So what happened to the Caucasian / Celts? Wiped out, didn't procreate as fast, driven West, didn't like the smell 0f Chinese cooking?
There are records of the Han that the Yuezhi were split, the Greater Yuezhi allied with the Han and the Lesser Yuezhi were driven back west. Most were eventually assimilated. The Chinese skeletons did not begin appearing in the region until around 100-200BC. The Uyghurs adopted the reconstructed image (known as 'The Beauty Of Loulan') of one of the mummies as the mother of their country and her image is now on their money and etc. The great wall was built to keep these people out of China...the wall didn't keep anyone out.
Paul Ratchnevsky quoted the contemporary Chinese Zhao Hong as saying that Genghis Khan differed from other Tartars in that he was tall and had long beard, and quoted Marco Polo as saying that Khubilai did have black hair but fair complexion 'ringed with red'. Rashid ad-Din, in 'Collected Chronicles', said that Genghis Khan was amazed to see that Khubilai had black hair while the rest of their family had red hair and said his grandson must have taken 'his old uncles' features.
Genghis Khan belonged to the Borjigid clan which was a branch of the Kiyats to which the Jurchens (Jurchids), Changsi'ut and the Kiyat-Sayar also belonged.
The importance of the Borjigids lies in the legend that after the death of Dobun-mergen, the alleged ancestress Alan-ko bore Bodunchar after being visited by a strange 'golden glittering man'. Rashid ad-Din alluded to a foreign origin of the visitor and described him as having red hair and blue-green eyes.
Paul Ratchnevsky speculated that the mysterious visitor could be a Kirghiz since the Kirghiz people were said to be tall and possessed red hair and green eyes.
Note that Rashid ad-Din's writings came from secondary sources and rumors and that Yuan Shi (History of Yuan Dynasty) only recorded that Bodunchar had grey eyes against the chestnut-colored eyes of his brothers and half-brothers. Nothing is mentioned of hair or skin of Bodunchar or Genghis Khan."