Here is an MDS plot of 454 individuals from 32 East Eurasian/substantially East Eurasian populations:
Population labels have been placed in the (x,y) spot of the population averages. This corresponds -usually- to the midpoint of blobs of individuals from that population, but some populations have a few European-admixed individuals, and hence their population average is transposed. Consult the legend for color/point information for the 32 populations.
The most striking feature of this plot is the extreme homogeneity of the vast majority of East Eurasians. They may occupy a tiny blob on the left of the plot, but the various ethnic groups of China, the Japanese, and the Cambodians all appear to be very close to each other; indeed you can hardly see their labels in the mass of points. Here is a blowup of that part of the plot.
These dots represent the overwhelming mass of East Eurasians, and indeed the biggest single set of human groups. The scattering of populations to the right of the main MDS plot are, in comparison, demographically insignificant, some of them numbering less than 1,000 individuals.
What this plot shows, in tangible form, is a picture of mankind's past: before the invention of agriculture, most humans lived in small tribes, scattered across the world. We can be fairly certain that the action of genetic drift and natural selection would have created a cornucopia of human diversity, with high between-group diversity due to high levels of genetic drift.
Out of all this variety, some tribes of hunters made the transition to agriculture, growing in numbers, filling the areas they exploited, and expanding into new ones. The hunters were on their way to extinction, but new tribes formed at the fringes, those of pastoral nomads exploiting animals to thrive where neither farmer nor hunter could.
In the world of farmers, with growing population densities and expansion came the breakdown of isolates: this led to a further homogenization of the farmers' gene pool, as different tribes that had adopted the new way of life lost all trace of their past tribal identities and formed new ones based on the common language of the agriculturalists and the new way of life.
With more human bodies in the farming communities, came more novel mutations, and hence more of the raw materials of selection.
Coupled with the new challenges of agriculture, for which man is unaccustomed to, the social challenges of living close to many others in villages, and, later, cities, the cognitive challenges of new symbolic systems of communication, selection further reduced diversity in key aspects of human appearance and behavior, while maintaining it, or even increasing it in others, such as resistance to pathogens.
In western Eurasia this process was pushed to its limits, and there are virtually no nomads or hunters to be found there anymore. Africa was explored by Europeans just in time to find living hunters such as the San and Pygmies still in existence there. A few centuries more, and perhaps they, too, would have beeen absorbed into the mass of expanding farmers.
The few dots of European- and Chinese-admixed individuals that deviate from their own populations are a reminder of what would have happened if genetic science had not arrived at the scene when it did. For better or worse, the odds are stacked against most of these peoples surviving as distinct entities. The numbers are against them, and, sooner, or later, they will be assimilated.