That's an impossible question to answer because I don't know the date or dates you're using for "Mesolithic" or the exact geography of "in Asia". We should be getting some samples from Central Asia and the Caucasus soon, and that should answer some of the questions we have, such as whether they were derived for SLC 24A5 and 42A5 for that matter, and that may bring us closer to when and where the West Eurasian type depigmentation mutations occurred and started to spread.
However, we do know that a Samara hunter gatherer from 8,000 years ago who was R1b was derived for both SLC24A5 and SLC42A5, although the phenotypes in the Yamnaya R1b were more mixed, as although they were fixed for derived SLC24A5, they had much lower levels of SLC42A5 than do modern Ukrainians and they carried no depigmentation snps that could be interpreted as coding for blonde hair or blue eyes.
What you don't seem to grasp is that in the long time and over the long miles from the tropics to more northerly latitudes there was selection for one depigmentation snp after another. It took
time. According to this paper, for example, " we estimate that the onset of the sweep shared by Europeans and East Asians at
KITLG occurred approximately 30,000 years ago, after the out-of-Africa migration, whereas the selective sweeps for the European-specific alleles at
TYRP1, SLC24A5, and
SLC45A2 started much later, within the last 11,000–19,000 years, well after the first migrations of modern humans into Europe. We suggest that these patterns were influenced by recent increases in size of human populations, which favored the accumulation of advantageous variants at different loci."
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3525146/
Do you get it now?
(If I had to guess right at this moment, I think the mutation on SLC 24A5 might have occurred somewhere around the Caucasus and the Caspian Sea and then spread from there. The one on SLC42A5 looks ANE related to me, but whether it occurred in the Caucasus as well and then moved north with ANE like populations or actually occurred in the northern reaches of far eastern Europe I don't know. I think it's also very likely that the spread to fixation was a post Neolithic occurrence. )