Since I am not a scientist, I can speculate all I want; right? Ian Logan has an array of interesting charts. But here I am concerned only with the U5b charts:http://www.ianlogan.co.uk/discussion/gifs/U1_gif.htmhttp://www.ianlogan.co.uk/sequences_by_group/u5b2_genbank_sequences.htm
There are three branches of U5b2 on his chart. The smallest twig, I have no idea where it comes from, other than ultimately western Europe; as it is currently represnted by only one sample (sample #150) on the Logan tree. It has more coding region mutations than any of the other U5b2's, and my haplotype is right behind. This may have to do with adapting to new and stressful environments after the migration into a colder climate, or other environmental stresses. I only have coding region mutations for #150 and #160.
The largest branch (everything below 4732) has representatives in both Iberia (Spain) and Italy (Ice Age refugia), along with derivatives in the Briitish Isles where they settled after the Big Melt. Most U5b2's in the Mitosearch collection appear to be of British Isles derivation, and seem to belong to this branch.
My own twig (everything under 11653,12634,13630) is only shown by two examples on his tree (#160 & #235). But another haplotype in this branch, not shown, is readily seen in a scientific paper (Fraumene) and is found on the island of Sardinia (2 people). I propose that this twig, which includes my mtDNA haplotype, had their Ice Age refuge in Italy; especially in the upper Po River Valley. At least it would seem that the area around Milan was a staging area before those ancient U5b2's migrated back north up over the Alps and on into Northern Europe. Others of this bunch seem to have gravitated down to the seashore. I don't know whether this twig was so restricted in range or not. My maternal line seems to trace back to the German Rhineland; origin of many colonial Pennsylvania Germans. I have the impression that U5b2, as a group, were never very populous. But they managed to spread far and wide in western Europe, probably integrated in with the Celts.
When the time came to move north after the big melt, my group may have taken the Ticino River up to Lake Maggiore and on up to the Gotthard Pass, following the migrating birds. From there they took one or more valleys down the Rhein River drainage system into Germany. A smaller bunch may have traveled up the Adda River to Lake Como (or farther east); then up and over into valleys leading down to the Danube River. This might explain U5b samples popping up in central Europe. A more likely scenario would be that some U5b women moved down the Danube with the eastward expansion of the Celts in pre-Roman times (Ellis,"The Celts").
The largest Branch of the three sub-branches under U5b2 (everthing under 4732) would most likely have moved northward from southern France and Iberia. They may have gotten an earlier start than my branch, who would have had to wait for Alpine glaciers to recede somewhat. The final cold episode of the last Ice Age, the Younger Dryas, ended around 10,000 years ago. After that the climate warmed up rapidly, with all available haplogroups participating in the northward surge into new hunting grounds. There seems to have been an earlier push northward, judging from the Older Cheddar Man skeleton in England, dated to 12,000+ years. And camp fires in northern France have been dated to around 13,000 years ago (Sykes), marking the line of advance. Were some among them U5b2 women ancestors of ours? Another guess is they were U5b1 reindeer hunters slowly on their way to Finland, having not yet evolved into U5b1b. Or maybe that vangard belonged to haplogroups H or V. What happened when the Younger Dryas thousand year cold snap kicked in? Did everyboody retreat back to their refugia?
There were earlier modern humans north of the Alps, of course, but the maximum cold period prior to about 13,000 years BP drove all or most of them south to their repective refugia. For a peek at our probable haplogroup U/U5 ancestors, read the article starting on page 28, in the Sep/Oct 2007 issue of ARCHAEOLOGY magazine titled: "The Dawn of Art; A controversial scholar claims modern culture was born in the foothills of the Alps." Small carvings of animals and flutes are examples at two neighboring limestone cave sites in Swabia, as well as a carving of a mythological Lionman. That may indicate that cave lions were found around there. The author is talking 35,000 to 40,000+ years ago, during the Upper Paleolithic Aurignacian.