A recent study of human skulls covering a period of about 600 years shows the human skull has changed significantly. Facial features, particularly the lower jaw, have decreased markedly in size, and more intriguingly the size of the cranial vault has increased “by more than a centimetre” (presumably they mean a cubic centimetre). What does this mean? I don’t think anyone knows, but I’d guess the increase in processed foods means less need for strong jaws, and the increased brain size might explain why IQ scores are increasing over time.
Update: The increase in IQ over time is known as the Flynn effect. There are some good links here. Cranial capacity is proportional to body size, as discussed here. I haven’t been able to find the publications of the original study, so I can’t tell if they controlled for increases in body size. If not, then the increase in cranial capacity is likely not significant, as people today are generally larger than in the Middle Ages.