…and nobody knows why although there’s no shortage of conjecture. From an article at Discovery magazine:
John Hawks is in the middle of explaining his research on human evolution when he drops a bombshell. Running down a list of changes that have occurred in our skeleton and skull since the Stone Age, the University of Wisconsin anthropologist nonchalantly adds, “And it’s also clear the brain has been shrinking.”
“Shrinking?” I ask. “I thought it was getting larger.” The whole ascent-of-man thing.
“That was true for 2 million years of our evolution,” Hawks says. “But there has been a reversal.”