August 08, 2004

On the History of Inner Asia, as Reflected in Its Intestinal Flora

Ladakh is the closest I've been to Central Asia; it's a part of the Indian state of Jammu and Kashmir, bordered to the west by Pakistan, the north by China, and the east by Tibet. It's an incredibly beautiful place, in a very forbidding way --- immense, sharp-edged mountains sloping into dusty, almost Martian plains of rock. The desolation is only relieved by the rivers, along which run narrow, intensely green strips of vegetation. In such a landscape, even the Indus looks like a creek, and one appreciates why Paradise was not just a garden but a walled garden. My father, sadly, has never digitized any of his photos, but the pictures on this site give a little sense of what it's like.

As those pictures make clear, for a long time the politically-dominant religion has been a version of Buddhism imported from Tibet. The population is a bit less than half Buddhist, and a bit less than half Muslim, with Hindus making up the balance. Historically, Buddhism has been dominant since the first or second century AD, when it was introduced from India and Central Asia under the Kushans, when the population seems to have been largely Indo-Iranian. (Contemporary Chinese sources refer to them as "Hu", the same word used for the Indo-European populations of Central Asia, Afghanistan, etc.) Around a thousand years ago, as a kind of side-effect of the formation of the Tibetan Empire in Central Asia, a different, lamaist form of Buddhism was introduced by conquerors from Tibet, along with a new script, a new language, and a new aristocracy. (There are still a few areas speaking Dardic Indo-Iranian languages, related to Kashmiri, and practicing a substantially different form of Buddhism.) Islam is even more recent, the first major in-roads having begun under the Mughals, in part through some concessions from Aurangzeb wrung out of the Ladakhi princes in exchange for defending them against a joint Tibetan-Mongol invasion. (A really nice history, if you can find a copy, is Janet Rizvi's Ladakh, Crossroads of High Asia.)

All of which brings us to today's paper, and the promised intestinal flora.

Thierry Wirth, Xiaoyan Wang, Bodo Linz, Richard P. Novick, J. Koji Lum, Martin Blaser, Giovanna Morelli, Daniel Falush and Mark Achtman, "Distinguishing human ethnic groups by means of sequences from Helicobacter pylori: Lessons from Ladakh", Proceedings of the National Academy of Sciences (USA) 101 (2004): 4746--4751 [link]
Abstract: The history of mankind remains one of the most challenging fields of study. However, the emergence of anatomically modern humans has been so recent that only a few genetically informative polymorphisms have accumulated. Here, we show that DNA sequences from Helicobacter pylori, a bacterium that colonizes the stomachs of most humans and is usually transmitted within families, can distinguish between closely related human populations and are superior in this respect to classical human genetic markers. H. pylori from Buddhists and Muslims, the two major ethnic communities in Ladakh (India), differ in their population-genetic structure. Moreover, the prokaryotic diversity is consistent with the Buddhists having arisen from an introgression of Tibetan speakers into an ancient Ladakhi population. H. pylori from Muslims contain a much stronger ancestral Ladakhi component, except for several isolates with an Indo-European signature, probably reflecting genetic flux from the Near East. These signatures in H. pylori sequences are congruent with the recent history of population movements in Ladakh, whereas similar signatures in human microsatellites or mtDNA were only marginally significant. H. pylori sequence analysis has the potential to become an important tool for unraveling short-term genetic changes in human populations.

H. pylori is a bacterium essentially co-extensive with the human race, colonizing roughly half of all adult human stomachs. It's notorious because most ulcers are caused by H. pylori infections, though most infections do not cause ulcers, for reasons which are totally unclear. It's a good bug for genomics, because it has lots of easily-detectable genetic variation. For the present purposes, what's important is that it mutates much more rapidly than human genomes do, and it's mainly transmitted within families, i.e., your H. pylori is very probably a descendant of your parents' H. pylori. Since it is an "unusually sexual bacterium", one in which recombination of genes from different sources is common, your H. pylori genes generally combine those of both of your parents' bugs, just your own genes combine those of your parents. Hence, differences in Helicobacter genomes should track fairly recent population movements fairly accurately.

In this case, it seems that H. pylori really does reflect the population history. While Muslim and Buddhist Ladakhis are genetically indistinguishable, bacterially they're quite distinct, in the way their abstract describes. (It's not clear to me what, exactly, counts as the "Near East" for them; my guess is that it would include the Iranian Plateau, which would make sense historically.) Their more detailed findings suggest that "the late Muslim missionary wave appealed primarily to animists and other nonconverted Ladakhi, whose bacterial genomes had acquired fewer foreign nucleotides than their [Tibetan] Buddhist counterparts. The latter scenario agrees well with the geographic restriction of Muslims to the the Kargil and Suru Valleys as well as the capital city". This agrees, though Wirth et al. don't mention it, with the accounts of historians like Rizvi. All in all, this is a really cool paper, and I look forward to seeing this technique applied more broadly, though I'm glad I'm not the one collecting bacterial samples from dyspeptic patients in a Third World hospital at 11,000 feet.

Afghanistan and Central Asia; Biology; The Natural Science of the Human Species; Writing for Antiquity

Posted by crshalizi at August 08, 2004 13:43 | permanent link

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