I think we have to start thinking about the idea that humans in the last 30, 40, or 50,000 years have been domesticating ourselves. If we're following the bonobo or dog pattern, we're moving toward a form of ourselves with more and more juvenile behavior. And the amazing thing once you start thinking in those terms is that you realize that we're still moving fast. I think that current evidence is that we're in the middle of an evolutionary event in which tooth size is falling, jaw size is falling, brain size is falling, and it's quite reasonable to imagine that we're continuing to tame ourselves. The way it's happening is the way it's probably happened since we became permanently settled in villages, 20 or 30,000 years ago, or before.
That's from Taylor's interesting new book Not a Chimp: The Hunt to Find the Genes that Make Us Human. Taylor does stress that this hypothesis is speculation rather than established fact.
By the way, our skulls are becoming thinner, a process known as gracilization.