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Archiver > GENEALOGY-DNA > 2009-10 > 1255120497


From: Alan R <>
Subject: Re: [DNA] R-L21 vs. R-U152
Date: Fri, 9 Oct 2009 20:34:57 +0000 (GMT)


As far as I understand, no DNA technique other than phylogeny is capable of inferring direction of spread of a clade.  I think its accepted that intraclade variance dating is of little use in this respect.  So, within a single clade it appears that it will always be impossible to infer direction of movement by only looking at it and its distribution.  All that appears to leave is the geographical spread of a clade relative to the phylogenetically upstream clades (and interclade dating) and the archaeological record with which to infer direction of spread.  They seem to be the only tools we have.  It is important to remember too that the archaeological record only presents a very few periods with windows for a hugely important population input like L21 and R1b1b2 in general to have taken place.  I think people tend to think there are are plethora of possibilities but if the archaeological record is in any way to be taken seriously, there are
very very few potentially population defining events or phases inferred in the record that could explain L21's core distribution.  Its more of a case of choosing between 2 or 3 options than a complete mystery IMO. 
 
There are no recorded north to south demographically crucial movements in any prehistoric period that could explain L21's general distribution today.  We know from Vince V's work that the general phylogeny of R1b is from old in the SE to young in the NW and that it was no earlier than the Neolithic.  This direction is in line with most of the crucial post-Mesolithic populating events of European prehistory identified archaeologically.  There is absolutely no evidence that there was a sudden dramatic reverse movement back towards the south from a northern set off point.  We know that L21 and S28 are branches of S116 which is in turn a descendant of the S116- S21- clades which are mainly found in the south-east so there is little question that S116 spread east to west and then at some point spawned L21 on that journey.  Distribution and phylogeny practically demands that this mutation happened among S116* people who were located north of the Alps
and whose ancestors had in turn come from a phylogenetically upstream source somewhere in the south-east. 

I do not think this is all as mysterious as people often feel.  Its really a case of working out the detail and the timing from a small group of options. The devil is in the detail but if you combine DNA, archaeology etc, the basic very broad picture of a spread from SE to west and north-west seems firm.  There is no need to look for invisible sudden reverse movements to explain L21's distribution.  L21 is best represented on the continent on the right general trail across Europe leading to the expected departure shores to the isles that archaeologists had independently concluded on long ago.  Its also geographical common sense that the settlers crossed from the nearest parts of the continent.  Northern France is intervisible with England and NW France is by far the closest continental area to Ireland.   

L21 appears both in phylogenetic and distributional terms to be closely allied with S28.  The story of one is unlikely to be radically different from the other.  One is weighted more northerly than the other but they have a major overlap in the former Gaulish Celtic heartland's and both feature a major drop off as the more solidly Germanic areas are entered..  The idea of explaining S28 in a radically different way from L21 seems unfounded to me and is simply born of an early over-focus on L21 in the isles.  L21 is probably as common (maybe more-France is still very under-sampled) in absolute numbers in France than it is England, where about 80% of the isles population lives.  So, the isles peak is very localised to Ireland and Irish Sea Britain, not a general isles phenomenon. 

In my opinion, L21 is a far too casually ignored 'gift horse' SNP in terms of really deep ancestral origins.  It appears to link the isles with a trail on the continent likely indicating the last stages of the ancestral R1b1b2 trail through west-central and NW European before they reached the sea and took to the boats for the isles.  I don't think its going to get better than that.  I suppose you cant rule out the possibility that L21 was split into currently unidentified clades by the time the journey to the isles was made.  My pet theory is that the L21 mutation blossomed as Linearbandkeramik (LBK) spread from the Danube to the Rhine and then passed from the Middle Rhine into north-central France and finally in the form of LBK-derived cultures to the Channel shores and the isles.   This was a long process from Danube to the isles.  There was probably over 1000 years between the arrival of LBK on the Rhine and the first crossing
of Neolithic man to the isles from France c. 4300BC.  It doesn't seem impossible that some SNPs occurred during that 1000 year period but as i think almost all L21 that crossed to the isles would have crossed from the north shores of France (or perhaps the Low Countries) I don't think new clades would tell us anything mind blowing.  The only stretch of continental coast (other than anomalous Norway) that is known to have a lot of L21 also happens to be the closest to Britain and Ireland and is also where archaeology would point to so I think Occam's Razor would say that L21 must have crossed from Northern France where L21 seems to have its continental peak.    
 
Alan  



Janet said

Alan, Perhaps we are missing the forest for the trees. I tend to think
about L21 as a band across the north, and from there the spread went
to the south all along the length of that band. That gives us greater
densities in the north and lesser towards the south. Now where L21
began, and how it migrated within that band is yet to be determined.

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