I've read quite a bit about the Hakka in the past but admit that I've forgotten most.
It is my opinion (contrary to this link) that the Hakka are or are related to the Xiongnu...probably the southern branch of the Xiongnu that allied with the Han. Anyway, due to severe droughts in the north, they were allowed to migrate across China (in five different waves) to the south. To this day, the Hakka are known as 'the guests.'
What is generally not known is that during the migrations those Hakka with Caucasian features were discriminated against and killed losing most of these features before arriving in the south.
Btw, there are Hakka associations/groups in almost every country.
What I hadn't known before paying some attention to the Yakuts/Sakha was that in the Classical and Middle Ages periods they were technologically equal to the Chinese ON THE BATTLEFIELD. And so were the Hakka.
Nobody wanted to mess with them ~ probably why the Han let the Hakka cross China proper.
The guys with the Caucasion features were most likely those poor souls with more than their fair share of autosomal recessives left over from consorting with the Sa'ami!
To think, for hundreds of years the Norse thought of the Sa'ami as looking rather Asiatic ~ but actually, it was the other way around. The Asiatics looked rather Sa'ami!
This is the current avant garde theory on the origin of the Fenno-Ughric languages ~ that they reflect a Sa'ami origin (not the other way around), and are consistent with Europeans developing language and technology and MOVING EAST in Northern Asia.