You're referring to the Flynn effect. But the Flynn effect is a 20th century (post-WWII) phenomenon that describes an increase in the average intelligence test performance (and similar abilities like memory span). There are a number of explanations that have been proposed for this effect, the most convincing ones being improved nutrition and schooling. Either way, this effect does not apply on an evolutionary scale (or even a larger historical one) and it also represents a fairly narrow, gradual change rather than the broad, drastic change suggested in the OP. Also, in recent years, the Flynn effect appears to have reached a ceiling and is even reversing in some countries.