“Self-domestication, if it occurred, must have happened something like 800,000 years ago, and there is no evidence whatsoever that human brains shrank at that time.”

So where does that leave us? Unfortunately, to understand why brains shrank, you would need to pinpoint exactly when the shrinkage began. But the fossil record makes this an almost impossible task. Older fossils are harder to find, so the record is heavily skewed towards newer specimens. For some poorly preserved species, we are currently dependent on a few or even a single cranium.

“What we do know is that back in the Pleistocene, human brains were about the same size as Neanderthal brains, which is quite a bit bigger than the average size of human brains today,” says…