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

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Abstract

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.

Introduction

The questions of when the first anatomically modern human populations arrived at the Tibetan Plateau (TP), how and when they gradually conquered and occupied its higher elevations, and when and where genetic adaptations to high altitude stress took place remain mostly topics of speculation. Interpretive scenarios vary widely as a result, yet despite their many differences these speculations share a common thread: they all refer to the Tibetan Plateau as if the plateau was a singular geomorphic, hydrological, and environmental phenomenon. It is not. Rather, elevational, ecological, climatological, and geomorphological conditions vary significantly across the TP, in both east–west and north–south directions, and these significant differences have a dramatic impact on the distribution and adaptive responses of modern Tibetans. These modern constraints on where people live, how they make their living, and how they interact with one another likely have deep roots in the past, and shed light on the archeological and biological questions posed above. Here, I briefly review some of these environmental constraints and discuss how these variables affect our interpretations of when, where, and how humans populated the TP.

Section snippets

Tibetan Plateau geomorphology

Geomorphically, there are really two TPs: the high, relatively flat, plateau above ~ 4000 m and the plateau margins between ~ 1500–4000 m (Fig. 1):

The high plateau — The Tibetan Plateau proper consists of an elevated platform, > 30% of the contiguous U.S. and ~ 25% of the entire European continent in area, that is sharply bounded on all sides by extensively faulted, rugged, heavily dissected mountainous escarpments. Interior mountains occur, particularly the Tanggula Mountains that run east–west

Climatic and biotic constraints

Precipitation on the TP is a product of two major weather systems (Fig. 3): the South Asian, or Indian, Monsoon, whose moisture originates in the Indian Ocean and flows north and slightly east across the plateau (Goswami, 2005), and the East Asian Monsoon, whose moisture originates in the western Pacific and flows in a northwesterly direction across the TP (Hsu, 2005). A third weather system, the prevailing Westerlies/continental winter monsoon, impacts the far northern TP regions but has a

Human population distribution on the Tibetan Plateau

Not surprisingly, the distribution of modern human populations on the TP largely mimics these patterns of elevational differences, moisture availability and environmental productivity (Fig. 5). As of the 2000 census, a very large majority of the human population in the Tibetan Autonomous Region (TAR) and Qinghai Province, the two province level areas constituting most of the TP, lived below 4000 m (Liao and Li, 2003, Liao and Sun, 2003). Most of these people are located in intermountain valleys

Implications for genetic adaptations and “colonization” models

These basic environmental constraints on the human occupation of the TP are generally well known to most students of the colonization process and have been discussed elsewhere (e.g., Aldenderfer and Zhang, 2004). However, their implications for our understanding of that process are less well integrated into most models of how and when historical patterns of subsistence and settlement patterns came to be. The implications of these constraints are important for two separate, but related, areas of

Summary and conclusions

Conceptualizing the Tibetan Plateau as being composed of two distinct regions, the high relatively flat, arid upper plateau above 4000 m and a marginal zone of dissected mountains cut by extensive river drainage systems at elevations of 1500–4000 m, is useful in helping understand where and when early human foragers first began to colonize the TP and where and when genetic adaptations to the stress of high altitude occupations began. Most of the modern Tibetan population lives in this marginal

Acknowledgments

This stems largely from discussions with team members of the Tibet Paleolithic Project including Jeffrey Brantingham, Xing Gao, John Olsen, Charles Perreault, and David Rhode, and from the work of the TPP over the last 15 years. I learned as much or more from students at the Qinghai Salt Lake Institute, Chinese Academy of Science as they did from me. I thank Sergio Ayala and Guoqiang Li for assistance with illustrations, Evelyn Seelinger for making the text presentable, and Rowan Flad, Charles

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