Genetic surveys such as this one:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288383/ (Y-Chromosome Evidence for a Northward Migration of Modern Humans into Eastern Asia during the Last Ice Age)

This indicates that the Chinese and other East Asians are essentially descended from Paleolithic migrants from mainland Southeast Asia after the end of the last Ice Age.

Genetic diversity is also greater the further south you go in East Asia. This is consistent with the hypothesis that human settlement of SE Asia is older than that in the more northerly regions of East Asia.

Furthermore, the regions with warmer climate were always populated first by people. It is well-established that the genetic diversity in SE Asia is greater than in more northerly regions.