come out of Africa originally (The article did not endorse that notion, but it is commonly stated
that way. The Afro-Asiatic (Semitic) languages are ordinarily included with the Nostratic)
X is already generally excerpted from this package and it is entirely possible A and B were no more present than X was. We do know now that Americans crossed back into Siberia (Kamchitka) and this is reasonably associated with the introduction of A2a into Siberia: A could have been absent from East Siberia and Beringia, and could have been introduced from another point of origin entirely (A is related to X) We do have what looks like an expansion of the s and Ds in that area at that time.
Here again is a copy of the Linguistic families chart which started this speculation.