"Am I correct that the tarim basin back in that era contained several glacier-fed lakes that would have provided the "mummies" ancestors the means for agriculture? That land is so inhospitable today."
Yup, they grew grapes and other things. There are still stumps today of their fruit orchards. The best book to read on these people is The Tarim Mummies, by J P Malloy and Victor Mair...another good one is The Mummies Of Urumchi by Elizabeth Barber.