A new study, published in the journal Science, has documented human adaptations to living at high altitudes on the Tibetan Plateau. This vast elevated region in Central Asia includes most of Tibet and Qinghai Province, together with a part of the Ladakh region of Jammu and Kashmir. Measuring 1,000 km (620 miles) from north to south and 2,500 km (1,600 miles) east to west, it has an area of 2,500,000 sq. km (970,000 sq. miles) or roughly five times the size of France. With an average elevation exceeding 4,500 m (14,800 ft.), the Tibetan Plateau is the world’s highest and largest plateau, for which reason it is popularly known as “the roof of the world”.
The first evidence for human presence on the Tibetan Plateau dates to around 20,000 years ago, reaching altitudes of up to 4,300 metres above sea level. Artefacts include stone tools, small hearths and animal remains. It is thought that these finds were associated with single-use campsites occupied by hunters in pursuit of game. The first farming villages appeared 5,200 years ago.
In order to ascertain when and at what altitude food production became sufficient to sustain a long-term presence, researchers recovered artefacts, animal remains and plant remains from 53 sites located on the northeastern corner of the plateau. They obtained radiocarbon dates for charred cereal grains recovered from each site, which suggested that the sites fell into two groups. The first group dated from between 5,200 to 3,600 years ago, and the second group from between 3,600 to 2,300 years ago. None of the first group of sites exceeded an altitude much above 2,500 metres above sea level, but the majority of second group of sites ranged in altitude from between 2,500 to 3,400 metres above sea level.
The first group of sites were interpreted as reflecting the widespread settlement of the region by farming communities of the Yellow River and its tributaries. Millet accounted for all but a tiny percentage of cereal grains recovered from these sites. By contrast, at the higher altitude sites in the second group, the dominant crop was barley, together with some. Barley was grown alongside millet at the lower altitude sites in the second group. Sheep remains were present at those sites lying at or higher than 3,000 meters above sea level.
The presence of crops and livestock suggests that there was now a more sustained human presence at high altitudes. Barley is more frost-resistant than millet, but it also has a longer growing season of typically six months. The presence of houses and tombs at these sites further supports a sustained and likely year-round human presence. Thus this second phase of settlement of the Tibetan Plateau saw the northern Chinese millet joined and in some cases replaced by two Southwest Asian crops – barley and wheat. The introduction of these new crops enabled Tibetan farmers to exploit the harsher conditions of the higher elevations of the Tibetan Plateau.
Some sites outside the study region were located at even greater altitudes. For example, Changguogou on the southern-central part of the plateau has an elevation of 3,600 metres above sea level. Southwest Asian crops included barley, wheat, oats, rye and peas, in addition to millet.
The changing patterns of exploitation of the Tibetan Plateau might have been driven by climate change. The warm, wet conditions of the Early and Middle Holocene enabled both early hunter-gatherer exploitation of the plateau and the subsequent expansion of millet agriculture into the region. These conditions gave way to a colder, dryer climate, which did not favour millet and forced farmers to look for hardier alternatives. Not only did barley and wheat do well in the cooler conditions, they also grew at the higher elevations the farmers had previously been unable to exploit.
References:
1. Chen, F. et al., Agriculture facilitated permanent human occupation of the Tibetan Plateau after 3600 BP. Science (2014).
