- The journals simply arranged it long in advance. This would comport with their treatment of the chimpanzee genome announcement, which occurred in Nature but had coordinating papers in Science in the same week.
- Science found out about the Nature paper and moved up its commentary to take advantage of the press coverage. This would be relatively easy since the technical comment takes up so little journal space.
- Nature found out about the microcephaly exchange and moved up publication of the long-delayed (6 month review) bone paper to beat the press.
The result of the timing is pretty clear: Nature got a head start on saturating the press, which put out stories on the new bones starting on Tuesday. The first article mentioning the Science exchange appeared on Thursday, and the story has not been widely picked up. Outcome: the vast majority of press accounts emphasize the additional evidence for small body size, but don't include the strongest evidence for pathology published to date.
This outcome doesn't, of course, necessarily favor any of the above hypotheses, especially since a technical comment cannot expect to get the kind of attention of a regular research paper. And Nature always comes out a day before Science.
But Tuesday is not a day before Thursday: it's two days. The stories started on Tuesday because somebody broke the Nature press embargo. According to Google News, the first stories to break on Tuesday were from Reuters and from Nature's own online news site.
So who did it?