Can't tell if you're joking or serious.

In the latter case, the first humans (H. Habilis) had about 1/2 of H. Sapiens brain to work with, and a much smaller fraction of neocortex.

If that doesn't satisfy you, let's say I was speaking about some sort of human ancestor before that, which would have been about as dumb as chimps, unless you require proof of their dumbness as well.