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).
Thursday, January 14, 2010
288. Aftermath 3
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, based on a theory developed by Stanley Ambrose, who proposed a volcanic winter reaching all the way to Africa:
Since the book in which Jones' discussion appears was written prior to the discovery of the artifacts below and above Toba tuff, she would not have been aware of the new evidence pointing to South Asia, rather than Africa, as the center of the bottleneck in question.
Jones is summarizing Ambrose's theory, by the way and not her own. Both she and Petraglia have been extremely circumspect in discussing the meaning of the Toba event for human evolution, and their summations of speculative theories of this sort have been balanced by references to very different interpretations by Clive Oppenheimer (NOT Stephen, though the difference isn't always made clear in their book), Gathorne-Hardy, Harcourt-Smith and others.
C. Oppenheimer argues, for example, against the possibility that the effects of Toba would have been so extreme as to have a major impact on Africa, as Ambrose assumed. His objections don't extend to South Asia, however, so they can be discounted for our purposes. Gathorne-Hardy and Harcourt-Smith studied fauna on the Mentawai Islands, southeast of Toba, and found no evidence of any disruption of rainforest or animal life during this period. In response, Jones cites evidence of heavy Toba fallout throughout the region, "suggesting that (Toba tephra) traveled in all directions apart from to the south-east" (ibid.), a conclusion that makes sense in terms of evidence I've already cited that Out of Africa migrants upwind from the heaviest Toba cloud might have suffered only minimally from its effects -- which would explain the survival of an "African signature" in parts of Southeast Asia, Indonesia, Melanesia, the Philippines, etc., but nowhere to the north and west, where the Toba fallout would have been the most devastating.
While Ambrose is focused on the possible effects of Toba in Africa, prior to the Out of Africa migration, his description of its impact, both immediate and lasting, can just as easily be applied to the hypothesis I've been examining (actually Stephen Oppenheimer's hypothesis), centered not in Africa, but South Asia. In fact, if modern humans had already formed colonies in South Asia at the time, and beyond, in Southeast Asia and perhaps also Island Southeast Asia, that would, as I see it, make Ambrose's hypothesis much more convincing.
A major human population bottleneck in South Asia would in fact explain "the diversity that is seen in modern human populations today" much more effectively, since this region can be seen as a kind of hub, from which future migrations could have emanated, in literally every direction (including southeast, by sea, to Indonesia, the Sahul, Melanesia, etc.)
To understand the explanatory power of an event of this sort, which, as I argued in the previous post, could have had a lasting impact on both the morphology and culture of succeeding generations, we need to understand a very basic problem posed by the Out of Africa migration itself. Because a straightforward, unbroken migration from Africa to Asia and from there to all other parts of the world could not have produced the highly structured, morphologically and culturally differentiated populations we now see.
In order to understand this better, let's compare the world as a whole with Africa, which has had a very different history. 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). In North America 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.
Central America, South America, Southeast Asia and Oceania, however, are more difficult to pin down -- for reasons I'll be discussing presently.
(to be continued . . . )
Posted by DocG at 2:30 PM