The paper, to its credit acknowledges that the "effective mutation rate" depends on population growth history as I have argued a year and a half ago. The authors write:
Owing to the uncertainties associated with the estimate of the evolutionary effective microsatellite mutationrates, depending on the haplogroup demographic history,37 we considered two different population models: (1) a constant size population and (2) a single rate of m=0.01 for exponential population growth. After calibration for the specificmicrosatellites used in this study,13 we found evolutionary effective mutationrates of 7.9x10-4 and 1.3x10-3, respectively.and:
As I noted in haplogroup sizes and observation selection effects haplogroup sizes provide a sanity check to assumptions about population growth history:
As an upper limit, we used the coalescence time of the R-M343/P25 haplogroup (12.9 ky, 95% CI=11.6–14.3 ky, under a conservative scenario of constant population size), which, on the basis of the accumulated nucleotide and microsatellite diversity (Table 1; Figure 2), most likely originated outside Africa. The coalescence time of the seemingly African-specific haplogroup R-V69 (6.0 ky, 95% CI=4.2–8.2 ky, under the hypothesis of an expanding population) was used as a lower limit.
Haplogroups do not reach commonly-observed present-day sizes under the assumption of constant population size. Inferences of age based on such an assumption are a very conservative upper limit. However, the assumption of m=0.01 also does not result in "large" present day haplogroups (see previous link).
Thus, I suppose that the age of R-V88 is younger than 4.2–8.2 ky, and could be as young as ~3-4ky in a rapidly expanding population. To determine how fast R-V88 actually grew, we must take into account its present-day demographic size (how many people in the world now possess it). The final estimate must be consistent with both the demographic size and the current Y-STR variance.
I don't have data on R-V88 prevalence today, but it really doesn't take a very large haplogroup in order to infer a very fast growth rate, and a Y-STR variance accumulation rate (effective rate) close to the germline one. Therefore, I am guessing that R-V88 is also one of a growing palette of haplogroups that expanded during the Bronze Age.
European Journal of Human Genetics doi:10.1038/ejhg.2009.231
Human Y chromosome haplogroup R-V88: a paternal genetic record of early mid Holocene trans-Saharan connections and the spread of Chadic languages
Fulvio Cruciani et al.
Although human Y chromosomes belonging to haplogroup R1b are quite rare in Africa, being found mainly in Asia and Europe, a group of chromosomes within the paragroup R-P25* are found concentrated in the central-western part of the African continent, where they can be detected at frequencies as high as 95%. Phylogenetic evidence and coalescence time estimates suggest that R-P25* chromosomes (or their phylogenetic ancestor) may have been carried to Africa by an Asia-to-Africa back migration in prehistoric times. Here, we describe six new mutations that define the relationships among the African R-P25* Y chromosomes and between these African chromosomes and earlier reported R-P25 Eurasian sub-lineages. The incorporation of these new mutations into a phylogeny of the R1b haplogroup led to the identification of a new clade (R1b1a or R-V88) encompassing all the African R-P25* and about half of the few European/west Asian R-P25* chromosomes. A worldwide phylogeographic analysis of the R1b haplogroup provided strong support to the Asia-to-Africa back-migration hypothesis. The analysis of the distribution of the R-V88 haplogroup in >1800 males from 69 African populations revealed a striking genetic contiguity between the Chadic-speaking peoples from the central Sahel and several other Afroasiatic-speaking groups from North Africa. The R-V88 coalescence time was estimated at 9200–5600 kya, in the early mid Holocene. We suggest that R-V88 is a paternal genetic record of the proposed mid-Holocene migration of proto-Chadic Afroasiatic speakers through the Central Sahara into the Lake Chad Basin, and geomorphological evidence is consistent with this view.