The Xiongnu were the bad asses of the steppes. The Yuezhi(sp) were the top dogs until the Xiongnu split them into the greater and lesser Yuezhi and the lesser Yuezhi migrated away and are probably the people we call Schytians.
The greater Yuezhi were assimilated by the Xiongnu and warred with the Han Dynasty and the Han finally defeated the Xiongnu. The Great Wall Of China was begun to keep out the Xiongnu. The Yuezhi were a Caucasian tribe and some believe the Xiongnu were also or at a minimum a mixed race group.
It is my opinion that the Hakka ('the guests') were a mix of all these racial groups and eventually migrated all the way across China after a serious drought in the north. Some migrated toward Europe and may be the Picts of Scotland(?).
Thanks, blam...when you mention the Picts, you're getting to the home turf of my Y chromosome.
Now how do we know that?
What about the old theory that the Xiongnu are the same as the Huns (known in European history in the 4th and 5th centuries)? The main problem might be finding anyone who could be shown to be of Hunnic ancestry to make the comparison. (The Kaiser's family doesn't count--they weren't really Huns.)