One argument I've seen frequently is that humans offset our smaller crania by losing the sloping foreheads of earlier hominids. Neuron count in the cerebral co

One argument I've seen frequently is that humans offset our smaller crania by losing the sloping foreheads of earlier hominids. Neuron count in the cerebral cortex seems to predict intelligence better than total neuron count (e.g. putting humans ahead of elephants, and chimps ahead of giraffes), with only one odd case (pilot whales) beating humans. So if we lost overall capacity but retained most or all of our cerebral cortex size, the change might be explained. (Full disclosure: a quick look says most primate brains are right near 26% cerebral cortex, while humans are around 18%. It's possible intermediate hominids were even lower, but without knowing why the difference exists I can hardly assert that.)