Identical twins aren't so identical, not even their brains

That's been known to be true for some time. Depending on when the single fertilized egg divided, identical twins can have separarate amniotic sacs, umbilical cords and placentas, the same sac, the same cord as well, or even the same placenta.

And now, we're learning that identical twins may not have entirely the same genetic makeup, not even in brain cells. (And, perhaps this is due to chimera issues, which got new light in late 2013?)  As a result, twin studies for illnesses, behavior, etc., may be called into a bit of question, and future twin studies more carefully controlled for subjects.

At the same time, the authors occasionally slip into quasi-teleological language while wondering why evolution "allows" this. Their proposed answer as to why this may happen is interesting, but could serve to have that language nuanced better. Also, even if you allow for the teleological/personifying language, that may not be the reason why this happens; maybe epigenetic events are at least partially involved, even with identicals. After all, they don't share 100 percent the same environment.

And, it may turn out that on a statistical average, such transposition isn't favorable or unfavorable, and that we're talking about a ramped-up genetic drift. Or maybe more modest transposition was more favorable, and now, the degree of favorability has lessened. From what little I know, genetic studies like this are kind of like studying individual frames, or at best, snippets, from a movie, when the backdrop for the movie may have been radically different at another point.

Update, May 2, 2021: Fascinating Smithsonian piece here. If you're familiar with what chimeras are, namely a human fetus that has absorbed another in the womb, the discovery that moms can become chimeras from their fetuses is ... fascinating.