the artifacts suggests that they survived the immediate effects of the disaster, this does not mean they were able to survive its long-term effects.

As both the archaeological and genetic evidence suggests, much of the Indian subcontinent, especially the east coast, directly in the path of the volcanic plume, could have been depopulated, only to be repopulated at a later time from the East, as Oppenheimer suggests, by people who would also have been affected by the disaster, but to a lesser extent. It is these Toba survivors who would most likely have experienced severe population loss, resulting in bottlenecks, both genetic and cultural. Populations even farther to the east and southeast, and also farther to the north (assuming there were any at that time) would also have suffered, but to a much lesser extent, and would thus show fewer signs of genetic, morphological and cultural change. This does indeed seem to be the case, though the situation is obscured by the considerable movement of various peoples into and out of this region for many thousands of years since.

It's possibly for this reason that, as Oppenheimer noted, “some of the best, if not the only archaeological evidence for dating the beachcomber's trek along the coast of the Indian Ocean, comes not from India, South Arabia, or Africa, but from the later parts of the trail -- the Malay Peninsula, New Guinea, and Australia” (The Real Eve, 159), those areas least affected by the volcanic fallout.

The Toba Effect

The possible effects of a Toba-induced bottleneck on modern humans ca 74,000 years ago, were succinctly summarized by Michael Petraglia's collaborator, Sacha Jones:

This bottleneck would have greatly reduced modern human diversity as well as population size. With climatic amelioration, population explosion out of this bottleneck would have occurred, either ~ 70 ka, at the end of a hypercold millennium . . . or ~10ka later with the transition from OIS (Oxygen Isotope Stage) 4 to warmer OIS 3. Post Toba populations would have reduced in size such that founder effects, genetic drift and local adaptations occurred, resulting in rapid population differentiation (Ambrose 1998). In this way the Toba eruption of ~74 ka would have shaped the diversity that is seen in modern human populations today (“The Toba Supervolcanic Eruption,” in The evolution and history of human populations in South Asia, ed. Petraglia and Allchin, 2007, p. 177 -- my emphasis).

In the aftermath of a disaster such as the Toba eruption, many groups in the path of the huge, thick ash cloud would not have survived. For those who did, life would have drastically changed. For one thing many if not most, if not almost all, of their population may have been killed outright, simply suffocated in a sea of ash. Some might have been in a position to retreat to the depths of certain caves, where the worst effects of the ash cloud might not have penetrated. When emerging, they would have been faced with a world largely depleted of both vegetation and wildlife.

To get a sense of what life would have been like in an environment suddenly deprived of almost all the usual sources of food, water and communal support, we can consider the fate of the group Colin Turnbull, in his book The Mountain People, called the “Ik.” Ik society was, at that time, undergoing severe stress due to external conditions they could not control, and the stress had very definite and very dire effects on a people who had become increasingly desperate with hunger and other forms of deprivation, to the point that their cultural values were disappearing into a mode of existence based, as one might expect, on the philosophy of “every man for himself” or “dog eat dog.”

While the Ik may be seen as victims of a characteristically modern, “post-colonial” situation, the radical changes recorded by Turnbull can give us an insight into what could have happened at certain times in the past, when a particular population is suddenly placed under tremendous stress to the point that the most basic cultural norms begin to break down. Of special significance for us is the relative scarcity of musical references in the book. Whenever singing is mentioned, it is almost always solo singing, not surprising in an atmosphere where social cohesion is breaking down and “every man for himself” has become the norm. 


The only group singing noted by Turnbull among the Ik is the singing of Christian hymns, and that takes place only when a group is expecting a consignment of food from some missionaries (who never show up). He has nothing to say about what their music might have been like in the past, but if the Ik were a typical African tribe, we can be almost 100% sure that group singing -- and dancing -- would have been a common, if not everyday event. In the context described in the book, however, occasions for such activities, either for pleasure or for traditional ritual purposes, no longer exist.

As Petraglia's findings suggest, there appear to have been Toba survivors, though they would certainly have been only a small fraction of the population directly in the path of the disaster. It's also possible that the artifacts he discovered were from survivors in a neighboring area, where the fallout wasn't quite as heavy, who moved into this area at a later time. In any case, anyone trying to survive in the post-Toba environment would have been faced with extraordinary difficulties, paralleling in some ways the hardships faced by the Ik.

And, as with the Ik, it’s not difficult to imagine such a disaster leading to a population dying out completely. But what if it doesn't die out? What if there are survivors who manage to begin anew at some point, what will they be teaching their children? What aspects of their old culture are likely to survive, what are likely to be lost and what new elements are likely to be introduced? Under such dire circumstances it's difficult to imagine that a highly interactive, group-oriented musical tradition such as P/B could have survived. And it's not difficult to imagine how it could have been replaced by something much simpler, as was apparently the case among the Ik.

And other traditions reflecting the African origins of the migrants may also have been lost. If the most gifted and experienced wood carvers had been killed, then the African wood carving traditions might have died with them. If the leading shamans died, then the most elaborate rituals might have died with them. It's important to realize that once a tradition is lost, to the point that there is no longer anyone to hand it down to the younger generation, then it is gone forever.

It's not difficult to see, moreover, how some of the original core values, inherited from HBP, might also be lost. What does it mean to share meat when the only meat available might be from mice or rats, hardly enough to feed one person, let alone an entire group? Egalitarian values might also go by the boards in a situation where the strong can only survive at the expense of the weak. And the weak survive only if protected by someone stronger -- and more aggressive. Once such a situation is established, it's very easy to see how it could become a self-perpetuating tradition.

Instead of an egalitarian ethic, steeped in non-violence, a new system of values, based on the survival of the strongest, most assertive and most competitive individuals, and their subservient followers, could emerge. Once such a tradition is established, it would be almost impossible to go back to the old way of doing things and even of thinking. Even if things might improve over time, to the point that the society is no longer stressed, and no longer dependent on strong, aggressive leaders, it might not matter, because traditions tend to perpetuate themselves long after they have lost their original purpose and even their meaning.

Genetically it's not simply a matter of a “population bottleneck” forming among relatively neutral mitochondrial or Y chromosome genes, but the favoring, if only by chance, of certain genotypes and phenotypes (so called “racial” characteristics). All societies contain a certain amount of morphological variation, but when almost everyone is struck down by a disaster, or chooses to leave in search of better conditions elsewhere, then the genetic and morphological characteristics of the survivors become the new norm, which could be quite different from the old one. 


For example, if only a few members of a particular migrant colony happened to have what we would now consider “Mongoloid” features, and that group happened, by sheer chance, to survive, while most of the others died or migrated elsewhere, then such a development could lead to the establishment of a new “race,” with “Mongoloid” features exclusively. Thus, it’s not difficult to see how an event such as Toba could have been the trigger for certain very fundamental changes, cultural, genetic and morphological, which could explain the highly structured differences we now see among different populations in different parts of the world.

We see, in East Asia, people who've been described as having “mongoloid” features, and a highly distinctive, extraordinarily sophisticated culture, unlike any other on earth. In Central Asia, we see very different people, mostly horse nomads. Northern Asia is dominated by so-called Paleosiberian people, mostly reindeer herders, who span the entirety of the circum-polar world, from the Lapplanders of Europe all the way to the Inuit of North America. In Europe we find, again, people who are unique, both physically and culturally unlike any others anywhere in the world (though there are some intriguing morphological links with, for example, the Ainu of Japan).

Among native Americans we find people who, again, have been described as “mongoloid,” possibly because they are descended from the same ancestral group that gave rise to both the East Asians and Paleosiberians. But North American Indian culture is very different from that of East Asia or Paleosiberia -- and equally unique. In southeast Asia, Indonesia, the Philippines, Melanesia, Australia, etc. we find a more complex mix of people with morphologies often characterized as either more or less “negroid,” or mongoloid, or some mix thereof.

Which is why I find the notion of a major bottleneck very early on, due to Toba or perhaps some other serious event, so compelling. Because it would seem as though only such an event -- at such an early stage -- could have had the sort of major impact needed to produce the large-scale patterns of difference we now see. In my view, it is only when we pay attention to such large scale distributions of certain traits, characteristics, traditions, etc. that we can find useful clues with the potential for recreating historical events that might otherwise seem totally beyond our reach.

It is, admittedly, not always easy to consistently reconcile all the details of the various genetic, social and cultural distributions in this vast region, not to mention the many different interpretations of such evidence. But the explanatory power of this event is potentially so strong that further research is certainly justified. Toba can no longer be on the back burner, it must come to the forefront of our attention.

---

1. Though tribal populations constituted roughly 50% of the 2572 total sample for India (4600 samples for Asia overall were analyzed, and considerably more added for Figure 11), they are, very unfortunately, not represented on the isofrequency maps (A, B and C), for reasons explained as follows in the "methods" section:

In relatively small and isolated groups (e.g. tribal groups) random genetic drift might seriously affect the haplogroup frequencies, which may become uninformative when a whole region (e.g. state) is considered . . . Therefore, the tribal data were excluded from the haplogroup isofrequency maps calculation. When illustrating the spread of mtDNA haplogroups native to West Eurasia, East Eurasia and India (Figure 11, panel D) we present these data as pie diagrams. The respective sample size and origin are indicated adjacent to the diagrams. (p. 22)

The effect of the tribal data on maps A, B and C may, to some extent, be inferred from the pie charts in map D. And, indeed, the picture presented in D, of a major division between South (mostly green and blue) and Southeast Asia (mostly orange), is consistent with what we see in the maps above.