A recent study showed that survivors had brains that were 53% larger, which was perhaps useful in avoiding predators. The problem is, it is genuinely unclear what role brain size plays in intelligence.
Small size — which includes having a small brain — hones the ogre gnat’s remarkable neurological abilities
The human brain has actually been shrinking in the last 30,000 years, the same period that has also shown great intellectual achievements.
This is further indirect evidence— not, of course, drawn out here—that the human brain did not develop the way it did simply to enable survival.