Constructed EI

To better elucidate the comprehensive natural environment conditions of the QTP, this study selected the vegetation type, elevation, river classification (river and lake), accumulated temperature of ≥ 0 °C, and longitude indicators as geographical and environmental factors based on the activity characteristics of the microlithic hunter-gatherers (Table ​(Table4).4). Using the reclassification and raster calculator tools in the spatial analysis of Arc GIS, we constructed an EI of the QTP to comprehensively characterize the natural environment of the QTP. Using this approach, a higher index would indicate better environmental conditions that are more suitable for human survival and living. The specific construction method is as follows:

Classification and evaluation of geographical factors.

Due to the diversity of vegetation types, only typical representatives were selected for “Typical types of vegetation” column heading. “River” Column heading represents grades 1, 3, 4, and 5 rivers, respectively; 5 km, 7.5 km, and 10 km represent buffer areas 0–5 km, 5–7.5 km, and 5–10 km from river, respectively.

Previous studies have shown that a mixture of forest-steppe was the dominated vegetation in which hunter-gatherers existed, followed by steppe⁵⁴. Due to the vegetation density (forest) or relative lack of vegetation (desert), we concluded the sites that would not be conducive to human activities. The former would be difficult for prehistoric humans to conduct productive activities and transportation, while the latter would be more difficult to provide the necessary resources to support human societies owing to its inherent low biological productivity. Accordingly, the highest evaluation of temperature forests and steppe was 9, each type of temperature steppe was approximately 6–8, broad-leaved and coniferous forests were approximately 4–7, shrub was 6, meadow vegetation was 5, alpine vegetation and desert were approximately 1–2, and swamp and non-vegetation regions were 0.

In general, an elevation of 1,600–2,400 m a.s.l may result in a human response to the relatively low atmospheric oxygen. With an elevation above 3,000–3,600 m a.s.l, humans would have obvious signs of hypoxia. However, if humans are in an adaptive elevation ladder and have enough time, they will gradually adapt to these hypoxic plateau conditions. When the elevation is more than 4,500 m a.s.l and the atmospheric pressure is close to one-half of the sea level, humans will suffer from obvious hypoxemia and significant, negative physiological responses. When the elevation is higher than 5,500 m a.s.l, humans will suffer from a severe decline in function, with some damage being irreparable⁵⁵. According to elevation, values from 0–9 were evaluated (Table ​(Table4),4), namely, elevation classification. With this ranking, lower evaluation numbers indicate elevations that are more difficult for humans to adapt to and survive in.

Rivers were divided into four grades 1, 3, 4 and 5 with each river used as a buffer area according to 5 km or 10 km distance from the main river. In general, the higher the grade is, the higher the evaluation of the region near the river.

The accumulated temperature of ≥ 0 °C plays an important role in characterizing the climate of the plateau. Specifically, plateau regions with accumulated temperatures ≥ 6,500 °C belong to tropical zones, < 500 °C belong to frigid zones, and approximately 1,500–4,200 °C represent temperate zones. At present, temperature crops are grown in areas of ≥ 2000 °C, while chimonophilous crops are exclusively grown in areas of < 2000°C⁵⁶. Accordingly, the higher the accumulated temperature is, the higher the evaluation is.

Longitude is an important factor for prehistoric human to spread on the plateau. Since Last Glacial Maximum (LGM), the general direction of prehistoric human expansion to the QTP was a process going from east to west with the characteristics that the eastern sites have the early age results and the western sites have the late age results. The reasons for this expansion pattern are as follows. First, some scholars have proposed a “three-step jumping” model for human beings on their march into the plateau⁵⁷. In this model, the first step was the LGM with an age range of 24.0–16.0 cal. ka BP. During this time, human activities occurred in the grassland at an elevation below 3,000 m. The second step was from 15.0–11.2 cal. ka BP during the Last Deglacial period, when hunter-gatherers expanded to regions with elevations of 3,000–4,000 m a.s.l. During this expansion, they left behind short-term camp sites, which were used to search for small- and medium-sized animals. The third and final step occurred during the early and middle Holocene when humans spread to high elevation regions above 4,000 m a.s.l. Second, genetic studies have revealed that approximately 98% of the maternal genetic component of modern Tibetans can be traced back to the northern Chinese who prehistorically moved into the QTP^6,58. Tibetans have been recognized as Mongolians from East Asia, so there should be a view that people in northern China are expanded from east to west across the plateau⁵⁸. However, the possibility of people originating from other directions and entering the plateau cannot be excluded, owing to the limitation of scale and differences in what time they may have entered the plateau⁵⁹. Given this, these complexities have been omitted from the current discussion. Thus, we can conclude that the higher the longitude, the earlier the prehistoric age of human activity, and the higher the evaluation value.

Based on the above classification criteria and their application to the Analytic Hierarchy Process (AHP), each factor was evaluated, weighted, and the following relationship was constructed:

In the formula (³), “I” represents the plateau EI, “H” is the elevation classification, “R” is the river classification, “P” is the vegetation classification, and “T” is classification of accumulated temperature of ≥ 0 °C. The higher the EI is, the more suitable it is for human survival and occupation. It should be pointed out that we used modern geographical factors to construct this EI, which are—to some extent—different from past environmental and geographical factors, but the changes of the natural environment are systematic and continuous, and the present natural environment is the inheritance of historical evolution, therefore the present natural environment has inseparable connections to its past form. Thus, the simulation used in this study plays an important role in such work.