content: Hell, Neanderthals had much larger brains than modern humans do. This shrinking brain phenomenon seems to correlate with the time period that they left the gene pool.  A comment below points out that living in safer, larger groups in warmer, more hospitable climates allows humans to survive with a smaller brain. Modern humans are much more social than Neanderthals, living in larger groups than our cousin subspecies. The extinction of Neanderthals is related to the inability to fight off the greater numbers of smaller brained and cheaper to produce(shorter gestation, less energy requirements=more children) modern humans.