Mapping out settlements of prehistoric populations presents some interesting problems for researchers. Unlike comparatively more recent migrations and settlements with written records or a comprehensive mythos, prehistoric movements of peoples are far harder to trace. Likewise, in comparatively remote regions, archaeological evidence that can fill in the gaps of written records are difficult to come by, and what artifacts are available are subject to intense debate and interpretation. Recently, however, scientists have been using maternal and paternal DNA to trace the ancient origins of modern day ethnic groups to build a strong genetic record of population movement that provides a foundation for constructing a prehistoric chronology.
Xuebin Qi, a member of Bing Su's Comparative Genetics division of the State Key Laboratory of Genetic Resources and Evolution of the Kunming Institute of Zoology (KIZ),Chinese Academy of Sciences (CAS), undertook a study on one such prehistoric migration that has been the source of contention for decades—the peopling of the Tibetan Plateau. Modern day Tibetans, numbering some 5 million, survive on the highest plateau in the world, in places at an altitude sometimes exceeding 3,500 meters. Despite these harsh conditions, Tibetans are well suited to the region, possessing several unique genetic adaptations that allow them to survive the thin air of the plateau.
By sampling some 6,109 of these ethnic Tibetans from 41 different geographic populations around the entire region, Su’s team attempted to reconstruct the prehistoric colonization and demographic history of modern humans settlement on the Tibetan Plateau. Through a comprehensive phylogenetic analysis of both paternal (n = 2,354) and maternal (n =
6,109) lineages as well as genome-wide SNP markers (n = 50), they found genetic evidence supporting two distinct major prehistoric migrations of modern humans into the region, separated by almost 20,000 years. The evidence suggests that the first migration occurred during the Upper Paleolithic (~30,000 years ago), followed by a younger and more recent population expansion during the Neolithic (~7,000-10,000 years ago).
While the evidence of two distinct settlements is in interesting novel finding itself, Su’s team also found evidences supporting a relatively young (~ 7,000 -10,000 years old) shared Y chromosome
and mitochondrial DNA haplotypes between Tibetans and Han Chinese. While an interesting finding, drawing an definitive conclusions from this shared genetic legacy are fraught with complications. In attempts to reconstruct the demographic history and chronology of modern human settlement on the Plateau, Su’s team systematically screened paternal and maternal lineages in Tibet and found two major Y-chromosomal haplogroups (D-M174 and O-M175) and another and four major mtDNA haplogroups (M9a, D, A and F). The majority of both of these
paternal (87.80%) and maternal (90.99%) haplogroups are found not only in Tibetans but also in the founding lineages of East Asians, many of which are shared between Tibetans and Han Chinese. However, in the course of the study they did not observe Y-chromosomal haplogroups in Tibetans coming from Central Asia and South Asia, even though the Tibetan Plateau is located in the vicinity of these two regions, suggesting that the Himalayas served as a natural barrier for human migration.
Taken on the whole, the genetic data indicates that Tibetans have been adapted to a high altitude environment since initial colonization around 30,000 years ago, before the Last Glacial Maximum, when water was more plentiful and widespread than it is in the region today. This settlement was then bolstered by a population expansion that coincided with the establishment of farming and yak pastoralism on the Plateau in the early Neolithic, which may have been necessary innovations to support the expansion of the population in such a harsh climate.
The complete study “Genetic evidence of Paleolithic colonization and Neolithic expansion of
modern humans on the Tibetan Plateau” was recently published in Molecular Biology and Evolution, available here [HYPERLINK NOT AVAILABLE FOR ADVANCED COPY—FIND AND REMOVE BEFORE POSTING ONLINE].
(By Andrew Willden)
