This is in contrast to the Flynn effect: The Flynn effect is the substantial and long-sustained increase in intelligence test scores measured in many parts of the world from roughly 1930 to the present day http://en.wikipedia.org/wiki/Flynn_effect
Hmm, so as we adapt and excel at our new surroundings we lose skills that are no longer necessary?
Just look at all the vestigial crap we still carry around [1]: appendix, tailbone, wisdom teeth, inside corner of the eye, etc. We don't use these yet we don't loose them. And loosing these is propably much, much easier than intelligence, because the development of things like the appendix is controlled by much fewer factors than intelligence.
[1] Which might still have a purpose that we don't know yet, for example the appendix might be important in immune response: https://www.scientificamerican.com/article.cfm?id=what-is-th...