Sand grains shed light on the peopling of Tibet

When did humans establish a permanent presence on the Tibetan Plateau, which reaches a height of some 4500 m? The physiological challenges posed by the altitude suggest that a long period of acclimatization would have been required to conquer such an environment. Consequently, estimates of the dating of initial human settlement vary widely. Indeed, it is thought that the Tibetan Plateau may have been the last region on Earth that was permanently settled by members of our species.

Strikingly, early settlers have left behind hand- and footprints in the limestones found near the hot springs of Chusang at an elevation of 4270 m. The hydrothermal springs in this region transport dissolved calcite minerals from deep underground, which precipitate as so-called travertine limestone when the waters reach the surface. The evidence strongly suggests that the footprints made while the travertine was being formed.

These impressions in the rock have now been dated by an international research team led by geologist Michael Meyer of the Geology Institute at Innsbruck University (Science, January 6th 2017, DOI:10.1126/science.aag0357). In addition to the conventional radiocarbon method and the uranium-thorium technique often employed by geologists, Meyer and his colleagues made use of thermoluminescence-based dating in their study. This last method essentially determines the age of rocks by measuring how much light is stored within their mineral matrix, as Michael Meyer explains.

Exposure to sunlight and other forms of radiation raises electrons in the atoms of minerals to higher energy levels, and the excited electrons can become trapped for long periods in defects in the crystal lattice. When the mineral is buried, natural radioactivity within the rock supplies the energy that drives the process. However, the excited state is quite labile, and when the electron falls back into its ground state the absorbed energy is lost in the form of thermoluminescence.

For thermoluminescence dating, the geologists isolated sand grains associated with the limestone impressions and exposed them to laser light in the laboratory under controlled conditions. This input of energy is sufficient to displace the trapped electrons, thus allowing them to fall back into their original ground state. The resulting thermoluminescence signal can be measured with high precision, and is proportional to the age of the rock.

Meyer’s team in Innsbruck specialises in the dating of single grains of quarz, which are individually stimulated by laser light. In this case, the total luminescence emitted by 1000 such grains was sufficient to enable them to determine a reliable date for the samples from Chusang.

“Based on our calculations, these impressions are between eight and twelve thousand years old. This corresponds to the Early Holocene, i.e. the beginning of the present warm period, when the climate on the plateau was characterized by monsoon-like conditions,” Meyer says. The wetter and warmer conditions will have promoted plant growth, and thus the survival of animals, at progressively higher elevations. Hence the dating studies suggest that the people who settled in the area responded to the amelioration of climatic conditions in order to exploit new hunting grounds at ever higher altitudes.

Life at elevations exceeding 2500 m requires specialized physiological adaptations. Indeed, a specific mutation in a gene that is linked to the ability to cope with low oxygen levels has been shown to have been positively selected in the Tibetan population. Some geneticists have suggested that this mutation may be as much as 30,000 years old. Moreover, as Meyer is careful to point out, the impressions dated in the study may not record the earliest settlement of the high plateau of Tibet by prehistoric humans. Nevertheless, the dating of the hand- and footprints at Chusang is a milestone in the investigation of the history of human life in the harsh environment of Tibet’s high – and now very dry – hills.