This might be consistent with the finding that homo sapiens 30,000 years ago (such as those in found in Europe, often called Cro-Magnon) had larger brains that moderns homo sapiens. One argument that I have heard about this is that humans might, nowadays, be more clever about using their brains, using better algorithms, or simply are more specialized, doing brilliant in some areas, while losing some skills that no longer seem relevant. There is also the possibility that homo sapiens are, in some sense, "domesticate" and have lost brain size for the same reason that domesticated cows have lost brain size.