286. Aftermath

I've already considered some of the genetic evidence to support the notion of a Toba-induced bottleneck, but let's dig a bit deeper this time. It's important to understand that the usual signs of a genetic bottleneck, in the form of reduced haplotype diversity, won't apply in this case, because the initial Out of Africa exodus would itself have produced a very significant bottleneck, making detection of a second bottleneck only a few thousand years later virtually impossible. There are, nevertheless, other unusual aspects of the Asian genetic picture that are very difficult to explain unless such a bottleneck in fact occured. Whether we are content to accept Toba as the cause, or feel forced to reject that theory, it's very difficult to explain away all the many signs that, regardless of the cause, a major bottleneck, centered in India, did in fact occur at a very early, and crucial, period in human history.

Some of the key problems are discussed in an unusually thorough and critical study by Richard Cordaux et al, Mitochondrial DNA analysis reveals diverse histories of tribal populations from India, 2003. While it is often assumed that the tribal peoples of India are directly descended from the initial wave of Out of Africa migrants, Cordaux et al found little sign of that:

Our analyses of mtDNA variation in tribal populations of India indicate that groups in different geographic regions have different demographic histories. In general, southern tribes have reduced mtDNA diversity and mismatch distributions strongly indicative of recent bottlenecks. The distinctiveness of southern groups is also emphasized by the MDS analyses and AMOVA. However, it is difficult to distinguish from these data between old and severe bottlenecks or more recent and less severe bottlenecks (my emphasis).

The picture they found is, in fact, more consistent with -- you guessed it -- a gap.

three typical east-Asian mtDNA haplogroups (A, B and F) are absent or virtually absent from non-northeast India . . . Furthermore, the fourth typical east Asian mtDNA haplogroup M has a different structure in India as compared to other Asian areas. This suggests that, although they show close affinities, the east Asian and Indian mtDNA gene pools are fairly distinct. This result is consistent with the suggestion that the east Asian and Indian mtDNA pools have been separated from each other for about 30 000 years (262 -- my emphasis).

To extend the context of their research, the authors refer to another study of special interest for our purposes, Phylogenetic Star Contraction Applied to Asian and Papuan mtDNA Evolution, by Peter Forster, et al (2001), in collaboration with the noted archaeologist, Colin Renfrew. These authors consider a very interesting question that we haven't yet discussed,

the question of whether modern European, Asian, Papuan, and Australian mtDNA types derive from an uninterrupted demographic expansion of the out-of-Africa founders (strong Garden of Eden model), or whether an initial expansion was followed by the formation of regional gene pools, which, after a period of isolation and drift, expanded demographically and geographically to form the present mtDNA variation in different continents and regions (weak Garden of Eden model) (1864).

These authors also see a gap, which for them suggests a weaker version of Out of Africa, in which a "period of isolation and drift" is followed by migrations in various directions to produce the differentiations, genetic and otherwise, with which we are now aware. While their notion of a "weak Garden of Eden model" strikes me as rather vague, the gap they see makes considerable sense, especially since they are among the few to notice the importance of Melanesia, and in particular, New Guinea, for the Out of Africa model: "The oldest expansion in Eurasia occurred 65,000, + or - 23,000 years ago . . . and is witnessed by mitochondrial descendants preserved in Papua New Guinea" (1875 -- my emphasis).

For them, this original expansion (consistent with the Toba date of ca 74,000 ya, if we take the + or - 23,000 seriously) is "about 20,000 years older than any mainland Asian cluster, although both the Papuans and the Asians are derived from the same two Eurasian founders [M and N -- VG]." On this basis, they propose a remarkable scenario "to account for the obvious phenotypic differences between Papuans and Asians despite their sharing a common mitochondrial ancestry":

The M and N founders derive from a single African migration but split at an early stage (possibly before reaching Europe, which lacks M) into proto-Papuan and proto-Eurasian. The proto Papuan M and N immediately expanded demographically and geographically along a southern route until reaching Papua New
Guinea, thus allowing Papuans to retain their overall genetic similarity to Africans (Stoneking et al. 1997). Meanwhile, proto-Eurasians spent 20 or more millennia genetically drifting to their present distinct European, Indian, and east Asian M and N types, as well as phenotypes (compare the common Papuan/Eurasian melanocortin receptor variants in table 1 of Harding et al. [2000]), long before expanding (1875).

Aside from the authors' fixation on New Guinea, by no means the only place in what could be called "greater Southeast Asia" to perpetuate both very old mtDNA haplotypes and very African cultural practices (not only P/B but also remarkably sophisticated visual art traditions), their scenario makes a good deal of sense. The "weak Garden of Eden" scenario is rarely discussed anymore, probably because it's so vaguely defined and so difficult to test. (What would it mean for proto-Eurasians to "genetically drift" for 20,000 years?) But the Toba bottleneck scenario might well make it unecessary, because the equivalent of 20,000 years of drift might have occured overnight.

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, after a few days, weeks, months or even years, 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," whom I wrote about as follows (in Post 176):

Ik society was, at that time, undergoing severe stress, 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" aka "dog eat dog."

(to be continued . . . )