... What I am tempted to read into that figure and lack of geographical patterning is that despite the exposive natuie of L21 expansion, there must have been a homeland where L21 existed long enough for several variants to emerge before the main dispersal took place and that that dispersal involved pretty homogenious mixes of L21 people. While variance seems to suggest all R1b1b2 emerged very quickly and that L21 came very hot on the heels of S116, there is no way of knowing how quickly L21 dispersed because there are no useful downstream SNPs until we are in the historic era. There is a gap of a couple of thousand years between the usual MRCA variance dates and the downstream SNPs.
So was L21 located in one area for a considerable time before dispersal? That would seem to be hinted at by the diagram. On the other hand variance calculations to date seem to contradict this as they show similar intraclade dates for L21 in both the isles and the continent.
It does seem that L21 must have been of fairly good size before the rapid expansion. I guess that is almost self-evident. It takes people to make people, but the point is their population must have expanded in place before it took off in all directions.
Given the time proximity of P312*, U152+ and L21+ expansions we know they must have been growing fast at an early time in their "careers".
In my mind I'm picturing a city with a number of large districts or suburbs that are growing extremely fast to the point where they finally burst in several directions. Perhaps a P312* subclade or two predominated a suburb that sent colonists out to the west to Iberia. Soon after or at about at the same time a suburb with predominately L21+ types moved north along the Rhine. Perhaps a little later, a U152 oriented district, perhaps from the city center, expanded north and south.
Of course, none of the city districts were pure anything and over time whole other different peoples (probably from the east) moved into the old city so the P312 types became diluted there or moved away. Perhaps the flight from the city to the new colonies was in search of the riches of "gold"... or more likely tin for making bronze. Almost sounds like 15th and 16th century European explorers and colonists.
Well, so where's the homeland for L21? It must not be far from the homeland of U152 and many P312* sub-clades. My opinion of where it's not, given where there isn't the strong mix of all three is:
1) Not in the Isles
2) Not in Iberia
3) Not in Scandinavia
Well, that's not narrowing it too much, but there must have been a valley on the continent where a P312*, U152, L21 people had ample land, good climate and the resources to grow and hone their wares.