Everybody's noticing the new article in PLoS Computational Biology about lactase persistence, which I've been emailed from several readers. Thanks for sending it, everyone -- it's always helpful even if I get it more than once!
The short version is that the authors place the origin in Germany around 7500 years ago, and using a 2-d forward-time dispersal model, find that fits well with the distribution of allele frequencies in Central Europe.
There's only one little problem: It's hard to see how the same scenario gets the allele to India. Or, for that matter, Ireland. The authors posit that Indian lactase persistence will be found to be caused by a "diversity" of alleles. They seem to have missed this paper that found a greater diversity of lactase-associated haplotypes "north of the Caucasus" -- consistent with an initial steppe dispersal. OK, that's two problems, and they're not little.
Their potentially interesting finding -- the dispersal of lactase persistence in their model didn't increase the diffusion of other central European genes -- should inspire more modeling. How independent can a strongly-selected allele be of its genomic background? Can selection cause demographic events without affecting unlinked neutral variation? I imagine we can explore this issue with differential equations.
Itan Y, Powell A, Beaumont MA, Burger J, Thomas MG. 2009. The Origins of Lactase Persistence in Europe. PLoS Comput Biol 5(8): e1000491. doi:10.1371/journal.pcbi.1000491
Enatteh NS and 26 others. 2007. Evidence of Still-Ongoing Convergence Evolution of the Lactase Persistence T-13910 Alleles in Humans. Am J Hum Genet 81:615-625. doi:10.1086/520705