Genes Promoting Fertility Are Found in Europeans. Here is the abstract in Nature. Bullet points:

• A gene inversion is found among Icelanders which seems to confer greater fertility on carriers.
  
• This gene is found in ~20% of Icelanders. The macroregional breakdown for the frequency of the inverted sequence is as such: Europe: 21%, Africa: 6% and Asia: 1%. Here is an image showing more detailed distributions.
  
• The inversion seems to promote recombination (evolved for "evolvibility"?).
  
• The inversion has been increasing in frequency Europe for the last 10,000 years.
  
• It is a very old genetic feature. The last common ancestor of variant H1 (normal) and H2 (inverted) is 3 million years ago. In other words, we are tickling the initiation of our lineage of homonids as distinct from the chimpanzees. The theorists propose as solutions to this age: balancing selection or that the inversion "jumped" from another homonid lineage (remember, the MHC genes have variants which predate our species, so this wouldn't be the first time).
  
• Also, the inversion seems to be correlated with longevity. As one of the scientists interviewed notes, there is often a tacit assumption that fertility and longevity are trade offs, so this is surprising.
  

So, what could have been happening in the last 10,000 years? And what's the problem if you have two copies of H2 (assuming balancing selection)? Well, if you followed the link to the image above, you will note that the frequency of two diagnostic SNPs shows that the Druze and Samaritans (two genetically isolated groups in the Middle East) are the modes. Ashkenazi Jews are somewhat lower, but still higher than generic Europeans. Also, the Irish are closer in frequency to Icelanders than Danes (recall that Icelanders are reputed to have some Irish admixture on top of the Scandinavian base), who have a lower frequency of the diagnostic markers, while Finns and Russians are lower down on the total pole than the Western European groups. Though the markers are nearly absent in East Asians (excluding an outlier in Taiwan which the researchers discard as a likely error), they are present at non-trivial rates among Africans and the peoples of the New World, in the latter case, in particular the Maya (those who have access to the full article can tell me if inspecting the SNP frequencies is leading me down the wrong path).

Let the speculation begin. I will throw out a guess and say it has something to do with a particular form of agriculture (based simply on the gradient out of the Middle East and the possible New World mode of the Maya).