Conceptualizing the Tibetan Plateau: Environmental constraints on the peopling of the “Third Pole”

Understanding the geomorphological and environmental constraints on the human use of the Tibetan Plateau (TP) is critical in reconciling genetic and archeological interpretations of the initial occupation of the world's “Third Pole.” The TP has essentially two major components: a high, relatively flat region above ~ 4000 m that constitutes the plateau proper and a mountainous region between ~ 1500–4000 m on the TP margins that is heavily dissected by the drainage systems of major Southeast Asian rivers. A biotic gradation ranging from subtropical environments to alpine steppe deserts is superimposed upon this geomorphology by the South and East Asian monsoons that lose strength as they move to the north and northwest. The distribution of the current Tibetan population mirrors these differences in environmental productivity, with the large majority restricted to river valleys in the TP margins below ~ 4000 m. The distribution of the initial TP occupants was likely similar, as was that of the earliest pastoralists, and any occupations dating to immediately before and after the Last Glacial Maximum are likely to be found in these TP marginal zones. Current archeological data suggests that the initial use of zones much above ~ 2500 m, even in these marginal areas, was by small hunting parties who occupied short-term camps before returning to lower elevation residential areas. Low incidences of cerebral edema and other similar short-term high altitude health issues during these trips would lead to genetic adaptations to high altitudes in their home population, even if that population occupied elevations well below 2500 m. When family groups of pastoralists began to seasonally occupy the higher elevations of the TP between ~ 8–5 ka, genetic adaptations to longer-term health issues, such as low birth weights, would occur at a faster rate, even if these family groups and the farming populations they interacted with occupied lower elevations for all or most of the year.