The story appears to be complicated, and there is some disagreement about basic points. The introduction to this article has a good recent summary: http://adva

The story appears to be complicated, and there is some disagreement about basic points. The introduction to this article has a good recent summary: http://advances.sciencemag.org/content/2/4/e1501385.full http://advances.sciencemag.org/content/2/4/e1501385.full