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:
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).
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 . . . )


German Dziebel said...

"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."

It could produce this kind of differentiation if the original migration was massive and the population was growing exponentially in all local regions, including America. High population densities would then lead to the easy survival of innovations. The hypothesis of a bottleneck or a series of bottlenecks outside of Africa makes it even harder to imagine how populations could regain cultural and linguistic diversity and take it to the whole new level unknown in Africa. This especially applies to the Americas, which displays very wide cultural and linguistic connections with such remote areas as Papua New Guinea, Australia and the Caucasus.

Another possibility is a series of waves out of Africa and then out of other regions into the regions farthest removed from Africa. This would increase cultural and linguistic diversity, too.

Depopulation would probably lead to the reversal to the most common and likely archaic form and the loss of all fledgling forms, rather than to the proliferation and fixation of new forms.

DocG said...

German: "It could produce this kind of differentiation if the original migration was massive and the population was growing exponentially in all local regions, including America." etc.

It's impossible for me to discuss such alternatives with you because your thinking is oriented in exactly the opposite direction from mine and as a result I can never be sure exactly what it is you are suggesting.

I can't accept Out of America as a meaningful solution to any of these problems, however it's an interesting theory nevertheless, and worth exploring, if for no other reason than as a methodological exercise. And who knows, there might be something in it.

But this is for you to do, not me, and not the two of us together, because I am now going in the complete opposite direction. I urge you, therefore, to set up your own blog, where you could explore all these possibilities as an extension of what you've already done in your book. I promise to read what you have to say and comment from time to time on your ideas -- but not here, as this is not the proper venue for the exploration of an Out of America hypothesis, sorry.

German Dziebel said...

Victor, you are missing a fundamental point (over and over again): I'm not arguing for out of America, but rather criticize the dominant interpretations of the out of Africa model. These two approaches are complementary but one can exist without the other. The way out of Africa is being developed, including here by you, has many weaknesses of methodological, logical and factual kind. In the end, humans could come from Africa (but the story should be different from that of rapid replacement), or they could come from Asia, or they could stay in their places in accordance with Multiregional theory (unlikely in yours and mine opinion). But I'm not using my out of America bias to judge your ideas. If I've invoked America here, only because it provides a good illustration of how demographic models considered in the out of Africa models can't explain the accrual of diversities on the other side of the world, including PNG, America and parts of Asia. Again, this doesn't mean, by itself, that humans came from America. It's a whole different exercise.

Unfortunately, I have a feeling that you are not capable or willing to write this blog in a way that utilizes the intelligence of your commentators in every next post. It's a shame, Victor, but your commitment to certain mental constructs, to use a neutral term, such as out of Africa, Toba or P/B style is in the way of a more nimble and data-driven approach to theory-building on all levels, from musical to genetic. You treat this blog as a "small business" and a "private practice" and you have a right to do so, but here's a trick: people don't treat your blog as an "exodus from Africa" rondo coming from under the fingers of an artistic prodigy. They treat it as an attempt at a truthful account of human evolution. They make it their "business," whether you are comfortable with it or not.

I wish I had time for such a blog of my own but I prefer traditional media. Now, you have my book, and I've read your blog. Our conversation is over.