The underlying biochemical problem would show up long before hunter gatherers, the equivalent of a 2-3% drop in IQ across billions of years is massive pressure

The underlying biochemical problem would show up long before hunter gatherers, the equivalent of a 2-3% drop in IQ across billions of years is massive pressure to find a solution. Brains are a major calorie sink that’s a big enough difference to matter. If it’s a 0.0001% drop then sure evolution would ignore that, but so should we.