Ko, Koyama, and Sng (2018) argue that a severe, recurring, and unidirectional external threat from the Eurasian steppe fostered political centralization in China, whereas Europe faced a wider variety of smaller threats from multiple directions (Scandinavia, Central Asia, the Middle East, North Africa) and remained fragmented. The geography made political fragmentation unsustainable for China: small states near the steppe were more vulnerable than large ones, so the steppe threat pushed toward unification. In Europe, no single threat axis dominated, so a system of multiple defensive states was viable. Out of seven major waves of nomadic invasion since the first century CE, China was involved in six; Europe was affected only twice (Chaliand 2005).
The steppe corridor also functioned as a two-way highway for technologies, crops, and animals. Exchange was carried not by single migrations but by a relay mechanism: mobile pastoralists exchanging goods and practices with neighbors, who exchanged with theirs (Frachetti 2012). The DOM2 horse lineage spread east from the Volga-Don (~2200 BCE, Librado et al. 2021), followed by Sintashta chariot technology and Seima-Turbino tin-bronze metallurgy. Wheat and barley moved east through the Inner Asian Mountain Corridor, while Chinese broomcorn millet moved west.
Wheat reached the Hexi Corridor by ~2000 BCE but remained marginal in core China for millennia. During the Han dynasty, official salaries were denominated in millet. Isotopic evidence from Eastern Zhou sites shows urban commoners eating wheat under subsistence pressure while elites consumed millet (Zhou et al. 2017, 2019). Only when rotary quern technology spread during the Tang dynasty (618–907 CE) did flour processing make wheat desirable (Lu et al. 2019; Guedes et al. 2015).
Feng Li (2006) shows that the upper Jing River valley was the battleground where the Xianyun repeatedly penetrated toward the Zhou heartland. The Duo You ding inscription records a major 840 BCE incursion; Zhou forces captured 117 Rong chariots. These were chariot-on-chariot engagements. In 771 BCE, the Quanrong sacked the Zhou capital Haojing, ending the Western Zhou.
Gnecchi-Ruscone et al. (2021, Science Advances) found that Scythian gene pools emerged from admixture of steppe MLBA populations (Sintashta/Srubnaya/Andronovo) with eastern Eurasian sources. The Scythians shared the chariot cultures' western steppe ancestry but with substantial additional eastern admixture. They were a genetically diverse confederation sharing a material culture (the "Scythian triad"), not a single ethnicity.
Qin originated as a horse-breeding operation on the steppe frontier (Feizi, ~891 BCE). The decisive military adaptation was Zhao's 307 BCE cavalry reform (胡服骑射). Qin followed. Meng Tian's ~215 BCE Ordos campaign triggered Xiongnu consolidation under Modun (~209 BCE). At Baideng (200 BCE), Modun trapped Emperor Gaozu, forcing the heqin policy. This was the mirror-empires dynamic in operation (Turchin 2009).
Our polygon combines two sources. For the western and Central Asian sections (27–82°E), we derive the boundary from the replication dataset published by Currie, Turchin, Turner, and Gavrilets (2020, Harvard Dataverse doi:10.7910/DVN/8TP2S7), using cells with maxSteppe_distance=0. For the eastern section (82–126°E), we hand-trace from Ko, Koyama, and Sng (2018) Figure 2, incorporating the Ordos Loop and Manchurian grassland terminus.