While I agree that the Mongol Empire played a major role in the admixture of Caucasoids and Mongoloids, we have very clear evidence now that the two races had been mixing in Central Asia long before that time.
J Hum Genet.
Genetic imprint of the Mongol: signal from phylogeographic analysis of mitochondrial DNA
Cheng B, Tang W, He L, Dong Y, Lu J, Lei Y, Yu H, Zhang J, Xiao C.
Mitochondrial deoxyribonucleic acid (DNA) from 201 unrelated Mongolian individuals in the three different regions was analyzed. The Mongolians took the dominant East Asian-specific haplogroups, and some European-prevalent haplogroups were detected. The East Asians-specific haplogroups distributed from east to west in decreasing frequencies, and the European-specific haplogroups distributed conversely. These genetic data suggest that the Mongolian empire played an important role in the maternal genetic admixture across Mongolians and even Central Asian populations, whereas the Silk Road might have contributed little in the admixture between the East Asians and the Europeans.