The reason that matters for the evolution of human brains, of course, is that pregnant and breast-feeding women just so happen to have brains that are doing very similar things to what human brains do at other major transitions in our body’s life cycle: they violently rearrange themselves. A pregnant woman’s brain will, quite reliably, shrink in volume by as much as 5 percent during her third trimester, followed by a steady rebuilding during the first few months after giving birth.[*25] Similar things seem to happen in other mammalian mothers, but it’s particularly dramatic in human brains.
