Inverted fertility?

Geneticists in Iceland have discovered a major genomic inversion that appears to promote female fertility and longevity. In some people, a 900,000-base-pair region on chromosome 17 is flipped around, and this inversion is most common in Europeans (20% of the population has it), but rare in Africans and Asians. The inversion also shows sign of having been favored by natural selection in the recent past, suggesting that there's something in the European environment that favors it. And very weirdly, the inversion diverged from its more common counterpart 3 million years ago - suggesting that either there is some selective advantage to having both copies, or that the inversion entered into the Homo sapiens population through interbreeding from a now-extinct hominid population shortly before modern humans left Africa.

How fascinating! It's got all the interesting elements of a population genetics story - signs of natural selection, clues of an unusual evolutionary history, a mysterious reproductive and survival advantage... The best part of the article is near the end:

Dr. Stefansson said his findings were empirical observations for which functional explanations have yet to emerge. It is not clear why the inversion should affect fertility or longevity, why it is favored in Europeans or how it has endured for three million years.

What an understatement! Part of the explanation is that the inversion increases recombination, which helps fertility. But why longevity? With such a large region, I'm sure there are a ton of possible explanations - a gene near the edge loses its promoter; the genes interact with the chromosomal packaging proteins differently; etc. I'm also puzzled by this: if this inversion is so great (have more offspring and live longer! a Darwinian dream), why hasn't it spread throughout the whole human population? Does it only work in Europe?

So many questions!

Update, 20 Jan: Keats' Telescope agrees that the analysis in the paper is valid and also wonders what on earth is going on with this inversion. This is the cool thing about science... every answer raises new questions.